| "Meteorology is a branch of physics," a weather expert remarked
in 1939, "and physics makes use of two powerful tools: experiment
and mathematics. The first of these tools is denied to the meteorologist
and the second does not prove of much use to him in climatological
problems." So many interrelated factors affected climate, he explained,
that you couldn't write it all down mathematically without making
so many simplifying assumptions that the result would never match
reality. It wasn't even possible to calculate from first principles
the average temperature of a place, let alone how the temperature
might change in future years. And "without numerical values our deductions
are only opinions."(2)
- LINKS -
| That didn't stop people from putting forth
explanations of climate change. A scientist would come up with an
idea about how certain factors worked and explain it all in a page
or two, helped along by some waving of hands. Some scientists went
on to build a few equations and calculate a few numbers. At best they
could show only that the factors they invoked could have effects of
roughly the right magnitude. There was no way to prove that some other
explanation, perhaps not yet thought of, would not work better. These
mostly qualitative "theories" (in fact, merely plausible
stories) were all anyone had to offer until digital computers came
into their own, late in the 20th century. Until then, the climate
community had good reason to keep theory at arms' length. Even those
who tried to think in general physical terms hesitated to call themselves
"theorists," an almost pejorative term in meteorology.
More discussion in
| The science did have a foundation, at least potentially, in simple
ideas based on undeniable physical principles. The structures that
scientists tried to build on these principles were often called "models"
rather than "theories." Sometimes that was just an attempt to hide
uncertainty (a paleontologist complained that "'model'... is just
a word for people who cannot spell 'hypothesis'").(3)
But calling a structure of ideas a "model" did emphasize the scientist's
desire to deal with a simplified system that one could almost physically
construct on a workbench — something that embodied a hands-on
feeling for processes. The great trick of science is that you don't
have to understand everything at once. Scientists are not like the
people who have to make decisions in, say, business or politics. Scientists
can pare down a system into something so simple that they have a chance
to understand it.
| The first job of a model
was to explain, however crudely, the world's climates as presently
seen in all their variety. After all, the main business of climatologists
until the mid-20th century was the simple drudgery of compiling statistics.
Knowledge of average and extreme temperatures and rainfall and so
forth was important to farmers, civil engineers, and others in their
practical affairs never mind guessing at explanations. But
people could not resist trying to explain the numbers. A textbook
would start off with the main factor, the way sunlight and thus warmth
varies with latitude (perhaps with some calculations and charts).
There would follow sections on the prevailing winds that brought rain,
and how mountain ranges and ocean currents could affect the winds,
and so forth. It was all soundly based on elementary physics. It was
a dry exercise, however, not so much a theory of climate as a static
| Asked about changes in climate, most climatologists
at mid-century would think of the extremes that people should plan
for the worst heat wave to be expected or the "hundred-year
flood." If there was any pattern to such changes, experts believed
it would be cyclical. Rather than try to build a physical theory,
those who took any interest in the question mostly looked to numerical
studies. Perhaps eventually someone would find correlations that pointed
to a simple physical explanation. The varying number of sunspots,
for example, might signal changes in the Sun that correlated with
| The simplest and most
widely accepted model of climate change was self-regulation,
which meant that changes were only temporary excursions from some
natural equilibrium. Through the first half of the 20th century, textbooks
of climatology treated climate in a basically static fashion. The
word "climate" itself was defined as the long-term average
weather conditions, the stable point around which annual temperature
and rainfall fluctuated.(5*) After all, in their records of reliable observations the
meteorologists found only minor fluctuations from decade to decade.
These records went back less than a century, but they supposed that
one century was much like the next (aside from changes that took place
over many thousands of years, like the ice ages, which were themselves
seen as excursions from the very long-term average). Climatologists
expanded this idea into a "doctrine," as one critic called it, "that
the present causes of climatic instability are not competent to produce
anything more than temporary variations, which disappear within a
few years."(6) A leading climatologist put it straightforwardly
in 1946: "We can safely accept the past performance as an adequate
guide for the future."(7)
| Almost everyone believed in the natural
world's propensity to automatically compensate for change in a self-sustaining
"balance." If climate ever diverged toward an extreme, before long
it would restore itself to its "normal" state. As evidence, the atmosphere
had not changed or at least not extremely radically
over the past half-billion years.(8) And scientists came up with plausible regulating mechanisms
(some of them are described below). The approach expressed a generally
sound intuition about the nature of climate as a process governed
by a complex set of interactions, all feeding back on one another.
But romantic views that stability was guaranteed by the supra human,
benevolent power of Nature gave a false confidence that every feature
of our environment would stay within limits suitable for human civilization.
Issues of complexity and stability in the social structure of
climate science are explored in a supplementary essay on Climatology as a Profession.
| Certainly people knew
that climate did change. There was abundant historical evidence of
variations lasting a few decades or centuries, random swings or (as
some thought) regular cycles. Perhaps periods of drought like the
American Dust Bowl of the 1930s recurred on some schedule, or perhaps
not. Most of these changes were poorly measured, but some climatologists
sought to understand them, more or less as a sideline. Far more impressive
were the ice ages of the past few million years, undeniable proof
that climate could change enormously. Looking farther back, geologists
found evidence of much earlier ice ages, as well as periods when most
of the Earth had basked in tropical warmth. Understanding these great
alterations of the far past posed a fascinating scientific puzzle,
with no apparent practical value whatsoever.
| Basic Physics (19th century)
| "As a dam built across a river causes a local deepening of the
stream, so our atmosphere, thrown as a barrier across the terrestrial
rays, produces a local heightening of the temperature at the Earth's
surface." Thus in 1862 John Tyndall described the key to climate change.
He had discovered in his laboratory that certain gases, including
water vapor and carbon dioxide ( CO2), are opaque
to heat rays. He understood that such gases high in the air help keep
our planet warm by interfering with escaping radiation.(9)
| This kind of intuitive physical
reasoning had already appeared in the earliest speculations on how
atmospheric composition could affect climate. It was in the 1820s
that Joseph Fourier first explained that the Earth's atmosphere retains
heat radiation. He had asked himself a deceptively simple question,
of a sort that physics theory was just then beginning to learn how
to attack: what determines the average temperature of a planet like
the Earth? When light from the Sun strikes the Earth's surface and
warms it up, why doesn't the planet keep heating up until it is as
hot as the Sun itself? Fourier's answer was that the heated surface
emits invisible infrared radiation, which carries the heat energy
away into space. But when he calculated the effect with his new theoretical
tools, he got a temperature well below freezing, much colder than
the actual Earth.
| The difference, Fourier recognized, was due
to the Earth's atmosphere. Somehow it kept part of the heat radiation
in. He tried to explain this by comparing the Earth with its covering
of air to a box with a glass cover. That was a well-known experiment
the box's interior warms up when sunlight enters while the
heat cannot escape.(10) This was an over simple explanation,
for it is quite different physics that keeps heat inside an actual
glass box, or similarly in a greenhouse. (The main effect of the glass
is to keep the air, heated by contact with sun-warmed surfaces, from
wafting away, although the glass does also keep heat radiation from
escaping.) Nevertheless, trapping of heat by the atmosphere eventually
came to be called "the greenhouse effect."(11*)
until the mid-20th century would scientists fully grasp, and calculate
with some precision, just how the effect works. A rough explanation
goes like this. Visible sunlight penetrates easily through the air
and warms the Earth’s surface. When the surface emits invisible
infrared heat radiation, this radiation too easily penetrates the
main gases of the air. But as Tyndall found, even a trace of CO2,
no more than it took to fill a bottle in his laboratory, is almost
opaque to heat radiation. Thus a good part of the radiation that
rises from the surface is absorbed by CO2 in
the middle levels of the atmosphere. Its energy transfers into the
air itself rather than escaping directly into space. Not only is
the air thus warmed, but also some of the energy trapped there is
radiated back to the surface, warming it further.
|That’s a shorthand way of explaining the greenhouse effect
— seeing it from below, from "inside" the atmosphere.
Unfortunately, shorthand arguments can be misleading if you push them
too far. Fourier, Tyndall and most other scientists for nearly a century
used this approach, looking at warming from ground level, so to speak,
asking about the radiation that reaches and leaves the surface of
the Earth. So they tended to think of the atmosphere overhead as a
unit, as if it were a single sheet of glass. (Thus the "greenhouse"
analogy.) But this is not how global warming actually works, if you
look at the process in detail.
What happens to infrared radiation emitted
by the Earth's surface? As it moves up layer by layer through the
atmosphere, some is stopped in each layer. (To be specific: a molecule
of carbon dioxide, water vapor or some other greenhouse gas absorbs
a bit of energy from the radiation. The molecule may radiate the
energy back out again in a random direction. Or it may transfer
the energy into velocity in collisions with other air molecules,
so that the layer of air where it sits gets warmer.) The layer of
air radiates some of the energy it has absorbed back toward the
ground, and some upwards to higher layers. As you go higher, the
atmosphere gets thinner and colder. Eventually the energy reaches
a layer so thin that radiation can escape into space.
|What happens if we add more carbon dioxide? In the layers so high
and thin that much of the heat radiation from lower down slips through,
adding more greenhouse gas means the layer will absorb more of the
rays. So the place from which most of the heat energy finally leaves
the Earth will shift to higher layers. Those are colder layers, so
they do not radiate heat as well. The planet as a whole is now taking
in more energy than it radiates (which is in fact our current situation).
As the higher levels radiate some of the excess downwards, all the
lower levels down to the surface warm up. The imbalance must continue
until the high levels get warmer and radiate out more energy. As in
Tyndall's analogy of a dam on a river, the barrier thrown across the
outgoing radiation forces the level of temperature everywhere beneath
it to rise until there is enough radiation pushing out to balance
what the Sun sends in.
|While that may sound fairly simple once it is explained, the process
is not obvious if you have started by thinking of the atmosphere from
below as a single slab. The correct way of thinking eluded neary all
scientists for more than a century after Fourier. Physicists learned
only gradually how to describe the greenhouse effect. To do so, they
had to make detailed calculations of a variety of processes in each
layer of the atmosphere. (For more on absorption of infrared by
gas molecules, see this discussion
in the essay on Basic Radiation Calculations.)
<= Radiation math
|Despite Fourier's exceptional prowess in mathematics
and physics, he lacked the knowledge to make even the simplest numerical
calculation of how radiation is absorbed in the atmosphere.(12*) A few other 19th-century scientists attempted crude calculations
and confirmed that at the Earth’s distance from the Sun, our
planet would be frozen and lifeless without its blanket of air .(13) Tyndall followed with rich Victorian
prose, arguing that water vapor "is a blanket more necessary to the
vegetable life of England than clothing is to man. Remove for a single
summer-night the aqueous vapour from the air... and the sun would
rise upon an island held fast in the iron grip of frost."(14)
Tyndall needed no equations, but only simple logic, to see what many
since him overlooked: it is at night that the gases are most important
in blocking heat radiation from escape, so it is night-time temperatures
that the greenhouse effect raises the most.
| These elementary ideas were developed much further by the Swedish
physical chemist Svante Arrhenius, in his pioneering 1896 study of
how changes in the amount of CO2 may affect climate.
Following the same line of reasoning as Tyndall, Arrhenius pointed
out that an increase in the blocking of heat radiation would make
for a smaller temperature difference between summer and winter and
between the tropics and the poles. (Link
| Arrhenius's model used an "energy budget," getting temperatures
by adding up how much solar energy was received, absorbed, and reflected.
This resembled what his predecessors had done with less precise physics.(15)
But Arrhenius's equations went well beyond that by taking into account
another physical concept, elementary but subtle, and essential for
modeling real climate change. This was what one turn-of-the-century
textbook called "the mutual reaction of the physical conditions"
today we would call it "feedback."
An early example had been worked out by James Croll. This self-taught
British scientist had worked as a janitor and clerk in institutions
where he could be near the books he needed to develop his influential
theory of the ice ages. Croll noted how the ice sheets themselves
would influence climate. When snow and ice had covered a region,
they would reflect most of the sunlight back into space. The Sun
would warm bare, dark soil and trees, but a snowy region would tend
to remain cool. If India were somehow covered with ice, its summers
would be colder than England's. Croll further argued that when a
region became cooler, the pattern of winds would change, which would
in turn change ocean currents, perhaps removing more heat from the
region. Once something started an ice age, the pattern could become
Arrhenius stripped this down to the simple idea that a drop of
temperature in an Arctic region could mean that some of the ground
that had been bare in summer would become covered with snow year-round.
With less of the dark tundra exposed, the region would have a higher
"albedo" or reflectivity, that is, the ground would reflect
more sunlight away from the Earth. That would lower the temperature
still more, leaving more snow on the ground, and so forth. This
kind of self-reinforcing cycle would today be called "positive
feedback" (in contrast to "negative feedback," a
reaction that acts to hold back a change). Such a cycle, Arrhenius
suggested, could turn minor cooling into an ice age. Such gradual
processes were far beyond his power to calculate, however, and it
would be a big enough job to find the immediate effect of a change
|Arrhenius showed his physical insight at its best when he realized
that he could not set aside another simple feedback, one that would
immediately and crucially exaggerate the influence of any change.
Warmer air would hold more moisture. Since water vapor is itself a
greenhouse gas, the increase of water vapor in the atmosphere would
augment the temperature rise. Arrhenius therefore built into his model
an assumption that the amount of water vapor contained in the air
would rise or fall with temperature. He supposed this would happen
in such a way that relative humidity would remain constant. That oversimplified
the actual changes in water vapor, but made it possible for Arrhenius
to roughly incorporate the feedback into his calculations. The basic
idea was sound. The consequences of adding CO2
and warming the planet a bit would indeed be amplified because warmer
air held more water vapor. In a sense, raising or lowering CO2
acted mainly as a throttle to raise or lower the really important
greenhouse gas, H2O.
|The numerical computations cost Arrhenius
month after month of laborious pencil work as he estimated the energy
balance for each zone of latitude. It seems he undertook the massive
task partly as an escape from melancholy: he had just been through
a divorce, losing not only his wife but custody of their little boy.
The countless computations could hardly be justified scientifically,
given the large uncertainties in the data available to him for the
absorption of radiation and so forth. Moreover his model was crude,
neglecting crucial effects such as possible changes in cloudiness
as the moisture in the air changed with warming or cooling. Nevertheless
he came up with numbers that he published with some confidence.(17)
| "I should certainly not have undertaken these
tedious calculations," Arrhenius wrote, "if an extraordinary interest
had not been connected with them."(18) The prize sought by Arrhenius was the
solution to the riddle of the ice ages. He focused on a decrease
in CO2 as a possible cause of cooling. But he
also took the trouble to estimate what might happen if the amount
of gas in the atmosphere, at some distant time in the past or future,
was double its present value. He computed that would bring roughly
5 or 6 °C of global warming.
| This result is not far from the range that scientists would compute
a century later using vastly better models the current
estimate is that a doubling of CO2 will bring
some 3 degrees of warming, give or take a degree or two. Did Arrhenius
end up in the same range by sheer luck? Partly, but not entirely.
In the sort of simple physics and chemistry calculations where Arrhenius
had made his name, you can expect to come out roughly right if you
address a powerful physical effect in a straightforward way, starting
with decent data. The data Arrhenius fed into his calculations (based
on Samuel P. Langley's measurements of solar radiation reaching the
Earth's surface) were mostly in the right range. And Arrhenius included
all the obvious physics theory.
| But climate is not a simple physical system. A true calculation
of greenhouse effect warming requires measurements far more accurate
and far more complete than Langley's. The details of exactly what
bands of radiation are absorbed by CO2 and water
molecules might have happened to be arranged so as to produce a markedly
higher or lower amount of warming. As for theory, Arrhenius's model
planet was mostly static. He deliberately left aside factors he could
not calculate, such as the way cloudiness might change over the real
Earth when the temperature rose. He left aside the huge quantities
of heat carried from the tropics to the poles by atmospheric movements
and ocean currents, which also might well change when the climate
changed. Most important, he left aside the way updrafts would carry
heat from a warmer surface into the upper atmosphere. In 1963, when
a scientist made a calculation using roughly comparable assumptions,
but with the aid of improved data on the absorption of radiation and
an electronic computer, he found a far greater greenhouse warming
indeed impossibly greater. The assumptions left out too much
that was necessary to get a valid answer.(19*) (See below)
| Yet Arrhenius could have felt some confidence that he had not overlooked
any terribly potent effect. Calculations aside, since the atmosphere
keeps the surface of the Earth warm — in fact, roughly 40°C
warmer than a bare rock at the same distance from the Sun —
a few degrees sounds like about the right effect for a change in the
atmosphere that modestly altered the balance of radiation. Arrhenius
also knew that in past geological ages the Earth’s climate had
in fact undergone changes of a few degrees up or down, not many tens
of degrees nor mere tenths of a degree. While neither Arrhenius nor
anyone for the next half-century had the tools to show what an increase
of CO2 would really do to climate, he had given
a strong hint of what it could possibly do.
|A crude idea of how the amount of CO2 could
affect radiation was only the first half of a calculation of global
warming. The other half would be a model for figuring how the amount
of CO2 itself might change. A colleague of Arrhenius,
Arvid Högbom, had already published some preliminary ideas. That
stimulated an American geologist and bold thinker, Thomas C. Chamberlin,
to ponder the role of the gas in climate change. Arrhenius's 1896
paper spurred Chamberlin to publish "a paper which, I am painfully
aware, is very speculative..." The speculations revolved around the
great puzzle of the ice ages. Chamberlin later remarked how ice ages
were "intimately associated with a long chain of other phenomena to
which at first they appeared to have no relationship." He was the
first to demonstrate that the only way to understand climate was to
understand almost everything about the planet together not
just the air but the oceans, the volcanoes bringing gases from the
deep interior, the chemistry of weathered minerals, and more.
| Chamberlin's novel hypothesis
was that ice ages might follow a self-oscillating cycle driven by
feedbacks involving CO2. The gas was originally
injected into the atmosphere in spates of volcanic activity. It was
steadily withdrawn as it combined with minerals during the weathering
of rocks and soil. If the volcanic activity faltered, then as minerals
leached the gas out of the atmosphere, the planet would cool. Feedbacks
could make a temporary dip spiral into a self-reinforcing decline.
For one thing, as the land cooled, bogs and the like would decompose
more slowly, which meant they would lock up carbon in frozen peat,
further lowering the amount of CO2 in the air.
Moreover, as the oceans cooled, they too would take up the gas
warm water evaporates a gas out, cold water absorbs it. The process
would stop by itself once ice sheets spread across the land, for there
would then be less exposed rock and bogs taking up CO2.
Reversing the process could bring a warming cycle.(20)
| Chamberlin seemed only
to be adding to the tall pile of speculations about ice ages, but
along the way he had pioneered the modeling of global movements of
carbon. He made rough calculations of how much carbon was stored up
in rocks, oceans, and organic reservoirs such as forests. He went
on to point out that compared with these stockpiles, the atmosphere
contained only a minor fraction and most of that CO2
cycled in and out of the atmosphere every few thousand years. It was
a delicate balance, he warned. Climate conditions "congenial to life"
might be short-lived on geological time scales.(21)
| Chamberlin quickly added that "This threat of disaster is not,
however, a scientific argument..." He was offering the idea more for
its value "in awakening interest and neutralizing inherited prejudice,"
namely, the assumption that the atmosphere is stable. Other scientists
were not awakened. While some admitted that geological processes could
alter the CO2 concentration, on any time scale
less than millions of years the atmosphere seemed to be unchanging
The CO2 model, "recommended
to us by the brilliant advocacy and high authority of Prof. T.C.
Chamberlin," briefly became a popular theory to explain the very
slow climate changes of the past. Within a few years, however, scientists
dismissed the theory for what seemed insuperable
problems.(22) It appeared that there was already enough CO2
in the air so that its effect on infrared radiation was "saturated"
— meaning that all the radiation that the gas could block
was already being absorbed, so that adding more gas could make little
difference. Moreover, water vapor also absorbed heat rays, and water
was enormously more abundant in the atmosphere than CO2.
How could adding CO2 affect radiation in parts
of the spectrum that H2O (not to mention
the CO2 itself) already entirely blocked? These
studies with the crude techniques of the early 20th century were
inaccurate. Modern data show that even in the parts of the infrared
spectrum where water vapor and CO2 are effective,
only a fraction of the heat radiation emitted from the surface of
the Earth is blocked before it escapes into space. And that is beside
the point anyway. The greenhouse process works regardless of whether
the passage of radiation is saturated in lower layers. As explained
above, the energy received at the Earth’s surface must
eventually work its way back up to the higher layers where radiation
does slip out easily. Adding some greenhouse gas to those high,
thin layers must warm the planet no matter what happens lower down.
Through the first half of the 20th century, however, hardly any
of the few scientists who took an interest in the topic thought
in this fashion. They were convinced by the subtly flawed viewpoint
that looked at the atmosphere as a single slab. Even Chamberlin
concluded that Arrhenius had failed to get his physics right. After
all, was it reasonable to imagine that humans could alter something
as grand as the world's climate by changing a tiny fraction of the
atmosphere’s content? The notion clashed with common ideas
that everyone found reasonable. Scientists were confident they could
dismiss changes like the global warming foreseen by Arrhenius, because
the climate was known to be self-regulating.
| Many Sorts of Models (1900-1960)
| While most people thought it was obvious from everyday observation
that the climate was self-regulating, scientists had not identified
the mechanisms of regulation. They had several to choose from.
| Through the first half of the 20th century,
one common objection to the idea of a future global warming was that
only a little of the CO2 on the planet's surface
was in the air. Vastly more was locked up in sea water, in equilibrium
with the gas in the atmosphere. The oceans would absorb any excess
from the atmosphere, or evaporate gas to fill out any deficiency.
"The sea acts as a vast equalizer," as one scientist wrote, making
sure all fluctuations "are ironed out and moderated."(23)
| If the oceans somehow failed to stabilize the system, there was
another large reservoir of carbon stored up in organic matter such
as forests and peat bogs. That too seemed likely to provide what one
scientist called "homeostatic regulation."(24) For if more CO2 entered the atmosphere,
it would act as fertilizer to help plants grow more lushly, and this
would lock up the excess carbon in soil and other organic reservoirs.
| Beginning in the 1950s,
a few scientists attempted to work out real numbers to check the idea.
They constructed primitive models representing the total carbon contained
in an ocean layer, in the air, in vegetation, and so forth, with elementary
equations for the fluxes of carbon between these reservoirs. These
were only one of a number of "bookkeeping" studies, begun early in
the century and increasingly common by the 1950s, that added up the
entire atmosphere's stock of heat, energy, and various chemicals.
The implicit aim was to balance each budget in an assumed equilibrium.(25) There was little solid data for any
of these things, least of all the biological effects. Scientists could
easily adjust numbers until their models showed self-stabilization
by way of CO2 fertilization, as expected.
| Regardless of the CO2 budget, scientists expected
other feedbacks would regulate the world's temperature. In particular,
any increase of temperature would allow the air to hold more moisture,
where it would create more clouds, which would reflect sunlight away,
moderating the heat and doubtless restoring the equilibrium. Such
was the view of no less an authority than the President of the Royal
Meteorological Society, Sir George Simpson, K.C.B., F.R.S. In 1939
he explained that "the change in the cloud amount is the predominating
factor in the regulation of the temperature of the atmosphere. The
atmosphere appears to act as a great thermostat, keeping the temperature
nearly constant by changing the amount of cloud."(26*) That was about as simple as a physical model could get.
| Climate changes had
undoubtedly happened in the past, and slightly more complicated models
were needed to explain these. The most widely accepted style of explanation
invoked altered "weather patterns." The atmosphere could shift to a
different arrangement of winds, lasting decades or perhaps centuries,
with different storm tracks and precipitation. Such changes could
plausibly be caused by slow geological movements. The raising or lowering
of a mountain range would obviously alter winds and temperatures,
and opening or closing a strait would of course redirect ocean currents.(27)
Perhaps changes of geography were all that geologists needed to explain
the major climate changes in the Earth's history.
| These changes would be mostly regional, not
global, but many experts thought of climate changes as mostly local
affairs in any case. This view was in line with the traditional climatology
that explained the current distribution of deserts, rain forests,
and ice caps in terms of the location of mountain ranges and warm
or cold ocean currents. It was only necessary to take the reasoning
about prevailing winds, the tracks followed by storms, and so forth,
and apply it to a different geography. The result was what one expert
described as "a large amount of literature which is both geological
| Through the first half of the 20th century, scientific theories
on climate change continued to revolve mainly around attempts to explain
the ice ages. The explanations by geological rearrangements remained
the favorite type of theory, "never seriously challenged," as one
authority said in 1922.(29)
On the other hand, nobody ever made these explanations precise, and
they remained more a kind of story-telling than useful science.
| An important example of work on the topic was an idea developed
by the meteorologist Alfred Wegener in the 1920s. It happened that
Wegener loved geology as much as meteorology (he was also dedicated
to studies in Greenland, where he disappeared on an expedition in
his fiftieth year). In collaboration with another meteorologist, Wladimir
Köppen, Wegener worked through the geological evidence of radical
climate change. Traces of ancient ice caps were found in rock beds
near the equator, and fossils of tropical plants in rocks near the
poles. Wegener hoped to resolve the puzzle with his controversial
claim that continents drifted about from tropics to Arctic and back.
Along the way the two meteorologists worked out a climate change theory.
| They started off from Arrhenius's idea that the key variable, albedo,
depended on whether snow melted or persisted through the summer. The
great sheets of ice that reflected away sunlight could persist only
if they rested on land, not ocean. So the authors figured that the
recent epoch of ice ages had begun when the North Pole wandered over
Greenland, and ice ages had ceased once it moved on into the Arctic
|Wegener and Köppen
went into further detail using a theory that had been hanging around
since the 19th century. Croll had suggested that ice ages could be
linked with regular cycles in the Earth's orbit, the kind of thing
astronomers computed. Over many centuries these shifts caused minor
variations in the amount of sunlight that reached a given latitude
on the Earth. The variations gave rise to ice ages, Croll argued,
whenever enfeebled sunlight allowed excess snow accumulation. In the
1920s, Milutin Milankovitch began to develop these astronomical calculations
and plug them into equations that simulated the global climate. His
energy budget model was like Arrhenius's, but paid closer attention
to how much sunlight was received at each latitude in each season,
and what that would mean for ice and snow. Milankovitch found that
it was summers with weaker sunlight, in other words colder summers,
that counted for keeping the reflective snow in place not cold
winters, as Croll had supposed. Wegener and Köppen took up these
ideas, insisting that they were "nearly self-evident, and yet contested
by some authors!"(30)
| From then on, everyone who worked on climate
change took into account possible changes in albedo due to ice and
snow. For example, when G.S. Callendar took up the question of greenhouse
warming in 1938, in a discussion at a meeting of the Royal Meteorological
Society he noted that in recent decades temperatures had been rising
noticeably in the Arctic. That led him to suggest cryptically that
an increase of CO2 might be acting "as a promoter
to start a series of imminent changes in the northern ice conditions."(31)
| Some experts offered more specific elaboration, backed up by a
few primitive calculations. The most striking came from a respected
British scientist, C.E.P. Brooks. He argued that once an Arctic ice
cap formed it would chill the overlying air, which would flow down
upon the surrounding regions. Behind these frigid winds the snows
would swiftly advance to lower latitudes. Wind patterns would thus
redouble the impact of the familiar cooling feedback caused by increasing
reflection of sunlight. (Link from below) Only two stable states of the polar climate
were possible, Brooks asserted one with little ice, the other
with a vast white cap on the planet. A shift from one state to the
other might be caused by a comparatively slight perturbation, say,
a change of ocean currents that put a little extra heat into the Arctic
Ocean. Such a shift, he warned, might be shockingly abrupt.(32)
Scientists were beginning to recognize that feedback might grossly
magnify the smallest change. The meteorologist W. J. Humphreys,
for one, wrote in Atlantic magazine in 1932 that the current
situation was close to the conditions where ice sheets had ruled.
Thus "we must be just teetering on an ice age which some relatively
mild geologic action would be sufficient to start going." As an
example, he suggested that if a very wide sea-level canal were built
across Panama, currents flowing through it might shut off the Gulf
Stream, bringing "utterly destructive glaciation" to Northern Europe.
Or dust thrown into the air by a series of volcanic eruptions like
the famous Krakatau explosion might block enough sunlight to allow
the formation of ice sheets. This ice, scientists now understood,
might reflect enough sunlight to sustain the cold.
Humphreys also mentioned (following Chamberlin and others) that
additional feedbacks could reduce the main greenhouse gases. Colder
oceans would evaporate less water vapor into the air, and the colder
water would also tend to take up more of the "Earth's blanket" of
CO2. However, like nearly all the scientists
of his time, Humphreys did not consider changes in CO2
particularly important. Believing that adding or subtracting the
gas could have little effect on radiation, when they thought about
climate change they concentrated on volcanic dust, reflective ice
sheets and the like.(33)
|These models evidently left much room for
chance. Some pointed out that ice sheets should be self-sustaining
only in certain geological periods, when gross geographical changes
such as uplifting of mountain ranges had created a suitable configuration.
Even then, Brooks pointed out, "if the Arctic ice could once be swept
away, it might find some difficulty in re-establishing itself."(34) He told a Life magazine reporter in 1950 that
the Arctic ice had declined to a "critical size" and might no longer
be able to chill the air enough to maintain itself. Melting might
increase, and over centuries the seas might rise by tens
|Even when experts worked these ideas up in a few equations, the
results were scarcely quantitative, but only qualitative and indeed
speculative. Overall, theory remained in much the same state that
Simpson, as Director of the British Meteorological Office, had criticized
back in 1922. Writers on climate, he had said, each pushed their own
individual theory, and biased the evidence in their own favor. "There
are so many theories and radically different points of view," he complained,
"And new theories are always being propounded."(36)
| Simpson himself did
not resist the temptation to propound a personal theory, which can
serve as an example of the general style of argument of the times.
In 1937, he pointed out that, paradoxically, an increase of solar
radiation might bring on an ice age. The logic was straightforward.
A rise in the Sun's radiation would warm the equator more than the
poles. More water would evaporate from the tropics and the rate of
the general circulation of the atmosphere would increase. This would
bring more snowfall in the higher latitudes, snow that would accumulate
into ice sheets. The albedo of the ice sheets would cool the poles.
Furthermore, "the ice which enters the sea from these regions will
have a large effect on remote regions," cooling the entire hemisphere.
Of course, if the Sun grew brighter still, the ice sheets would melt.
Simpson worked out a complicated model of double-peaked glacial cycles,
driven by a supposed long-term cycling in the level of solar
radiation.(37) It was no more convincing than
anyone else's ideas. At a time when scientists were unable to explain
the observed general circulation of the atmosphere, not even the trade
winds, theories about climate change could be little more than an
| To wrestle with complex systems, for centuries scientists had imagined
mechanical models, and some had physically constructed actual models.
If you put a fluid in a rotating pan, you might learn something about
the circulation of fluids in any rotating system like the ocean
currents or trade winds of the rotating Earth. You might even heat
the edge of the pan to mimic the temperature gradient from equator
to pole. Various scientists had tried their hand at this from time
to time since the turn of the century.(38) The results seemed encouraging to the leading meteorologist
Carl-Gustav Rossby, who invited young Athelstan Spilhaus to join him
in such an experiment at the Woods Hole Oceanographic Institution.
In their pan they produced a miniature current with eddies. If this
represented an ocean, the current would have looked like the Gulf
Stream, if an atmosphere, like a jet stream (a phenomenon not understood
at that time). But they could not make a significant connection with
the real world.(39)
| Rossby persevered after
he moved to the University of Chicago in 1942 and built up an important
school of meteorologists. His group was the pioneer in developing
simple mathematical fluid-dynamics models for climate, taking climate
as an average of the weather seen in the daily circulation of the
atmosphere. They averaged weather charts over periods of 5 to 30 days
to extract the general features, and sought to analyze these using
basic hydrodynamic principles. The group had to make radical simplifying
assumptions, ignoring essential but transient weather effects like
the movements of water vapor and the dissipation of wind energy. Still,
they began to get a feeling for how large-scale features of the general
circulation might arise from simple dynamical principles.(40) In the 1950s, Rossby's
students and others moved this work onto computers.
| Meanwhile, to get another
peephole into the physics, Rossby encouraged Dave Fultz and others
to experiment with rotating mechanical systems. Funding came from
the Geophysics Research Directorate of the U.S. Air Force, always
keen to get a handle on weather patterns. The Chicago group started
with a layer of water trapped between hemispheres (made by sawing
down two glass flasks). They were delighted to see flow patterns that
strongly resembled the Earth's pattern of trade winds, and even, what
was wholly unexpected, miniature cyclonic storms. The group moved
on to rotate a simple aluminum dishpan. They heated the dishpan at
the outer rim (and later also cooled it in the middle), injecting
dye to reveal the flow patterns. The results, as another meteorologist
recalled, were "exciting and often mystifying."(41) The crude, physical model showed something rather like
the wavering polar fronts that dominate much of the real world's weather.(42)
| Meanwhile a group at Cambridge University carried out experiments
with water held between two concentric cylinders, one of which they
heated, rotating on a turntable. Their original idea had been to mimic
the dynamics of the Earth's fluid core in hopes of learning about
terrestrial magnetism. But the features that turned up looked more
like meteorology. "The similarity between these motions and some of
the main features of the general atmospheric circulation is striking,"
reported the experimenter. The water had something like a little jet
stream and a pattern of circulation that vacillated among different
states, sometimes interrupted by "intense cyclones."(43)
It seemed reminiscent of certain changing wind patterns at middle
latitudes that Rossby had earlier observed in the atmosphere (the
"Rossby waves" seen in the meanderings of the jet stream and elsewhere).
He had explained these patterns theoretically with a simple two-dimensional
| Following up with his
own apparatus, Fultz reported in 1959 the most interesting result
of all. His rotating fluid sometimes showed a symmetric circulation
regime, resembling the real world's "Hadley" cells that bring the
regular mid-latitude westerly winds. But at other times the pattern
looked more like a "Rossby" regime with a regular set of wiggles.
This pattern was somewhat like the standing waves that form in swift
water downstream from a rock (in the real Earth, the Rocky Mountains
act as the rock). Perturb the rotating fluid by stirring it with a
pencil, and when it settled down again it might have flipped from
one regime to the other. It could also flip between a Rossby system
with four standing waves and one with five. In short, different configurations
were equally stable under the given external conditions.(44)
This was realistic, for the circulation of the actual atmosphere shifts
among quite different states (the great trade winds in particular
come and go with the seasons). Larger shifts in the circulation pattern
might represent long-term climate changes.
| Fultz hoped that this kind of work would lead meteorologists to
"the type of close and fruitful interaction between theory and experiment,
mostly lacking in the past, that is characteristic of the older sciences."(45)
But in fact, fluid theory was wretchedly incapable of calculating
the behavior of even this extremely simplified model system. Anyway
the model was only a crude cartoon of the atmosphere interesting
to be sure, but unable to lead to anything definite about our actual
planet. The real contribution of the "dishpan" experiments was to
show plainly the tricky instability of any such system.
| The physical models
reinforced a growing suspicion that it was futile to attempt to model
a pattern of global winds on a page of equations, in the way a physicist
might represent the orbits of planets. This mathematical research
plan, pursued ever since the 19th century, aimed to deduce from first
principles the general pattern of atmospheric circulation. But nobody
managed to derive a set of mathematical functions whose behavior approximated
that of the real atmosphere.(46) The huge ignorance of scientists
was nakedly visible to the public, which looked with bemusement on
the farrago of simplistic theories that science reporters dug out
and displayed in magazines and newspapers.
| There was an alternative approach. In the 1950s, a few groups began
to build detailed numerical models of the general circulation of the
atmosphere on computers. The main impetus was to predict daily weather,
but some scientists hoped eventually to learn something about climate.
The models worked in a generalized way, but they were still more qualitative
than quantitative, and their reproduction of the Earth's actual climate
| Maurice Ewing and William Donn accordingly followed the traditional route,
qualitative argument, when they undertook an attack on the puzzle
of the ice ages. The two scientists had been interested for some time
in natural catastrophes such as hurricanes and
tsunamis.(47) Provoked by recent observations
of a surprisingly abrupt end to the last ice age, they sought a mechanism
that could produce rapid change. Also influencing them was recent
work in geology indications that over millions of years the
Earth's poles had wandered, just as Wegener had claimed. Probably
Ewing and Donn had also heard about speculations by Russian scientists
on how diverting rivers that flowed into the Arctic Ocean might change
the climate of Siberia. In 1956, all these strands came together in
a radically new idea.(48*)
|Our current epoch of ice ages, Ewing and Donn argued,
had begun when the North Pole wandered into the Arctic Ocean basin.
The ocean, cooling but still free of ice, had evaporated moisture
and promoted a pattern of severe weather. Heavy snows fell all around
the Arctic, building continental ice sheets. That withdrew water from
the world's oceans, and the sea level dropped. This blocked the shallow
channels through which warm currents flowed into the Arctic Ocean,
so the ocean froze over. That meant the continental ice sheets were
deprived of storms bringing moisture evaporated from the Arctic Ocean,
so the sheets began to dwindle. The seas rose, warm currents spilled
back into the Arctic Ocean, and its ice cover melted. And so, in a
great tangle of feedbacks, a new cycle began.(49*) (Link from below)
| This theory was especially interesting in
view of reports that northern regions had been noticeably warming
and ice was retreating. Ewing and Donn suggested that the polar ocean
might become ice-free, and launch us into a new ice age, within the
next few thousand years or even the next few hundred years.
|| <=Modern temp's
| The theory was provocative, to say the least.
"You will probably enjoy some criticism," a colleague wrote Ewing,
and indeed scientists promptly contested what struck many as a far-fetched
scheme. "The ingenuity of this argument cannot be denied," as one
textbook author wrote, "but it involves such a bewildering array of
assumptions that one scarcely knows where to begin."(50)
Talk about a swift onset of glaciation seemed only too likely to reinforce
popular misconceptions about apocalyptic catastrophes, and contradicted
everything known about the pace of climate change. Critics pointed
out specific scientific problems (for example, the straits are in
fact deep enough so that the Arctic and Atlantic Oceans would exchange
water even in the midst of an ice age). Ewing and Donn worked to patch
up the holes in their theory by invoking additional phenomena, and
for a while many scientists found the idea intriguing, even partly
plausible. But ultimately the scheme won no more credence than most
other theories of the ice ages.(51) "Your initial idea was truly a great
one," a colleague wrote Ewing years later, "...a beautiful idea which
just didn't stand the test of time."(52)
| Ewing and Donn's theory
was nevertheless important. Published in 1956, and picked up by journalists
who warned that ice sheets might advance within the next few hundred
years, the theory gave the public for the first time a respectable
scientific backing for images of disastrous climate change.(53) The discussions also pushed scientists
to inspect data for new kinds of information. For example, the theory
stimulated studies to find out whether, as Ewing and Donn claimed,
the Arctic Ocean had ever been ice-free during the past hundred thousand
years (evidently not). These studies included work on ancient ice
that would eventually provide crucial clues about climate change.
Above all, the daring Ewing-Donn theory rejuvenated speculation about
the ice ages, provoking scientists to think broadly about possible
mechanisms for climate change in general. As another oceanographer
recalled, Donn would "go around and give lectures that made everybody
mad. But in making them angry, they really started getting into it."(54*)
| Feedbacks Causing Cycles or Catastrophe
| Norbert Wiener, a mathematical prodigy, had
interests ranging from electronic computers to the organization of
animals' nervous systems. It was while working on automatic control
systems for antiaircraft guns during the Second World War that he
had his most famous insights. The result was a theory, and a popular
book published in 1948, on something he called "cybernetics."(55)
Wiener's book drew attention to feedbacks and the stability or collapse
of systems. These were timely topics in an era when electronics opened
possibilities ranging from automated factories to novel modes of social
communication and control. Through the 1950s, the educated public
got used to thinking in cybernetic terms. Climate scientists were
swimming with the tide when they directed their attention to feedback
mechanisms, whereby a small and gradual change might trigger a big
and sudden transition.
| At the start of the
1960s, a few scientists began to think about transitions between different
states of the oceans. Study of cores drilled from the seabed showed
that water temperatures could shift more quickly than expected. A
rudimentary model of ocean circulation constructed by Henry Stommel
suggested that under some conditions only a small perturbation might
shift the entire pattern of deep currents from one state to another.
It was reminiscent of the shifts in the dishpan fluid models.(56) All this was reinforced by the now
familiar concept that fluctuations in ice sheets and snow cover might
set off a rapid change in the Earth's surface conditions.(57)
| Similar ideas had been alive in the Soviet
Union since the 1950s, connected to fabulous speculations about deliberate
climate modification making Siberia bloom by damming the Bering
Straits, or by spreading soot across the Arctic snows to absorb sunlight.
According to the usual ideas invoking snow albedo, if you just gave
a push at the right point, feedback would do the rest. These speculations
led the Leningrad climatologist Mikhail Budyko to privately advance
doubts about how feedbacks might amplify human influences. His entry-point
was a study on a global scale. Computing the balance of incoming and
outgoing radiation energy according to latitude, Budyko found the
heat balance worked very differently in the snowy high latitudes as
compared with more temperate zones. It took him some time, Budyko
later recalled, to understand the importance of this simple calculation.(58) It led him to wonder, before almost
any other scientist, about the potentially huge consequences of fossil
fuel burning as well as more deliberate human interventions.
| In 1961, Budyko published
a generalized warning that the exponential growth of humanity's use
of energy will inevitably heat the planet. The next year he followed
up with more specific, if still quite simple, calculations of the
Earth's energy budget . His equations suggested that climate changes
could be extreme. In the nearer term, he advised that the Arctic icepack
might disappear quickly if something temporarily perturbed the heat
balance. Budyko did not see an ice-free Arctic as a problem so much
as a grand opportunity for the Soviet Union, allowing it to become
a maritime power (although he admitted the longer-term consequences
might be less beneficial).(59)
Even setting aside ice-albedo effects, interest in feedbacks
was growing. Improvements in digital computers were the main driving
force. Now it was possible to compute feedback interactions of radiation
and temperature along the lines Arrhenius had attempted, but without
spending months grinding away at the arithmetic.
|Meanwhile, new data on CO2
absorption made such calculations desirable. A few scientists took
a new look at the old ideas about the greenhouse effect. Nobody fully
grasped that the arguments about “saturation” of absorption
of radiation were irrelevent since adding more gas would make a difference
in the crucial thin layers from which radiation did escape into space.
But the way radiation traversed the high, thin layers was attracting
increasing scientific attention. New data showed that there was not
enough water vapor and CO2 in the atmosphere
to saturate the gases’ absorption of radiation in the upper
atmosphere (actually it was not even saturated at lower levels, although
that didn’t matter). A few physicists decided that it was worth
their time to calculate what happened to the radiation in detail,
layer by layer up through the atmosphere. (The details are discussed
in the essay on Basic Radiation Calculations, follow link at right.)
In 1963, building on pioneering work by Gilbert Plass, Fritz Möller
produced a model for what happens in a column of typical air (that
is, a "one-dimensional global-average" model). His key assumption
was that the humidity of the atmosphere should increase with increasing
temperature. To put this into the calculations he held the relative
humidity constant, which was just what Arrhenius had done long ago.
(The simplest alternative would have been to assume that the water
vapor concentration, the actual amount of water held in the world's
air, remained unchanged as climate changed. That was roughly equivalent
to the old assumption that a little global heating would make more
clouds, which would promptly cool things back to equilibrium.)(60) Möller got the same amplifying
feedback that Arrhenius had found. As the temperature rose more
water vapor would remain in the air, adding its share to the greenhouse
|When he finished his calculation, Möller
was astounded by the result. Under some reasonable assumptions, doubling
the CO2 could bring a temperature rise of 10°C
or perhaps even higher, for the mathematics would allow an
arbitrarily high rise. More and more water would evaporate from the
oceans until the atmosphere filled with steam! Möller himself
found this result so implausible that he doubted the whole theory
(in fact it failed to include essential features of the atmosphere).
Yet others thought his calculation was worth noticing. The model,
as one expert noted, "served to increase confusion as to the real
effect of varying the CO2 concentrations."(61) (Link from above)
| Increased confusion is valuable when it pushes
scientists to get a better answer. Möller's disturbing calculation
was one stimulus for taking up the challenging job of building full-scale
computer models that would take better account of key processes. By
1967 one team had removed the runaway by adding more realism. (In
their model, heat energy could move up from the planet’s surface
not only through radiation but through convection, the rise of heated
air.) But it would take another decade or two of hard work before
computer models would offer a reasonably convincing simulacrum of
the present global climate, let alone a changing one.(62)
| Crude models of climate change became common
during the 1960s, and some of them showed uncomfortably plausible
possibilities for disaster. One reason these drew attention was that
climate scientists were beginning to admit that there was no such
thing as a "normal" climate. By now they had many good long-term weather
records, and analysis showed that weather patterns did not always
swing back and forth around a stable average. The traditional model
of a self-regulating balance of nature was gradually yielding to a
picture in which climate continually changed. Feedbacks were no longer
seen as invariably helpful, ever restoring an equilibrium. Rather,
they might push the system into a fatal runaway.
| The scientists were not causing a change
of attitude so much as reflecting one that was sweeping through the
world public. Many people were taking up the idea that humanity was
liable to bring down global disaster on itself, one way or another.
Crude calculations pointed to ruinous consequences from the spread
of pesticides, radioactive materials, and above all nuclear war. People
no longer saw all this as mere science fiction for teenagers, but
as plain scientific possibility.
| Alongside the occasional models of spectacular
climate catastrophes, scientists continued to develop more workaday
studies of how this or that force or feedback might influence climate.
The subject remained a minor out-of-the-way field, salted with individualists
who dreamed of winning honor by discovering the key to the ice ages
or a way to predict droughts. As the Director of Research of the United
Kingdom Meteorological Office remarked in 1963, nobody had yet produced
a quantitative model that could show even "that the climate of the
Earth should be distributed as it is." Without such a model for the
present state of climate, so much the worse for understanding climate
change any discussion "is necessarily conjectural and inconclusive."
That was no wonder, he pointed out, when even the most basic data,
like the Earth's budget of incoming and outgoing radiation energy,
were known only approximately. "With theory so rudimentary and the
data so incomplete... the subject has largely been left as a topic
for armchair speculation."(63)
| Another expert tallied significant theories
about causes of climate change extant in 1960 and came up with 54
distinct hypotheses. When a colleague looked again in 1968, he found
the total had mounted to 60. "There is nothing to suggest that an
end to the speculation on climatic change is in sight," he sighed.
"It seems that we have a long way to go before the correct answer
can be affirmed."(65) The few and scattered scientists who tried to do scientific
work on climate change usually distrusted all the primitive models,
including their own. Hardly anyone pursued a given idea except the
author, who usually just presented a paper or two before moving on
to more productive work.
| As the 1960s proceeded, scientists found it harder to get any respect
at all for a physical model unless it incorporated at least a few
equations and numerical results. Such calculations, involving ice
sheets or CO2 or whatever, became increasingly
common, even if the product was often little better than hand-waving
dressed up with graphs. As the power of computers rose, people began
to possibilities for building models that would work out the whole
atmospheric circulation numerically. Such models were not easily built,
however. One problem was that computers were still too slow to handle
millions of numbers in a reasonable time. But a worse problem was
pure ignorance of how to build a general circulation model. An infinitely
fast computer would be no use unless it began with the correct equations
for complex effects like the way moisture in the air became raindrops
| Many people preferred to keep on developing
simple models of climate instability. Such models were easy and satisfying
to grasp, and however qualitative and speculative they might be, they
offered genuine insights. The best of these insights would eventually
be incorporated into the gigantic computer models. Meanwhile some
climate scientists took advantage of computers in a less expensive
and arduous way, putting them to work on simple models and working
out the numbers in minutes instead of weeks.
| Among various simplified models that were written down in a few
equations and run through a calculation, the most important was built
in the late 1960s by Budyko. He was drawn to the issue by the climate
modification proposals that had concerned Soviet climatologists since
the 1950s, the grand schemes to divert rivers from Siberia or spread
soot over the icepack. Budyko and his colleagues recognized that existing
models were far too primitive to predict how such activities might
alter climate. At first, they tried instead to make predictions using
the simplest sort of empirical model. They would study past climates,
compiling statistics on what had happened during years when the icepack
was a bit smaller, the temperatures a bit warmer, the atmosphere a
bit dustier. The way weather patterns had shifted in the past might
well indicate how they would shift in response to future interventions.
This resembled the traditional weather prediction method of "modeling"
tomorrow's weather by comparing today's weather with similar maps
from the past. The approach was also a natural extension of traditional
climatology, with its piles of statistics and its idea of climate
change as a simple question of changed weather patterns.
| In service of this program,
Budyko's institute in Leningrad had been laboriously compiling old
temperature figures from around the world. He noticed an apparent
correlation over the past century between fluctuations in global temperature
and variations in atmospheric transparency, due to dust from occasional
volcanic eruptions. Other climatologists reported similar findings
in the late 1960s. Apparently temperature was sensitive to any haze
of particles that lingered in the atmosphere. Budyko was well aware
of vigorous ongoing debates over the general warming trend that had
been reported for some regions, and he was starting to expect that
human industry would cause an accelerated warming. Moreover, studying
satellite data on the albedo of different parts of the Earth, he found
dramatic differences depending on snow cover. Combining these separate
concerns, he worried that a change in sea ice, or a similar feedback
mechanism, "can multiply a comparatively small initial change in air
temperature created by men's activities."(66)
| To pin down the idea, in the mid 1960s Budyko
constructed a highly simplified mathematical model. It was a "zero-dimensional"
model that looked at the heat balance of the Earth as a whole, summing
up radiation and albedo over all latitudes. When he plugged plausible
numbers into his equations, Budyko found that for a planet under given
conditions that is, a particular atmosphere and a particular
amount of radiation from the Sun more than one state of glaciation
was possible. If the planet had arrived at the present after cooling
down from a warmer climate, the albedo of sea and soil would be relatively
low, and the planet could remain entirely free of ice. (In particular,
as Donn was continuing to insist, once the Arctic Ocean was free of
its ice pack it would be less likely to freeze over in winter).(67)
But the Earth had come to the present by warming up from an ice age,
keeping some snow and ice that reflected sunlight, and so it could
retain its chilly ice caps.
| Under present conditions, the Earth's climate
looked stable in Budyko's model. But not too far above the present
temperatures and snow cover, the equations reached a "critical point."
The global temperature would shoot up as the ice melted away entirely.
That would give a uniformly and enduringly warm planet with high ocean
levels, as seen in the time of the dinosaurs. And if the temperature
dropped not too far below present conditions, the equations hit another
critical point. Here temperature could drop precipitously as more
and more water froze, until the Earth reached a stable state of total
glaciation the oceans entirely frozen over, the Earth transformed
permanently into a gleaming ball of ice! Budyko thought it possible
that our era was one of "coming climatic catastrophe... higher forms
of organic life on our planet may be exterminated."(68)
(Link from below)
M. Budyko on
a glacier expedition
Photo G. R. North, 1976
| Others were on the same trail, independently
of Budyko's work in Leningrad communications were sporadic
across the Cold War frontiers. Already in 1964, a New Zealand ice
expert, Alex Wilson, had offered some thought-provoking if schematic
calculations. Instability in the Antarctic might cause ice shelves
to spread swiftly across vast tracts of the southern oceans, then
melt away, raising and then lowering the Earth's albedo. He proposed
that this "provides the 'flip-flop' mechanism to drive the Earth into
and out of an ice age."(69*)
The following year Erik Eriksson in Stockholm wrote a set of differential
equations involving temperature and ice cover. The mathematics revealed
instabilities that might lead to either "an explosive growth" or "a
very rapid retreat of ice." As Eriksson explained in a 1965 conference
on climate change, the system had a "'flip-flop' mechanism."(70*)
<=Sea rise & ice
| That was an extreme example of what the American
meteorologist Edward Lorenz had begun to call "intransitive" effects.
Under given external conditions, the atmospheric system could get
itself locked into one persistent state or into another and quite
different state. The choice might depend on only minor variations
in the starting-point. These ideas were no doubt provocative, but
so blatantly primitive and speculative that few scientists spent much
time thinking about them.
|What did at last catch attention was the drastic
outcome of an energy-budget model published in 1969. The author, William
Sellers at the University of Arizona, built on Budyko's and Eriksson's
ideas. Rather than attempt another grand but rudimentary global model,
Sellers computed possible variations from the average state of the
actual atmosphere, separately for each latitude zone. The model was
still "relatively crude," as Sellers admitted (adding that this was
unfortunately "true of all present models"), but it was straightforward
and elegant. Climatologists were impressed to see that although Sellers
used equations different from Budyko's, his model too could approximately
reproduce the present climate and that it too showed a cataclysmic
sensitivity to small changes. If the energy received from the Sun
declined by 2% or so, whether because of solar variations or increased
dust in the atmosphere, it might bring on another ice age. Beyond
that, Budyko's nightmare of a totally ice-covered Earth seemed truly
possible. At the other extreme, Sellers suggested, "man's increasing
industrial activities may eventually lead to a global climate much
warmer than today."(71)
| The striking results published by Budyko
and Sellers kindled increased interest in simple models. While some
scientists gave them no credence, others felt that such models were
valuable "educational toys" a helpful starting point for testing
assumptions, and for identifying spots where future work could be
fruitful.(72) But did the Budyko-Sellers catastrophes
reflect real properties of the global climate system? That was a matter
of brisk debate.(73)
=>Venus & Mars
| In the early 1970s,
some scientists did find it plausible that feedbacks could build up
a continental ice sheet more rapidly than had been supposed. The ice's
albedo was not the only feedback that might contribute. Ewing continued
to defend his theory that a North American ice sheet could build up
over only a few thousand years because of the increase in snowfall
if the Arctic Ocean became ice-free. Other climate experts consistently
rejected the idea. Aside from specific details, many continued to
doubt the basic picture of a climate sensitive to small perturbations.
For example, a 1971 climatology textbook pointed out that the Arctic
Ocean occupied less than 5% of the globe's surface, and asked, "Is
it not inherently improbable that the freezing and thawing of this
surface should have major repercussions over the
whole globe?"(74) Whether such magnified consequences were truly improbable
got different answers from different scientists. Some went so far
as to take seriously the idea offered by C.E.P. Brooks back in the
1920s, that thanks to feedback, frigid winds sweeping down from snow
fields could move the snowline rapidly southward year by year.(75) (See above: Ewing and Brooks.) Such a runaway freeze might
possibly be triggered soon, according to some, as smog and smoke emitted
by human industry increasingly shaded the Earth.(76)
| The opposite extreme
a self-sustaining heating of the planet might be even
more catastrophic, according to another set of calculations from a
few equations. In the early 1960s, telescope measurements had revealed
that the planet Venus was at a temperature far above the boiling point
of water. It stayed so hot because a dense blanket of water vapor
and CO2 maintained a ferociously strong greenhouse
effect. The furnace-like conditions not only kept water vaporized
in the atmosphere but also kept the CO2 there,
for the surface minerals, red-hot, would not absorb the gas. The system
was thus self-perpetuating. Perhaps Venus had originally been similar
to the Earth, only just enough warmer to begin evaporating gases into
its atmosphere greenhouse gases, which produced further warming,
and so forth. If so, the end had been a "runaway greenhouse." According
to one calculation, the Earth would need to be only a little warmer
for enough water to evaporate to tilt the balance here as well. If
our planet had been formed only 6% closer to the Sun, the authors
announced, "it may also have become a hot and sterile planet." This
was published in 1969, the same time as the work of Budyko and Sellers.(77)
<=>Venus & Mars
| By 1971, the risks to climate were under vigorous discussion in
the small community of climate scientists. When Budyko presided over
a large meeting in Leningrad, a rare occasion when most of the leading
American, Western European and Soviet experts all met together, he
put the issue to them forcefully. At the conclusion of the conference,
where the organizer would traditionally sum up with some bland remarks,
"Instead of general words," Budyko recalled, "I presented in short
form an idea which proved to be absolutely unacceptable to everybody:
the idea that global warming is unavoidable... The result was a sensation.
Everybody had very strong feelings, and extremely unfavorable... A
few very prominent men said, first, that it was absolutely impossible
to have any [effect] of man's activity on the climate... And absolutely
impossible to predict any climate change."(78)
It was not pleasant, Budyko later recalled, to present unconventional
ideas and provoke negative feelings, but the risk to the planet seemed
so grave that it was important to provoke scientists to study the
question and find whether the ideas were valid.(79)
| Budyko was not in fact
alone in his concerns. They were taken up in an influential report
(the "SMIC report") as the consensus of a major scientific meeting
held in Stockholm that same year, 1971. The experts concluded that
there was a possibility that a mere 2% increase or decrease of solar
radiation, helped by albedo feedback, could leave the planet either
totally ice-free or totally frozen.(80*) Budyko, Sellers, and others pressed
ahead, finding that under a variety of simple assumptions, any model
that gave a good representation of the Earth's present climate looked
unstable and could just as easily produce a radically different climate.(81*)
In 1972, Budyko calculated that a mere few tenths of a percent increase
in solar radiation input could melt the icecaps. More important still,
changing the level of greenhouse gases in the atmosphere would have
an effect similar to changing the Sun's radiation. His model indicated
that a 50% increase in CO2 would melt all the
polar ice, whereas reduction of the gas by half "can lead to a complete
glaciation of the Earth." Budyko went on to note that any changes
in CO2 caused by natural geological processes
had been overtaken by human activity. Sometime "comparatively soon
(probably not later than a hundred years)... a substantial rise in
air temperature will take place." As early as 2050, he calculated,
the Arctic Ocean's ice cover could be melted away entirely.(82)
| A mathematical model like those of Budyko
and Sellers, built out of only a few simple equations, is quite likely
to predict sharp changes. The more complex processes of the real world,
however, might become saturated at some point, or react so as to counter
any big shift. Scientists tended to be skeptical about this entire
genre of models. As one expert later remarked, many in the 1970s thought
the Budyko-Sellers instability was a nuisance "an artifact
of the idealized models, and the usual approach was to dismiss it
or introduce additional ad hoc mechanisms that would remove
it."(83) The few who pursued the calculations
found no easy way to avoid the catastrophic instability, but they
understood that it would take a much larger and more complete computer
model to produce credible results.(84) Sellers himself developed a somewhat
more elaborate model (although it still took only 18 seconds on the
computer to work out a year of climate change), and again he got a
planet that was highly sensitive to perturbations. But he admitted
that resolving the question must wait for some future "super-computer."(85) Besides, in the early 1970s the public
had become agitated about possible climate shifts, and it could seem
irresponsible to talk too loudly about world doom predicted by patently
|Some senior climatologists,
attacking "the glibly pessimistic pronouncements about the
imminent collapse of our terrestrial environment," stuck by
their traditional model of climate as a self-regulating system.
They continued to expect, for example, that a negative feedback
from cloudiness would stabilize global temperature. But others were
taking a new view of their field. Not only theoretical studies,
but a flood of data on past climate changes were hard to reconcile
with the old definition of “climate” as a long-term
average of weather. An average made sense only if you calculated
it over a period where things were roughly the same during the first
half as during the second half. But was there ever such a period?
As one prominent climatologist explained, "it cannot be ruled
out... that [climate] varies on all scales of time." He admitted
that "it can be argued that the very concept of climate is
sterile," unless you gave up "the classical concept of
In 1973, studies in wholly different fields
brought new credence to the idea that positive feedbacks could defeat
stability, with drastic results. A spacecraft reached Mars and sent
back images with dramatic evidence that although the planet was
now in a deep freeze, in the past there had been floods of water.
Carl Sagan and his collaborators calculated that the planet had
two stable states, and ice albedo feedback helped to drive the shift
between them. Enormous flips of climate were apparently not a mere
theoretical possibility but something that had actually befallen
our neighboring planet.(86)
For more on the way studies of other planets supported ideas
of radical climate change, see the supplementary essay on Mars
<=Venus & Mars
| Another field of study produced even more
telling news. By the mid 1970s, analysis of layers of clay extracted
from the seabed gave unassailable evidence that ice ages had come
and gone in a 100,000-year cycle, closely matching Milankovitch's
astronomical computations of periodic shifts in the Earth's orbit.(87) Yet the subtle orbital changes in the amount of sunlight
that reached the Earth were far too small to have a direct effect
on climate. The only reasonable explanation was that there were other
natural cycles that resonated at roughly the same timescale. The minor
variations of external sunlight evidently served as a "pacemaker"
that pinned down the exact timing of internally-driven feedback cycles.
| What were the natural cycles that fell into
step with the shifts of sunlight? The most obvious suspect was the
continental ice sheets. It took many thousands of years for snowfall
to build up until the ice began to flow outward. A related suspect
was the solid crust of the Earth. On a geological scale it was not
truly solid, but flowed like tar. The crust sluggishly sagged where
the great masses of ice weighed it down, and sluggishly rebounded
when the ice melted. (Scandinavia, relieved of its icy burden some
twenty thousand years ago, is still rising a few millimeters a year.)
Since the 1950s, scientists had speculated that the timing of glacial
periods might be set by these slow plastic flows, the spreading of
ice and the warping of crustal rock.(88)
Starting around the mid 1970s, scientists in a variety of institutions
around the world, from Tasmania to Vladivostok, devised numerical
models that indicted how 100,000-year cycles might be driven by feedbacks
among ice buildup and flow, with the associated movements of the Earth's
crust, albedo changes, and rise or fall of sea level. They rarely
agreed on the details of their models, which of necessity included
But taken as a group, the numerical models made it plausible that
ice-sheet feedbacks could somehow amplify even the weak Milankovitch
sunlight changes (and perhaps other slight variations too?) into full-blown
| From Small Models to Big Computers
| Many scientists had converted by now to a
new view of climate. No longer did they see it as a passive system
responding to the (name your favorite) driving force. Now they saw
climate as almost a living thing, a complex of numerous interlocking
feedbacks prone to radical self-sustaining changes. It might even
be so delicately balanced that some changes would be unpredictable.
To be sure, many people stuck to earlier views. In 1976 the Director-General
of the United Kingdom Meteorological Office told the public that "sensational
warnings of imminent catastrophe" were utterly without foundation.
"The atmosphere is a robust system with a built-in capacity to counteract
any perturbation," he insisted.(90) That was becoming a minority opinion.
A 1974 feedbacks
| While models of ice-albedo and ice-sheet
flow gave the most spectacular results, scientists developed a variety
of other simple models. Most important and technically challenging,
some labored to improve calculations of how radiation was transferred
through a column of the atmosphere. These one-dimensional calculations
were the underpinning for simplified energy-budget models incorporating
changes in ice cover, atmospheric CO2, and so
forth. Such models also provided basic elements to build into the
proliferating full-scale computer models of the general circulation.
| The huge computer models were taking over
the field from simpler models. By the mid 1970s, everyone understood
that it was hopeless to try to understand how climate changed by looking
at just one or another feature, or even several features: you had
to take into account all the mutually interacting forces at once.
Digital computers were reaching a point where they might be able to
do just that. Work increasingly concentrated on developing systems
to incorporate as components of comprehensive models. Some scientists
nevertheless continued to build elementary stand-alone models of various
features, using them to garner insights that would be necessary to
grasp the full climate system.
|| <=Models (GCMs)
| The most outstanding difficulty was the intricate problem of clouds.
Among other things, the authoritative 1971 SMIC report noted that
a climate change could alter not only the amount of clouds but also
their average height. The height determined the temperature of the
cloud surfaces, which affected how they radiated heat upward and downward.
The authors concluded that "clouds could act as a feedback mechanism"
responding to global warming, "but the direction of feedback remains
to be determined."(91)
had assumed with little thought that more clouds obviously would
reflect sunlight, and necessarily cool the Earth. But in 1967 a
pair of computer modelers, Syukuro Manabe and Richard Wetherald,
included in their calculations the way that clouds not only reflected
incoming sunlight but also intercepted radiation rising from below.
Like greenhouse gases, the clouds could radiate heat back downward
or as one writer put it, "trap" heat on the surface. (After
all, it's common experience that a cloudy night will typically be
warmer than a clear one). Also, by absorbing some of the radiation
coming from above or below, clouds would warm the layer of atmosphere
where they floated. In 1972, Stephen Schneider published a suggestive
attempt to discuss the complexities in detail. He argued that while
a greater amount of cloud cover would lead to net cooling, an increase
in the height of the cloud tops would lead to warming. Overall his
model was highly sensitive to small changes not to mention
being sensitive to its simplifying assumptions.(92)
Further rudimentary calculations showed that all sorts of subtle
and complex influences would determine whether a given type of cloud
brought warming or cooling.
| Another problem that
needed much more work was smoke, dust, and other aerosols. Tiny atmospheric
particles would not only strongly influence the formation of clouds,
but would interact with radiation on their own. Some observations
and primitive calculations showed that aerosols from volcanoes, and
perhaps from human activity too, would have to be included in any
realistic climate model. By the late 1970s, groups studying aerosols
had built simple one-dimensional models and slightly more advanced
models that averaged over zones of latitude. The models gave important
results: the net effect of injecting aerosols would be global cooling.
Confidence in the results was bolstered when James Hansen's group
used a simple model to compute the temporary cooling caused by the
haze from a volcano that had erupted back in 1963; their results matched
real-world data remarkably well. In particular, the model predicted
that the stratosphere should warm up while the lower atmosphere cooled,
just what was observed. To be sure, knowledge of aerosols was so uncertain,
and the normal fluctuations in climate were so great, that the volcano
"experiment" could not prove anything for certain. "Nevertheless,"
as one reviewer commented, "the good agreement is rather satisfying."(93)
| Another essential problem
that people studied in simple models was the circulation of the oceans,
which computers could not yet handle as part of a full-scale global
simulation. Some modelers recognized that real understanding of long-term
climate change would require models that coupled the atmosphere and
oceans. They continued to offer hand-waving models to suggest how
the interaction might behave. For example, in 1974 Reginald Newell
offered some ideas about rapid switching between two distinct configurations
for heat transport through the oceans. Newell's ingenious mechanism
involved the spread of sea ice over the ocean, but he noted that there
could also be important Budyko-style albedo feedbacks, and other effects
such as changes in the pattern of winds. His suggestions were "speculative,"
Newell admitted, "as indeed are all previous suggestions concerning
the course of the ice ages."(94)
| Simple models also remained
necessary for studying conditions beyond the range of general circulation
models. The big models had a problem. Any tiny initial error in the
physics or climate data tended to accumulate, adding up through the
millions of numerical operations to give an impossible final result.
The models gave stable results only when their initial parameters
were adjusted ("tuned") until the outcome simulated current conditions
realistically. Such a model was useless outside its range. By 1976,
a full-scale model had been built that not only simulated current
climate but also, with suitable readjustment, gave a rough reproduction
of conditions at the peak of the last ice age. (It was good enough
to confirm the long-held assumption that ice and snow albedo were
indeed important for sustaining an ice age.) But to dynamically compute
the whole range of climates as ice ages came and went was far beyond
this or any model's capacity. By the 1980s, the strangely regular
cycles of ice sheet advance and retreat over the past several hundred
thousand years were well determined, thanks to cores drilled from
seabeds and icecaps. Scientists who took up the old challenge of explaining
the cycles had no recourse but simple models a few equations
including the time-delay for laying down and melting ice sheets, plus
feedbacks such as changes in the level of CO2
in the atmosphere.(95)
| Going to still more extreme conditions, only
simple models could deal with the runaway changes in temperatures
and greenhouse gas concentrations that might make a totally glaciated
"snowball Earth." (See
above) In the 1980s, this threatening scenario became more credible.
It had increasingly puzzled computer modelers that their simulations
had a strong tendency to wander into this mode. Some began to wonder
why it had not actually happened.(96) Now geological evidence turned up suggesting
that at least once in the very distant past, the Earth's oceans really
had frozen over, or mostly so.
|| <=CO2 greenhouse
| For analyzing climate under current conditions,
mammoth supercomputer models took over the field toward the end of
the 1970s. The last great contribution of primitive one-dimensional
and two-dimensional calculations was to provide a check. The most
complex three-dimensional computer models seemed plausible once they
were found to behave much the same as the simple models. Tests using
a variety of small models came up with numbers close to the ones printed
out by the biggest ones. In short, introducing the myriad complexities
of a full-scale model did not change the plain lesson of global warming
contained in the elementary physics that stretched back to Arrhenius.
| During the 1980s, many scientists came to
believe that the Earth was getting warmer. But that said nothing about
the cause. A search got underway for "fingerprints" specific
patterns of climate that would point to the greenhouse effect (or
point away from it, to some other cause). As one example, both computer
models and simple reasoning declared that when gases in mid-atmosphere
blocked radiation coming up from the surface, that would leave the
stratosphere above the gases cooler. By 1988, "a number of intriguing
candidates are appearing that might be part of a fingerprint," a Science
magazine report said, but "no one is claiming a certain identification
of the greenhouse signal."(97)
|Whatever the cause of
warming, elementary reasoning could predict some important consequences.
Warming would not mean a slightly higher temperature on every day,
but a serious increase in "heat waves," runs of days of extreme heat
harmful in many ways but especially to farmers.(98) That was one of the things that attentive
journalists picked up from James Hansen's widely reported 1988 testimony
to the U.S. Congress: the number of deadly heat waves would shoot
up.(99) Furthermore, a warmer atmosphere would hold more moisture,
so it seemed likely that the whole grand cycle of weather from evaporation
to precipitation would intensify. The effects were debatable, but
most experts felt that a warmer world would have worse droughts, worse
floods, and perhaps worse storms possibly all very much worse
— although nobody could say just how bad these disasters could
be, let alone where they might strike.(100*)
| A wholly different approach was to "model"
the greenhouse world on similar climates of the past. Paleontologists
had traditionally studied rocks and fossils to find whether a region
had once been jungle, prairie or desert. In the 1980s, Budyko encouraged
Soviet geologists to extend this line of work into a detailed mapping
of the last warm interglacial period, especially in the territory
of the Soviet Union itself. They hoped that this would give an idea
of how the world's climate map would appear during global warming
in the 21st century.(101) This program was largely overtaken
by much more detailed data deduced from studies of ocean floor mud,
data precise enough to combine with computer models of climate. Still,
the Soviet studies did help to demonstrate that a warmer planet was
likely to have a very different geographical distribution of warm,
cold, wet and dry regions than at present.
|A completely different group of simple models
was meanwhile joining the discussion. It was essential to understand
how biological systems interacted with both global warming and an
increased CO2 level. What would changes in gases
and temperature and precipitation do to forests, wetlands, and so
forth? In particular, what changes might come in the emission of methane
gas, the absorption of CO2, the amount of dust
in the air, and so forth? This was important to climate science because
such things could react back on the climate system itself, perhaps
in a vicious circle.
| Most people
were more interested in another question: what might climate change
mean for agriculture, forestry, the spread of tropical diseases, and
other matters of human concern? By the early 1980s, some scientists
found the risk of climate change great enough to justify an effort
to work out preliminary answers. Simple models could give at least
a rough idea as to how global warming might affect, say, the production
of wheat in North America. A new area of research got underway with
the customary features research grants, conferences, articles
in interdisciplinary journals like Climatic Change.(102)
| Specialists in a variety of fields approached the issues with computer models.
They plugged in equations and data on such things as how farmers might
be forced to change the crops they grew, or how higher temperatures
might affect electricity production or wildlife. "Impact studies,"
as this field came to be called, was rudimentary compared with atmospheric
modeling. The underlying data came from only a few acres of woods
or fields at a few locations, and the equations did not go far beyond
hand-waving. Another and even simpler approach was the one pioneered
by Budyko's Leningrad group, finding what the weather had been like
in a given region during past periods of warmer climate, and asking
how such weather might affect modern life. Here too the data were
sketchy, and extrapolation to our own future little more than a guess.(103)
What these studies did show was that it was more likely than not that
a few degrees of warming would have important consequences for both
natural ecosystems and human society mostly nasty consequences.
| Such studies required expertise in botany
or agronomy or sociology as much as in geophysics. Nobody would be
able to predict precisely how the atmosphere would change, nor what
the impacts of the change would be, without understanding all the
interactions. The climate science community dreamed of a grand model
computing every factor together, not just the physics and chemistry
but the biology (would forests grow farther north and absorb more
sunlight?) and economics (would a rise of temperature promote more
fossil fuel burning, or less?). Such a comprehensive model lay far
in the future. Devising its multitude of component parts would take
many years of development using simple models.
| From the 1980s forward, many groups have done extensive
studies on how human and natural systems might interact with both
climate change and increased CO2. The details
fall outside the scope of these geophysical essays; for a brief summary
see the essay on the history of impact studies.
| After 1988
| Despite the rapid improvement in the huge general circulation computer
models, simple models continued to be useful. For one thing, they
could lend conviction when critics disputed the incomprehensibly intricate
computer models. As statisticians sought a definitive "fingerprint"
to demonstrate the arrival of greenhouse warming, of course they compared
their observations with the predictions from big models. But the conclusions
seemed more solid when they showed features that Tyndall and Arrhenius
had long ago predicted from elementary and ironclad principles. In
particular, the simplest physical logic said that the blocking action
of greenhouse gases would be most effective where outward radiation
was most important for cooling the Earth: warming would come especially
at night. (See above) And indeed a rise in the daily minimum temperature,
mainly due to rising night-time temperatures, was plainly observed
world-wide from the 1950s onward.(104*)
The predicted increase in extreme climate events also seemed to be
showing up in statistics, at least for the United States, although
nobody could be certain what caused it.(105)
| No less persuasive, Arrhenius and everyone
since had calculated that the Arctic must warm more than other parts
of the globe. The main reason was that even a little warming would
melt some of the snow and ice, exposing dark soil and water that would
absorb sunlight. Enhanced Arctic warming was a solid feature of models
from the simplest hand-waving to the most sophisticated computer studies.(106*) And in fact, it was in places like the Arctic Ocean,
Scandinavia, and Siberia that global warming became most noticeable
in the 1990s. The area of ocean covered by ice declined sharply, so
did the thickness of the icepack, treelines moved higher, and so forth.
Studies mustering large amounts of data from around the Arctic showed
that the 20th-century warming far exceeded anything seen for at least
the past 400 years.(107) Humanity's "large scale geophysical
experiment," as Roger Revelle had called it back in 1957, was producing
data almost as if we had put the Earth on a laboratory bench to observe
the effects of adding greenhouse gases. The data seemed to be confirming
the basic theories.
=>Venus & Mars
| These "fingerprints" of the greenhouse effect,
combined with more elaborate computer studies and with the evident
global surface temperature rise, did much to convince scientists and
attentive members of the public that global warming was underway.
Nevertheless, as a few critics pointed out, the changes might be caused
by other influences.(108) Further, even
if the main force was indeed the increase of greenhouse gas, patterns
such as the enhanced arctic warming did not necessarily confirm the
old simple models, for they could easily be a result of more complex
changes. If computer models agreed with old hand-waving arguments,
that did not alter the fact that the modelers were still far from
certain about the interactions with cloudiness
and so forth.(109)
| The most scientifically effective critic was a respected Massachusetts
Institute of Technology meteorologist, Richard Lindzen. He challenged
the way modelers allowed for water vapor feedback. This was the crucial
calculation showing how a warmer atmosphere would carry more water
vapor, which would in turn amplify any greenhouse effect. Lindzen
believed the climate system was more stable than that. He offered
an alternative scenario involving changes in the way drafts of air
carried moisture up and down between layers of the atmosphere. While
Lindzen's detailed argument was complex and partly impressionistic,
he said his thinking rested on a simple philosophical conviction
over the long run natural self-regulation must always win out. His
work also became, he confessed, "a matter of being stuck with a role."
It was important for somebody to point out the uncertainties.(110*)
found Lindzen's technical arguments convincing. Observations suggested
that the way the modelers handled water vapor, although far from perfect,
was not wildly astray.(111) Still, nobody could dismiss out of hand Lindzen's complaint
that computer results were based on unproven assumptions. The outputs
from every model showed significant errors. Evidently the modelers
had not properly represented all the real-world mechanisms. Simple
qualitative arguments would continue to be needed for checking the
plausibility of any big model.
| Back-of-the-envelope calculations
also continued to be useful for circumstances beyond the range of
the big computer models. Tuned to match current conditions, they had
a hard time reproducing any situation too different. Above all there
was the old problem of explaining ice ages what conditions
made a glacial epoch, and within such an epoch just what drove the
cyclic ebb and flow of ice? The usual combination of plausible arguments
and simple equations was applied in a variety of models, which now
incorporated not only ice sheets but also shifts in ocean currents.(112) Although computers were starting
to become capable of handling full-scale models for the ocean circulation,
here too there was still room for simple plausibility arguments.
<=Sea rise & ice
| Such arguments were
also the best way to study the interactions between glaciation and
CO2. Ice core measurements showed that during
recent glacial periods, CO2 and methane had gone
up and down roughly in time with the advance and retreat of the ice.
Was the cycle driven not by ice or ocean dynamics, but by emissions
of gas, as Chamberlin had speculated a century back?(113*) Perhaps the gases served as an amplifying feedback,
released into the atmosphere from seas and peat beds as a warm period
began? There were so many interactions that a climate modeler remarked,
"I have quit looking for one cause" of the glacial cycle. The mystery
of the ice ages, which had launched a century of studies of greenhouse
gases and climate, remained unsolved.
Meanwhile the old program of studying ancient climates for hints
on future conditions persisted. With computer models disagreeing
on how warming would affect specific regions, the experience of
past warm periods might give as good a guess as any. For the planet
as a whole, some scientists used paleoclimate data to get a rough
idea of the climate's "sensitivity"— how much temperatures
had actually changed, under the powerful influences that had operated
at one time or another in the geological past. The results made
for useful and encouraging comparisons with computer
|To get farther with the big models, it helped
to construct simpler models of the interactions between biological
systems and gases. Although actual measurements remained fragmentary,
the models of specific processes were improving rapidly. An example
(called "one of the most robust predictions of the new dynamic global
vegetation models") was a calculation that forests would tend to replace
northern tundra as the world warmed. Here, as so often, there were
complex and unexpected consequences. The dark evergreens would absorb
much more solar radiation than the pale tundra, amplifying the global
cores and other indicators showed that greenhouse gas levels during
the 20th century were rising much faster than any change detected
in the past. Was that why temperatures were likewise climbing, more
rapidly than any warming in the past millennium? The big general-circulation
computer models with their millions of numerical operations could
not reliably churn through a run of ten centuries. There was no way
to keep small initial errors from accumulating, a little more each
time the model ran through another year, until the whole computation
veered off into unreality. But a simple energy-balance model could
be adjusted until it responded smoothly to changes in gases, aerosols,
and the like in ways that faithfully mirrored the overall average
responses of the big models. Thus modeling came full circle, with
large computer systems used to calibrate a stripped-down version.
The result was some of the most convincing evidence yet that the greenhouse
effect was indeed upon us, rapidly growing more serious.(116) No matter how you manipulated any sort of model, if you
could get it to simulate the current climate, it was likely to show
warming if you put in greenhouse gases.
By now, some of the "simple"
models run on desktop computers were comparable to what had been
considered state-of-the-art for the most advanced computations in
the 1960s. Of course, at that time everyone had recognized that
those models were primitive. Such a simplistic model became far
more reliable and convincing once it was calibrated against a range
of different full-scale general circulation models. Then anyone
could run it through a variety of scenarios. It was thus, for example,
that scientists could take a set of different assumptions for how
much greenhouse gases humanity might emit over the coming century,
and get rough predictions for the range of temperature and sea-level
changes likely to result for each proposed policy. Or they could
get around the uncertainties in how the biosphere interacted with
climate by building models with boxes for various regions and types
of vegetation, then running these models through the entire range
of plausible responses in each box. Another example: to address
the large uncertainties in the parameters used to calculate such
things as cloudiness, thousands of people cooperated to run simplified
"screensaver" models using every reasonable combination
of such parameters (nearly all of them produced global warming,
and some got disastrously warm).(117*)
Simple models — hardly simple by the standards of 1970, perhaps,
but far more comprehensible than the enormous GCMs — also
found increasing use in estimating the impacts of global warming.
Specialized models were used, for example, to study how the strength
or frequency of storms might change. Others evaluated changes already
underway, as when a group calculated that global warming probably
had a hand in the unprecedented 2003 heat wave that killed thousands
There were larger reasons for simple models to remain important,
not only for science but for policy-making. The huge global computer
models, ever grander and more complex, represented an approach to
dealing with the world that some scientists found neither appealing
nor convincing. Simple models offered a variety of other approaches,
more comprehensible and more easy to verify within their special
domains. If they were not overlooked in the shadow cast by gargantuan
computations, they could add flexibility and plausibility to decisions
about future policies.(119)
|My own personal computer is running a real climate model in
its idle minutes. You can join the many helping this important
experiment: visit climateprediction.net.
The Carbon Dioxide Greenhouse Effect
Biosphere: How Life Alters Climate
Past Cycles: Ice Age Speculations
Rapid Climate Change
Chaos in the Atmosphere
Venus & Mars
1. Callendar (1961), p. 2.
2. Simpson (1939-40), p. 191.
3. Ager (1993), p. xvi.
4. A mid-century example of a technical text is Haurwitz and Austin (1944); and a more popular work, Hare (1953).
5. E.g., "By 'climate' we mean the sum total of the meteorological
phenomena that characterise the average condition of the atmosphere at any one place on the
Earth's surface." Hann (1903), p. 1; I surveyed a sample of
climate literature and textbooks, including, e.g., Blair (1942),
pp. 90-94, 100-101; George C. Simpson, preface to Brooks
(1922), pp. 7-8.
6. Huntington (1914), p. 479.
7. Landsberg (1946), pp.
297-98; for the history in general see Lamb (1995), pp. 1-3.
8. For example, Chamberlin
(1906), pp. 364-65.
9. Tyndall (1873), p. 117.
10. "The effect of solar heat on air contained in
transparent containers [enveloppes] has long since been observed."
Fourier (1824); Fourier
(1827); reprinted in Fourier (1890), quote p. 110; for discussion, see Fleming (1998), ch. 5. BACK
11. The term was first widely popularized in the
1960s, although already in 1896 Arrhenius somewhat inaccurately wrote,
"Fourier maintained that the atmosphere acts like the glass of a hothouse,"
Arrhenius (1896), p. 237; the word "greenhouse"
perhaps first appeared in this context in a study which explained how
greenhouses keep the warmed air from rising and blowing away, and that
this matters more than the fact that infrared radiation from within does
not escape through the glass (which is more like what happens in the atmosphere),
Wood (1909); for the science, see also Lee
(1973); Lee (1974); possibly the first widely
seen use of the phrase "greenhouse effect" was in a 1937 textbook (repeated
in later editions), wrongly describing "the so-called 'greenhouse effect'
of the Earth's atmosphere" as an effect "analogous to that of a pane of
glass." Trewartha (1943), p. 29. BACK
12. Fourier admitted that "we are no longer guided in this study
[of the temperature effects of the atmosphere] by a regular mathematical theory" Fourier (1827) (also in his 1824 paper); reprinted in Fourier (1890), p. 110.
13. Pouillet (1838).
14. Tyndall (1863), pp. 204-05.
15. For energy budget models, see Kutzbach (1996).
16. Croll (1875); "mutual
reaction" Hann (1903), p. 389.
17. Arrhenius (1896); see Crawford (1996), chap. 10; Crawford
(1997); reprinted with further articles in Rodhe and Charlson
18. Arrhenius (1896), p 267.
19. If Langley's measurements had been entirely accurate,
Arrhenius would have come even closer to the warming given by current estimates, according to
Ramanathan and Vogelman (1997). But S. Manabe (personal
communication) points out that Arrhenius got reasonable results in large part because he
underestimated the absorptivity of water vapor, and thus underestimated the crucial influence of
water vapor feedback on the heat balance, a feedback kept within bounds in the real world by the
upward convection of heat.
20. Chamberlin (1897),
"speculative" p. 653; see also Chamberlin (1898); Chamberlin (1899), "long chain" pp. 546-47; Tolman (1899); Fleming (1998), p.
90; Chamberlin (1923); for Chamberlin's work more generally,
Fleming (2000); for Högbom's contribution, Berner (1995).
21. Chamberlin (1897), quote p.
22. Gregory (1908), quote p.
347; similarly, "one can scarcely study it [the Chamberlin model] without profound admiration...
Nevertheless, we are unable to accept it in full...," Huntington and
Visher (1922), p. 42; for further background, Mudge
23. Lotka (1924), pp. 222-24.
24. Redfield (1958), 221,
referring to atmospheric oxygen and other elements but not carbon.
25. Nebeker (1995), pp.
26. Simpson (1939-40), p. 213;
the "rather surprising" conclusion that even a change in solar output could be thus compensated
was still accepted in 1956 by Rossby (1959), p. 11.
27. E.g., deflection of the Gulf Stream by a continent in the
Antilles, Hull (1897); glaciation from the raising of mountains,
28. Harmer (1925), quote in
discussion by Napier Shaw, p. 258.
29. Brooks (1922), p. 23.
30. Köppen and Wegener
(1924), "fast selbstverständliche und dennoch von einigen Autoren angefochtene," p.
3; Milankovitch published some of his in a work to which Köppen and Wegener referred,
Milankovitch (1920) ; for the full theory, see Milankovitch (1930); on energy-budget models 1920s-1960s, see Kutzbach (1996), p. 357-60.
31. Callendar (1938), p. 239.
32. Brooks (1925); Brooks (1949), chaps. 1, 8.
33. Humphreys (1932);
in his well-known textbook, Humphreys flatly denied the greenhouse activity
of CO2, Humphreys (1940),
p. 585. BACK
34. Brooks (1949), quote. p. 41,
see chap. 12; Brooks (1951), p. 1013.
35. Coughlan (1950). The
cause would be melting of ice on Greenland and other land masses, since the melting of floating
ice would not change sea level.
36. Simpson, preface to Brooks
(1922), pp. 8-9.
37. Simpson (1934); Simpson (1937); "ice which enters:" Simpson (1939-40), p. 215; Willett
(1949) elaborated Simpson's theory: each solar maximum would produce a single ice age,
38. References to work by Aitken, Exner, Taylor, Spilhaus, etc.
are in Fultz (1949); for a historical treatment, see Fultz et al. (1959), pp. 3-5.
39. A. Spilhaus, interview by Ron Doel, Nov. 1989.
40. Stringer (1972), p. 10.
41. Exciting: Smagorinsky
(1972), p. 27.
42. Fultz (1949); Fultz (1952); see also Faller (1956);
for background Lorenz (1967), p. 118; Lorenz (1993), pp. 86-94.
43. Hide (1953).
44. Fultz et al. (1959); Fultz et al. (1964); some of the results are shown in Lorenz (1967), see pp. 120-126.
45. Fultz et al. (1959), p. 102.
46. Eliassen and Kleinschmidt
(1957) reviews mathematical approaches and their frustrations.
47. In 1954, they sent foundations a proposal to study
geophysical catastrophes, which could be "more deadly than wars." Folder "Donn, William,"
Individual Files Series, prelim. box 242, Maurice Ewing Collection, Center for American
History, University of Texas at Austin.
48. Ewing and Donn cited in particular 1955 papers
on continental drift by S.K. Runcorn. According to Lamb
(1977), p. 661, the first to recognize that an ice-free Arctic Ocean
would lead to more snow near the ocean (based on observations of 20th
century warm years) and that this could lead to onset of glaciation was
O.A. Drozdov; the work was not published at once, and Lamb cites a later
publication, Drozdov (1966).
49. The process was accelerated because dark, open water
absorbed more sunlight. Ewing and Donn (1956); Ewing and Donn (1956). Besides this albedo effect, which Ewing
and Donn did not stress, it was later noted that sea ice is an excellent insulator, so that the air
over the ice is tens of degrees colder in winter than if the air were exposed to the water.
50. "enjoy": C. Emiliani to Ewing, 10 Oct. 1956, folder "Ice
ages Paper," prelim. box 52, Ewing Papers, University of Texas. Contested: e.g., Schell (1957); "ingenuity" Crowe
(1971), p. 493.
51. Typical critiques: Sellers
(1965), p. 213; and Crowe (1971), p. 493; Ewing and Donn (1958); see also Donn
and Shaw (1966).
52. Wallace Broecker to Ewing, 20 Jan. 1969, "Ewing" file,
Office Files of Wallace Broecker, Lamont Doherty Geophysical Observatory, Palisades, NY.
53. e.g., Science Newsletter
54. W. Broecker, interview by Weart, Nov. 1997, AIP; data: e.g., a biologist reported pollen evidence
that there was no open polar sea in the Wisconsin glacial period. Colinvaux (1964).
55. Heims (1980); Wiener (1956); Wiener (1948).
56. Stommel (1961).
57. E.g., Weertman offered calculations of ice cap instability in
support of Ewing-Donn, Weertman (1961).
58. Budyko, interview by Weart, March 1990, AIP.
Smagorinsky, interview by Weart, March 1989, AIP, credits Budyko for introducing
snow-albedo feedback with "hand-waving".
59. Budyko (1961) ; Budyko (1962) .
60. Möller (1963); Arrhenius (1896), p. 263.
61. Eriksson (1968), p. 74.
62. Manabe and Wetherald (1967) still used constant relative
humidity. Stimulus: Manabe and Wetherald (1975).
63. Sutcliffe (1963), pp.
64. [Duplicate note removed.]
65. Eriksson (1968), p. 68; the
earlier source he cites was Schwarzbach (1963).
66. Volcanoes: Budyko (1969);
his interest in the observed warming is reported by Kondratyev
(1988), p. 4; quotes and satellite data in Budyko (1972), p.
67. Donn and Shaw (1966).
68. Budyko (1968), quote p.
618; Budyko (1969), quote p. 618.
69. Wilson (1964), p. 148; he
pointed out that buildup of the Antarctic ice sheets was one of the few features of the Earth with
a time constant that might match the long Milankovitch periods, Wilson (1969).
70. The model was only mentioned casually at the conference,
not as the main point of Eriksson's presentation, was not published until
1968, and attracted little notice aside from helping to stimulate Sellers'
work. Eriksson (1968), "flip-flop," p. 77.
71. Sellers (1969), quote
p. 392, "rapid transition to an ice-covered Earth," p. 398.
72. Robinson (1971), pp. 209,
214; cited with approval e.g. by Schneider and Dickinson
73. Budyko (1972); Sellers (1973); North (1975).
74. Crowe (1971), p. 493.
75. Ives (1957); see also Ives (1958) ; Ives (1962); wind
feedback: Lamb and Woodroffe (1970).
76. Rasool and Schneider
(1971); Lockwood (1979), p. 162.
77. Ingersoll (1969); Rasool and de Bergh (1970).
78. Symposium on Physical and Dynamical Climatology, as
described in Budyko, interview by Weart, March 1990, AIP.
79. Budyko, interview by Weart, March 1990, AIP.
80. For support they pointed to semi-empirical studies of the
way polar ice had rapidly disappeared during the warming of the 1930s. Wilson and Matthews (1971), pp. 125-29; they cite Budyko (1971).
81. Out of five possible states, "The only completely stable
climate is one for which the Earth is ice-covered," according to Faegre (1972), p. 4; "multiple steady states do exist," and which one
would be found at a given time depended on the previous history, concluded Sellers (1973), p. 253.
82. Budyko (1972).
83. North (1984), p. 3390; see
also North (1975), p. 1307.
84. North (1984).
85. Sellers (1973), p. 241.
85a. Senior climatologists: e.g., Kellogg
(1971), pp. 131-32; concept of climate: Mitchell
(1971), pp. 134-35. BACK
86. Sagan et al. (1973).
87. Especially Hays et al.
88. E.g., Emiliani and Geiss
(1959). They emphasize these ideas were not especially original with them.
89. Examples: Weertman (1976)
(Northwestern Univ., IL, and the U.S. Army Cold Regions Research and Engineering
Laboratory, Hanover, NH); note also his pioneering calculation of ice
sheet buildup and shrinkage times, Weertman (1964);
Sergin (1979) (Laboratory for Mathematical Modeling of
the Climate, Pacific Institute of Geography of Academy of Sciences, Vladivostok,
but written while visiting NCAR, Boulder, CO); Budd
and Smith (1981) (Meteorology Dept., U. Melbourne); as a perhaps more
typical example, Young (1979) (Antarctic Division, Dept. of Science and
Technology, Kingston, Tasmania) conservatively showed a response time
of perhaps 20,000 years; an especially influential model involving ice
sheet buildup delay was Imbrie and Imbrie (1980); a good review is Budd (1981). BACK
90. Mason (1977), p. 23; see
91. Wilson and Matthews
(1971), p. 122.
92. Schneider (1972).
93. Examples of useful techniques are Wang and Domoto (1974); Coakley and
Chylek (1975); volcano: Hansen et al. (1978); reviewed by
Ramanathan and Coakley (1978), quote p. 484; Charlock and Sellers (1980); Coakley et
94. Newell (1974), p. 126.
95. Pisias and Shackleton
96. Gleick (1987), p. 170.
97. Kerr (1988), p. 560.
98. Mearns et al. (1984).
99. Hansen (1988).
100. Hurricanes (50% higher "destructive potential"
in future): Emanuel (1987); a trend had been detected of greater storminess
in the North Atlantic 1962-1988, Carter and Draper
(1988); for more recent work, e.g., Knutson
et al. (1998); droughts ("severe drought, 5% frequency today, will
occur about 50% of the time by the 2050s" in the U.S.): Rind et al. (1990); Karl et al.
(1995) reported a rise in extreme precipitation events. Whether the
hydrological cycle and tropical storms in particular will intensify is
still debated, see Ohmura and Wild (2002) and
the essay on "rising seas" here.
101. Zubakov and Borzenkova
(1990), pp. ix, 5-7.
102. For example, Kellogg and
Schware (1981); Rosenzweig (1985); Emanuel et al. (1985).
103. IPCC (2001), p. 748
includes 1980s references.
104. Karl et al. (1986); Karl et al. (1991); Easterling et
al. (1997). Warming should likewise be seen more in winter than in
summer, and there are indications this is happening. More studies are
being published every year, see the links page.
105. Karl et al. (1996).
Further studies are being published but the picture remains unclear.
106. The effect is much less in Antarctica, whose
thick ice cover is not easily changed, and whose climate depends largely
on surrounding ocean currents, but warming has been seen in the Antarctic
Peninsula. Arctic warming is also enhanced by increased transport of heat
energy in moisture carried from lower latitudes, and by thinner sea ice,
which allows greater conduction of heat from the Arctic Ocean into the
air. The first large model to demonstrate polar sensitivity was Manabe
and Stouffer (1980); a more recent example is Manabe
and Stouffer (1993). BACK
107. Overpeck et al. (1997).
108. Singer (1999).
109. Wang and Key (2003)
subsequently indicated that in fact circulation rather than radiation effects predominated in the
110. His essential argument was that warming would
make an increase in tropical thunderstorm clouds, whose downdrafts would
remove moisture from the upper atmosphere. Lindzen
(1990); Kerr (1989); "stuck with a role," quoted Grossman (2001); Lindzen has been accused of obfuscation,
taking extreme ideological positions, and unjust ad hominem attacks, see
e.g., Gelbspan (1997), pp. 49-54, but the accusation that Lindzen
has been in the pay of industry is itself an ad hominem attack based only
on lecture fees that Lindzen received. For the water vapor argument and
other areas of debate with Lindzen, see Hansen
et al. (2000), pp. 154-59. BACK
111. Del Genio et al. (1991);
satellite data show "the water vapour feedback is not overestimated in
models," Rind et al. (1991), p. 500; "there is no compelling evidence
that water vapour feedback is anything other than...it has generally been
considered to be," IPCC (1992), p. 114; Sun and Held
(1996); and the final nail in the coffin, Soden
et. al. (2005). BACK
112. For a short review and references, see Broecker and Denton (1989), p. 2486.
113. E.g., "the 100,000-year cycle does not arise from ice sheet
dynamics." Shackleton (2000).
114. Kerr (1999); "quit
looking:"Andre Berger. Kerr (2000). A landmark
work using paeloclimate data to derive sensitivity was Hoffert
and Covey (1992). BACK
115. Foley et al. (1994);
"robust:" Melillo (1999).
116. Crowley (2000).
117. The most important such use has been in the
Intergovernmental Panel for Climate Change reports, e.g., IPCC
(2001), ch. 9; Randall et al. (2007), section
8.8.. For another use see Wigley (2005). Among
the simplifications in the distributed model in the initial runs by www.climateprediction.net
was a "slab" ocean instead of a circulating-ocean model, see
Stainforth et al. (2005); Piani
et al. (2005) estimate 5th/95th probability percentiles for future
warming at 2.2 and 6.8 K. Similarly, Andreae et
al. (2005) used a one-dimensional model with parameters that began
with those from a big GCM and varied them within plausible limits, also
finding a possibility of extreme warming. A recent example of use of an
energy balance model calibrated against GCMs is Hegerl
et al. (2006). BACK
and Tuleya (2004); Stott et al. (2004)
et al. (1998). BACK
© 2003-2007 Spencer Weart & American Institute of Physics