There was a rain squall every afternoon when Christopher Columbus
anchored at Jamaica in 1494. He remarked that the island's lush carpeting
of forests caused these rains, for "he knew from experience that formerly
this also occurred in the Canary, Madeira, and Azore Islands, but
since the removal of forests that once covered those islands, they
do not have so much mist and rain as before."(2)
Columbus was claiming to see an impact of living creatures on climate
— in two senses. In the first place, humans are living creatures,
so anything we do is an effect of life. More directly, Columbus thought
the climate change was a result of alterations in the forms of life
covering the islands, from forest to grassland. Of course a change
in climate itself might bring such ecosystem alterations. But nothing
altered a region so quickly and dramatically as human civilization. |
- LINKS - |
Since the ancient Greeks, scholarly theories and folk beliefs had
speculated that chopping down a forest, irrigating a desert, draining
marshlands or grazing a prairie to bare dirt might change the temperature
and rainfall in the immediate vicinity. Americans in the 19th century
argued that settlement of the country had brought a less savage climate.
Sodbusters who moved into the Great Plains boasted that "rain follows
the plough." Some European scientists, however, agreed with Columbus
that deforestation made for a dryer, not wetter, climate.(3) |
|
By the end of the 19th century, meteorologists
had accumulated enough reliable weather records to test whether rain
did follow the plough, or perhaps fled from the axe. Both ideas failed the test.
Even the transformation of the entire ecosystem of Eastern North America
from forest to farmland had apparently made little difference to climate.
If the spectacular changes wrought by humankind could not alter a
region's climate, there seemed little reason to consider the impact
of other species. Through the first half of the 20th century, scientists
who studied climate treated ecosystems as passive. Deserts and forests
expanded or shrank in helpless response to climate changes. The cause
of these climate changes might be upheavals of mountain ranges, or
variations of the Sun, or other forces surely far mightier than the
meter or so of organic matter that covered some patches of the planet's
surface. |
Full discussion in
<=Public opinion |
A few scientists thought otherwise. The deepest thinker was the
Russian geochemist Vladimir I. Vernadsky. During his work mobilizing
industry during the First World War, he recognized that the volume
of materials produced by human industry was approaching geological
proportions. Analyzing biochemical processes, he concluded that the
oxygen, nitrogen, and carbon dioxide gas (CO2)
that make up the Earth's atmosphere are put there largely by living
creatures. More, he insisted that biological processes influenced
the chemistry of practically every element in the Earth’s crust.
In the 1920s, he published works that described how carbon cycled
through living matter. He argued that living organisms were a force
for reshaping the planet, comparable to any physical force. Beyond
this he saw a new and still greater force coming into play —
intelligence. A few scientists began to study how living creatures
affected the chemistry of the Earth's surface, notably in a "Biogeochemical
Laboratory" set up in the Soviet Union in 1929. Vernadsky's visionary
pronouncements about humanity as a geological force were not widely
read, however. They struck most readers as mere romantic ramblings.(4)
|
|
Carbon Dioxide and the Biosphere (1938 - 1950s)
TOP
OF PAGE |
|
The first barely credible
champion of an influence of life on climate was the British engineer
G.S. Callendar, who from 1938 on published arguments that human emissions
of CO2 were already producing a global warming.
A few scientists found this interesting enough to take a closer look
at how the gas, and indeed all forms of carbon, moved in and out of
the atmosphere. It had long been understood that the bulk of the planet's
carbon was locked up in lifeless chemicals. Since the 19th century
a few scientists had studied the age-long cycles as the gas was puffed
out by volcanoes, absorbed into minerals or the oceans, and deposited
in carbonate rocks. It hardly seemed worth mentioning that much of
this rock — millions of cubic kilometers of chalk, limestone,
and coal — had once been part of living creatures. But now scientists
were asking about carbon on the move, over a span of mere centuries.
For this they had to look at biology. |
<=>CO2 greenhouse
<=Simple
models |
Nothing much was known in the 1950s about
the slow movements of carbon in and out of the planet's biomass. Measurements
of radioactive carbon-14 brought a new source of data that stimulated
studies, but for more than a decade the data were too uncertain to
tell anything useful. The few people who took up the carbon question
had only vague estimates to work with, but that did not stop them
from reaching conclusions. They could calculate, in particular, that
the amount of carbon bound up in forests, peat bogs, and other products
of terrestrial life is several times greater than the amount in the
atmosphere (the lowly soils alone store two or three times more than
the air holds in CO2). Since these ecosystems
had been fairly stable over geological time, the stock of carbon bound
up in organic substances must have remained in rough balance with
the atmosphere over millions of years. |
<=Carbon dates |
The likely cause of stability was a fact demonstrated by experiments
in greenhouses and in the field — plants often grow more lushly
in air that is "fertilized" with extra CO2. Thus if gas were added to the atmosphere, plants should rapidly
take it up, turning it into wood and soil. Turning the argument backward,
in 1954 the biochemist G.E. Hutchinson figured that if atmospheric
CO2 had in fact increased as Callendar claimed,
that was probably due to emission from soils that were decaying following
the clearing of forests. This was the first time anyone had noticed
that deforestation — men with axes — might alter the atmosphere's
CO2. Hutchinson did not see it as a problem. It was a one-time
step, for once humanity finished converting the world's forests to
farmland, biomass uptake would soon restore a "self-regulating" equilibrium.(5)
(link from below) |
|
However plausible planetary self-regulation
might seem, scientists still wanted to check it rigorously. That meant
making a numerical model of the carbon system. They drew diagrams
with boxes — one box to represent the reservoir of carbon in
the atmosphere, other boxes for the oceanic and biological reservoirs
— and between the boxes they drew arrows to show the exchanges
of carbon. Applying a few equations and plugging in measurements of
radioactive carbon isotopes and other data, they made rough estimates
about how carbon moved about. (This box-and-arrow scheme has become
so common for visualizing the geophysical circulation of chemicals
that it seems natural and inevitable, but in fact it became familiar
only in the late 1950s.)(6*)
|
Box
model
<=Carbon dates |
Historians usually treat techniques as a stodgy foundation,
unseen beneath the more exciting story of scientific ideas. Yet techniques
are often crucial, and controversial. A case especially important
for biological studies is explored in a short essay on Uses of Radiocarbon Dating |
|
One of the first attempts to integrate the available data was a
model devised by Harmon Craig, an enthusiastic young scientist who
was in touch, by visit and letter, with Roger Revelle and others who
were doing parallel work. Craig's model boxes split the world-ocean
into two layers, the surface waters and the deeps. An arrow showed
carbon carried in water physically moving between the levels. Another
box and arrow showed the chemical exchanges of CO2 between the surface water and the atmosphere.(7) Meanwhile in Stockholm, two meteorologists
devised a model with a single box for the oceans but including separate
boxes for the reservoirs of carbon in living plants and in dead organic
matter such as forest litter.(8) In the following years several additional
models were published, as people added and adjusted boxes —
more ocean layers, perhaps, or separate boxes for ocean plankton and
terrestrial vegetation — each with its own estimates for the
uptake and release of carbon.(9) |
|
The first primitive models suggested that the systems should behave
in the manner long assumed. seawater and especially plants would
absorb or emit just enough CO2 to stabilize
the concentration of the gas in the atmosphere. But in fact the diagrams
and equations were so oversimplified that they only showed that it
was possible for the system to be self-regulating. On the
other hand, a widely noted model of biosphere absorption, constructed
by Erik Eriksson, oscillated all by itself under certain conditions.(10) This was characteristic of many models
built from a few simple equations, "as if their self-regulating properties
were defective in some way" (as a leading meteorologist put it).(11) Scientists expected that adding more realistic complexity
would add to stability. There might be short-term oscillations, but
over the long run, surely any extra carbon would be stored away in
biomass. Eriksson insisted that "the atmospheric concentration is
but little affected" by human input.(12)
|
|
More complex models
did not change these views. A modeler would draw up a system of carbon
reservoir boxes and write down five or so equations to describe how
they interacted. To get anywhere with this, the modeler had to make
simplifying assumptions of dubious validity. But with the poor data
at hand, there was little point in refinements, and none of the models
was pursued far.(13) The biological boxes were by far the most poorly understood
components. These early models tended to treat biomass almost like
a free parameter that the modeler could adjust, within the very broad
limits of what was known, to make the outcome fit the other data.
Thus the conclusions about stability relied not just on objective
calculations but also on what seemed plausible. |
=>Simple models
=>CO2 greenhouse
|
Can People Change Climate?
TOP
OF PAGE |
|
The scientists were under the sway of a firm belief that natural
systems are self-regulating. Biologists and ordinary people alike
had long assumed that communities of living creatures always managed
somehow to adjust their growth to counter any dangerous departure
from equilibrium — the indestructible "balance of nature." When
it came to the atmosphere, most geological experts thought that even
on a lifeless planet, the atmospheric balance would remain stable.
It seemed reasonable that chemical cycles would long ago have settled
down into some kind of equilibrium among air, rocks, and seawater.
Compared with those titanic kilometer-thick masses of minerals, it
hardly seemed necessary to consider the thin scum of bacteria and
so forth. Experts continued to calculate how the levels of atmospheric
gases, even oxygen, would be maintained by mineral processes that
had nothing to do with living creatures. In particular they figured
that the level of CO2 in the atmosphere was
locked down over the long run by geological forces — emission
from volcanoes balanced by absorption in weathering rocks. |
|
The planet's carbon cycle looked like just another example of the kind of
stable system that scientists had studied during their training. Chemistry
textbooks taught as an established principle (enunciated in 1888 by
Henri Le Chatelier, a French industrial chemist) that a system in
equilibrium responds to any stress in a way that tends to restore
its equilibrium. Le Chatelier's Principle reliably
regulated chemicals in laboratory flasks and in industrial plants.
Why not in the Earth's atmosphere as a whole?(link
from below) (see also above) |
<=>CO2 greenhouse
<=>Simple models
<=>Public opinion
|
In the 1960s, these views were standard, and few scientists imagined
that the planet's biology had much to do with its chemistry.(14*)
There were occasional doubts. A pioneering 1963 report on global warming, noting that human emissions of greenhouse gases were at a rate geologically unprecedented, warned: "It is not a cause for complacency that nature seems to have lots of checks... There may be processes... which will eventually be alarming." On the other hand, in 1966, when the U.S. National Academy of Sciences arranged a study
of possible climate change, the panel mainly considered urban and
industrial influences, that is, deliberate human excavation and emission
of materials. The experts remarked that changes involving living creatures
in the countryside, such as irrigation and deforestation, were "quite
small and localized," and set that topic aside
without study.(15) |
|
Yet as
the panel realized, the planetary environment was certainly affected
by human activity. During the 1960s, evidence
mounted that such human products as nuclear bombs and chemical pesticides
could inflict global harm. The comfortable traditional belief in the
automatic stability of biological systems was faltering. These feelings
connected with concern for the entire atmosphere when C.D. Keeling
published his data on changes in the level of CO2.
His measurements were so precise that from the outset, they showed
a seasonal "breathing" of the planet: plants in the northern hemisphere
took up carbon from the atmosphere in spring and summer, and returned
it to the air when dead leaves and grass rotted away in autumn and
winter. One could even use Keeling's data to figure how many tons
of carbon cycled through the plants each season.(16) The consumption was not keeping up with the quantities
of the gas that humans were putting into the atmosphere: year by year
the level ominously mounted. |
<=Public opinion
<=CO2 greenhouse
=>International
|
Keeling's curve was just one of many things that raised concern
about global biological effects. In the early 1970s, public sensitivity
redoubled following a series of climate disasters, especially a drought
in the African Sahel. Photographs of starving children, huddled in
a barren landscape of scrub, told a terrible story of expanding deserts
and changing climates. Was the Sahara desert expanding southward as
part of a natural climate cycle that would soon reverse itself, or
was something more dangerous at work? For a century, African travelers
and geographers had worried that overgrazing could cause changes in
the land that would turn the Sahel into a "man-made desert." During
periods of drought, missionaries and colonial officials blamed ignorant
native practices for the harm (few remarked that if anything would
make a permanent change, it would most likely be practices introduced
under the colonial regimes). The Sahara was not so much encroaching,
one scientist remarked in 1935, as taking advantage of "man's stupidity."(17) |
|
In 1975, veteran climate modeler Jule Charney
proposed that climate change was acting as man’s accomplice.
Noting that satellite pictures showed a widespread destruction of
vegetation in the Sahel from overgrazing, he pointed out that the
barren clay reflected sunlight more than the grasses had. He figured
this increase of albedo (surface reflectivity) would make the surface
cooler, and that could change the pattern of winds so as to bring
less rain. Then more plants would die, and a self-sustaining feedback
would push on to full desertification.(18)
|
<=>Public opinion
|
Charney was indulging in speculation, for computer models of
the time were too crude to show what a regional change of albedo
would actually do to the winds. It would be a few more years before
models and observations demonstrated what had long been suspected
— surface vegetation is an important factor in the climate.
For example, the Amazon rainforest generates much of its own rainfall
through evaporation. It would take a still later generation of models
to show that Charney's specific mechanism was valid to a degree.
It was an influence, but not the only one, in a complex set of interactions
involving other factors, such as variations in the surface temperature
of the Atlantic and Indian Oceans. (In the Sahel, the advance of
the desert halted and in the 1990s went into reverse, showing that
overgrazing did not by itself dominate changes. But the question
of human influence remained open. Later studies showed that along
with overgrazing, human emissions, not only greenhouse gases
but especially industrial haze, had caused changes in regional weather patterns
that contributed to the disaster.)
|
|
Despite the confusing details, scientists grasped the truth of Charney’s
main lesson. Human activity could change vegetation enough to affect
albedo, and a change in albedo could interact with other factors to
change climate. More generally, the biosphere did not necessarily
regulate the atmosphere smoothly through "negative" feedbacks that pulled the system back from any change.
It could itself be a source of the kind of "positive" feedbacks
that amplified changes. (For biological feedbacks, it was not good to be positive!)(19*)
|
|
Where Does the Carbon Go?
(1971-1980s) TOP
OF PAGE |
|
The science of biology was in no condition
to answer the questions that climate scientists were starting to bring.
A scientist’s funding and advancement depended on the publication
of conclusive studies that could be completed in a few years. To meet
that demand, most biologists concentrated their research projects
on one or another particular species if not a single molecule. Even
the pioneering scientists who had begun to consider larger systems
rarely undertook field studies that lasted as long as five years.
That was hardly enough to see how a biological community might respond
to climate change. Nevertheless the study of living communities in
all their complexity was gradually growing in scale and sophistication,
under the newly popular banner of ecology. The field was attracting
researchers who were curious about human impacts on the environment.
|
=>Climatologists
|
By the early 1970s, everyone had grown sensitive to a variety of
ways that humans were affecting the planet as a whole. The public
was becoming aware, in particular, that slash-and-burn farming was
eating its way through entire tropical forests. People realized that
only a small and diminishing remnant remained of the great ancient
forests of North America, and the same fate threatened the rest of
the planet's trees. Concern about the destruction of forests was on
the rise, although the concern was for the sake of wildlife, not climate.
|
|
Meanwhile a few scientists pointed out that the world's forests
were a significant player in global cycles of carbon and water. The
conversion of forests to croplands since the early 19th century had
given the first big contribution to the global rise of CO2.
(Decades later, scientists realized that deforestation also contributed
to cooling — for one thing, snow on exposed soil reflects more
winter sunlight than a forest does — so the net effect of deforestation
may have helped keep the 19th century cool.) Moreover, as anyone who
has walked sweating through a steamy jungle might understand, a forest
evaporating moisture can be wetter than an ocean, in the way it affects
the air overhead. The ancient ideas about climate change from deforestation
looked plausible again. Only now it was not just local weather, but
the entire global climate that could be affected. Just what kind of
changes would further deforestation bring? As one scientist who pioneered
study of the subject remarked, "it is difficult even
to guess."(20) |
|
There were a few things that could be measured with confidence.
Statistics compiled by governments on the use of fossil fuels told
how much CO2 was going into the atmosphere
from industrial production. And Keeling's measurements showed how
much of that remained in the air, to push the curve higher year by
year. The two numbers were not equal. Roughly half of the gas from
burning fossil fuels was missing. Where was the missing carbon going?
There were only two likely suspects. It must wind up either in the
oceans or in biomass. |
|
In 1971, the geochemist Wallace Broecker and colleagues developed
a model for the movements of carbon in the oceans, including the carbon
processed by living creatures. They calculated that something like
40% of new CO2 dissolved into the surface layer
of seawater, and they figured most of the rest would stay in the
atmosphere. While admitting that knowledge of biological interactions
was inadequate, they thought it likely that the "biosphere is not
an important sink" for swallowing up CO2.(21) However, more precise calculations indicated that the oceans
were not taking up all of the missing CO2.
"It seems impossible that any oceanic model can fully explain" the
missing carbon, Keeling wrote. The residue must somehow be sinking
into the biosphere. Perhaps trees and other plants were growing more
lushly thanks to CO2 fertilization?(22) |
|
If so, that was hard to check. The pioneering carbon box models
mostly concentrated on chemistry and did not attempt to calculate
whether any organisms might grow more abundantly when CO2
and warmth increased. Some ocean carbon calculations entirely left
out not only plants but all the terrestrial biota, that is, all
organisms on land. Plant biologists — a type of specialist
that had scarcely interacted with climate scientists — had
published few solid studies of carbon fertilization. (It was clear enough in greenhouses, but that said little about what would happen amid the complexities of a real forest.) The prevailing view had been established in the 1960s by Eugene Odum, the pioneering author of the dominant ecology textbook. In a mature ecosystem, Odum maintained, gains and losses of carbon precisely balanced one another. "Without much evidence to the contrary," a reporter noted, "Odum's paradigm held sway for several decades." It was not until the end of the 1990s that field studies showed that forests were dramatically gaining mass, presumably thanks to fertilization by the increased CO2 in the atmosphere, perhaps enhanced by warmer temperatures.(22a) |
|
What was clear in 1973, as Keeling pointed out, was that even with
good data on past and present conditions, any calculation of the
future fertilizer effect would be unreliable. Every gardener knows
that giving a plant more fertilizer will promote growth only up
to a certain level. Nobody knew where that level was if you gave
more CO2 to the world's various kinds of
plants. "We are thus practically obliged to consider the rate of
increase of biota as an unknown," Keeling warned.(23) As a sign of the uncertainty,
some rough calculations suggested that land plants might not be
a sink for CO2 at all. As Hutchinson had suggested back in 1954, deforestation
and other human works would increase decay in soils, so the land
biota could be a major net source of the gas.(24) |
|
The uncertainties became painfully obvious in November 1976 at
a "Workshop on Global Chemical Cycles and Their Alteration by Man"
held in Dahlem, Germany. The respected meteorologist Bert Bolin broke
with his earlier view that plants were not a major source of CO2.
He argued that deforestation of the tropics, plus the decay
of plant matter in soils damaged by agriculture, was releasing
a very large net amount of CO2 into the atmosphere
— somewhere around a quarter of the amount added by fossil fuels.
Since the level in the atmosphere was not rising all that fast, it
followed that the oceans must be taking up the gas much more effectively
than anyone had thought. Bolin admitted that "This result is difficult
to reconcile with present models of the role of the oceans."(25)
|
|
George Woodwell, a botanist who had recently joined the Marine
Biology Laboratory at Woods Hole to direct their Ecosystems Center,
went still further with calculations he had begun independently of
Bolin. Woodwell believed that deforestation and agriculture were putting
into the air as much CO2 as the total
from burning fossil fuel, or maybe even twice as much. His message
was that the attack on forests must be stopped, not just for the sake
of preserving nature but also to avoid disrupting the climate.(26) |
|
Broecker and other geochemists
thought Woodwell was making ridiculous extrapolations from scanty
data. Defending their own calculations, the geochemists insisted that
the oceans could not possibly be taking up so much carbon. "The subject
dominated the Dahlem conference," Woodwell recalled, "stimulating
much discussion."(27) The
arguments spilled over into general social questions of environmentalism
and regulation. People's beliefs about the sources of CO2
were becoming connected to their beliefs about what actions (if any)
governments should take.(28) |
=>Public opinion
= Milestone
|
Researchers tried to resolve the problem
scientifically, attacking it from many directions. In meetings, workshops,
and publications the experts met and wrangled, sometimes bitterly
but always politely. As occasionally happens in scientific debates,
opinions divided largely along disciplinary lines: oceanographers
plus geochemists versus biologists. The physical scientists like Broecker
pointed out that they could reliably calibrate their models of the
oceans with data on how the waters took up radioactive materials (fallout
from nuclear weapon tests was especially useful). Woodwell's biology
was manifestly trickier. His opponents argued that nobody really knew
what was happening to the plants of the Amazon and Siberia. When he
invoked field studies carried out in this or that patch of trees,
his opponents brought up more ambiguous studies, or just said that
studies of a few hectares here and there could scarcely be extrapolated
to all the world's forests. |
<=The oceans
|
Key data came from measurements of carbon
in old wood. (This used the fact that new radioactive isotopes cycled
through the atmosphere and plants, whereas fossil fuel emissions had
long since lost any radioactivity.) In 1978, Minze Stuiver used isotope
measurements to estimate that two-thirds of the CO2
added to the atmosphere up to 1950 had come from cutting down forests.
But global industry and population had been multiplying explosively. The situation had changed, and now nearly all the new
carbon was coming from fossil fuels. The ocean models were roughly correct.
|
<=Carbon dates |
This did not mean that forests were unimportant.
The way Keeling's CO2 curve swung up and down
with the seasons showed plainly that the springtime growth and autumn
decay of plant matter played a huge role in the atmosphere's carbon
budget. But averaged over a year, the gas emitted from decaying or
burned plants seemed to be roughly balanced by the amount taken up
by other plants. Maybe deforestation was balanced by more vigorous
growth due to fertilization by the increased CO2 in the atmosphere — "a chance compensation of opposed
effects."(29) |
<=Keeling's funds |
Woodwell denied this, and through the 1980s,
he continued to insist that tropical deforestation and other assaults
on the biosphere were contributing about as much net carbon to the
air as the burning of fossil fuels. Calling carbon dioxide "a major
threat to the present world order," he called not only for a halt
to burning forests but for aggressive reforestation to soak up excess
carbon. Saving the forests, more for the sake of wildlife than of
climate, was a popular idea in the growing environmental
movement — a movement in which Woodwell had long been a leader.(30) Other scientists, however, gradually concluded that his
claims were exaggerated. Eventually Woodwell had to concede that deforestation
was not adding as much CO2 to the atmosphere
as he had thought. |
<=>Public opinion
|
An important lesson remained. As a team headed by Broecker wrote
in 1979, Woodwell's claims that destruction of plants released huge
amounts of CO2 had been a "shock to those of
us engaged in global carbon budgeting." The intense reexamination
triggered by the claim had called attention to "the potential of the
biosphere." Broecker and others concerned with the geochemistry of
the oceans were especially frustrated by what they starting to call
the "missing sink" of carbon. The only areas so poorly
understood that they might hide such a huge feature of the system
were biological.(31) From the late 1970s onward, it was
clear that nobody could predict the future of global climate with
much precision until they could say how the planet's living systems
affected the level of CO2. |
|
Taking his own advice, Broecker began to look at seawater as a
container not only of chemicals but of life. In a pair of seminal 1982 papers he drew attention , he drew attention
to what was later called a biological "carbon pump."(32) The plankton that grow abundantly in
surface waters use carbon to build their bodies and shells. After
they die, fragments eventually snow down to the ocean floor, where
the carbon is buried in sediments. One might suppose that adding more plankton would
immediately reduce the amount of CO2 in the
atmosphere. Further investigation, however, showed that the short-term
effect is not straightforward. When creatures make calcium carbonate
for their shells, they alter the complex chemistry of seawater, which
actually ends up releasing more of the gas into the air. Scientists
had much to study in the many biochemical changes that occur as plankton
flourish and dissolve. |
|
In studying all this, Broecker and his colleagues
were not concentrating on what it meant for the contemporary carbon
budget. Their chief interest was what the burial of carbon over thousands
of years might mean for the swings between ice ages and warm periods.
Over the long run, the more carbon was buried, the less there should
be in the atmosphere. Could studying changes in the atmosphere's CO2 content lead them to the "holy grail"
of geochemical research, the mechanism that dominated ice age cycles?(33) Powerful support for this hope came from studies of ancient ice pulled up from boreholes in the Greenland and Antarctic ice caps. It turned out that during past ice ages the level of CO2 in the atmosphere had gone through big swings up and down, roughly in step with the rise and fall of global temperatures. Nobody could think of any physical or chemical effect strong enough to cause this. That left the biosphere, and above all the oceans. Ocean biology might be the key to explaining how global temperatures and CO2 had shifted together over the millennia. |
<=Climate
cycles
=>The oceans |
It was especially noteworthy that plankton
could grow only where they got enough trace minerals like iron and
phosphorus. Thus the global carbon cycle depended on the upwelling
of ocean currents bearing nutrients, and on the winds that blew mineral
dust out to sea. The patterns of upwelling, winds, and erosion were
not the same during glacial periods as during warm periods like the
present. Besides, changes in temperature would obviously affect the
growth of plankton directly. It was an outrageously tangled case of
interactions between biological activity and climate. |
<=Rapid change |
Broecker and several other scientists launched into increasingly
elaborate calculations of the connections between CO2
in the atmosphere, the chemistry of the various layers of ocean
waters, the plankton inhabiting those layers, and climates past,
present, and future. The biology and chemistry had so many complexities
and pitfalls that questions multiplied faster than answers, but
one thing was clear. In the future, as more and more CO2
from fossil fuels dissolved into the ocean — with levels already
well above anything found in measurements that went back half a
million years — there would be serious chemical changes. In
particular, the carbonated seawater would more easily dissolve
the compounds that made up shells. Whether that would endanger sea
creatures, and what would eventually result from all this, nobody
could guess.(34) |
|
Biological processes on land were easier to investigate, and
progress was steady. For example, in 1983 a pioneering study modeled
69 regional ecosystems separately, and concluded that changes in
land use since the 18th century had caused a net release of carbon
from soils.They confirmed Stuiver's finding that until around 1960 humanity had
released more carbon into the atmosphere by cutting down forests
and the like than through burning fossil fuels. The uncertainties
were large enough so that if you assumed the lowest reasonable level
for some factors and the highest reasonable level for others, it
was possible to balance the global carbon budget.(34a) |
|
Despite such efforts the argument over the
fate of CO2 remained unresolved. As one pair of authors complained, "from
meager statistical information and often ill-documented statements
in the literature, it is extremely difficult to calculate" what was
happening between the biosphere and the atmosphere.(35) Woodwell insisted that if not now,
then in the future, global warming would cause vegetation to release
overwhelming amounts of CO2. Through the 1980s,
debate continued as scientists came up with a wealth of new data and
new ideas, doing less to solve the carbon problem than to reveal ever
more complications. The complications were not only scientific. Calling
yet again for an end to deforestation, Woodwell pointed out that the
goal collided with powerful economic forces, not to mention corruption.
The necessary changes, he said, "require political advances rather
than scientific or technical insights."(36)
The story of the missing carbon is continued in a later
section. |
=>CO2 greenhouse |
Methane (1979-1980s)
TOP
OF PAGE |
|
Controversy over the
numbers in the planet's carbon budget, linked with growing concern
about global warming due to CO2, goaded scientists to study more closely the biological exchanges
of carbon. In 1979, a team reported that the burning of forests put
into the atmosphere significant quantities of CO2
and also other greenhouse gases. Methane gas (CH4) in particular had
a significant part to play in the global carbon budget.(37) And scientists had recently realized
that methane, molecule for molecule, was many times more effective
than CO2 as a greenhouse gas. |
=>World winter
<=Other
gases
|
The gas came mainly from living creatures: bacteria lurking everywhere
from soil to seawater to the guts of elephants. Everyone knew that
"swamp gas" bubbles out of wetlands in particular. Back in 1974, a
German geochemist had calculated that terrestrial bogs, not the oceans,
are the largest source of the methane in the atmosphere.(38) These natural emissions were much greater than the amount
of methane that escaped as humans extracted and burned natural gas.
Meanwhile, people were beginning to recognize that the world's wetlands
were rapidly changing under human impact. And that was not all. |
|
Studies found that animals could be a significant source of both CO2 and methane. Methane in particular was produced by bacteria in the
guts of cattle and other domestic animals and then burped into the air. The rapid increase
in meat and milk production added significantly to the rising level
of the gas in the atmosphere. (In later decades studies pinned this down, finding that animal husbandry contributed more than a tenth of humanity's greenhouse gas emissions. One component of this was another potent greenhouse gas, nitrous oxide, released from fertilizers used in growing fodder.) Rice paddies too had been spreading
swiftly, with methane bubbling up from the mud.(39) Even termites, found everywhere on
the planet that dead wood decayed, might be a significant source of
methane and CO2. (Later research showed that
termites are indeed a factor, but contribute considerably less than
domestic animals or rice paddies.)(40) Human activities would affect
these releases and uptakes of gas; the activity of soil bacteria and
termites, for example, were largest in areas disturbed by cultivation
or burning. And of course such things would also be affected by climate
change itself. All these interlocked effects would somehow have to
be taken into account. |
=>Simple models
=>Other gases
|
An especially thought-provoking calculation
showed that a huge reservoir of carbon was frozen in the deep permafrost
layers of peat that underlay northern tundras — perhaps half
as much carbon as in all the world's tropical forests and jungles.(41) As global warming reached these peat
beds, they might release a huge amount of CO2.
The soggy tundras, covering millions of squarekilometers and highly
sensitive to temperature change, might also emit massive quantities
of methane.(42) A similar danger turned up in an even more gigantic reservoir
of methane, at least partly of biological origin, that was locked
up in "clathrate" ices in the muck of deep sea beds. Global warming
would probably increase the emission of greenhouse gases from all
these sources. That raised an alarming possibility of an amplifying feedback
— more greenhouse warming, thus more emission, and so on up. |
<=>Other gases
|
A strong hint that this was a real concern showed up in the studies of ice from boreholes in the Greenland and Antarctic ice caps. Not only CO2 but also methane in the atmosphere had swung up and down roughly in time with the temperature swings. While for CO2 this meant mainly looking to marine life and perhaps soils and forests, for methane this pointed to changes in how tundra, wetlands and clathrates took up the gas as climate cooled, or released it in a warm period.(43) Reversing the sequence, if the greenhouse
gas abundances changed because of something happening in
the biosphere (for example, human activities), climate change would follow. |
<=>CO2 greenhouse |
Gaia (1972-1980s) TOP
OF PAGE |
|
Geoscientists had thought of carbon mainly as something to do with
volcanoes and the weathering of rocks, but from the early 1970s forward,
they understood that biology was a major player in the global carbon
budget. Indeed it dominated the game on the human timescale of centuries.
For other chemical elements, for example the cycle of sulfur through
the oceans and atmosphere, scientists still felt that simple mineral
chemistry must predominate. That changed during a research voyage
on the Atlantic Ocean that included James Lovelock, a wide-ranging
and exceptionally independent-minded researcher. His Ph.D. was in
medicine, but his most notable achievement at this point had been
inventing instrumentation for measuring rare gases even at tiny concentrations.
On the high seas Lovelock discovered that one such gas, dimethyl sulfide
(DMS), was a principal element in the global sulfur cycle. The main
source of DMS was ocean plankton.(44*)
|
|
Lovelock was already convinced that, as he
put it, "the atmospheric gases are biological products." His interest
was partly stimulated by gases that he found everywhere in the Earth's
atmosphere and that were undoubtedly produced by living creatures:
pollutants from human industry. But Lovelock based his thinking more
deeply on the most fundamental property of biology, the uphill march
of life against entropy.(45) |
<=Other gases |
Back in the 1960s, Lovelock had proposed measuring gases in the
Martian atmosphere as a way to look for traces of life In 1965 a pair of scientists had found that the oxygen in Earth's atmosphere comes mainly from photosynthesis in plants. Living creatures, Lovelock explained, could drive their planet's atmosphere
into "a state of disequilibrium." Mars lacked the free oxygen of our
own planet precisely because Mars was sterile. At this point in Lovelock's
thinking, a stable balance gave witness to dead minerals, whereas
the system of life plus minerals created a perpetual state of dynamic
imbalance.(46) |
|
Lovelock ran into trouble
when he tried to publish these ideas in 1966. At the time he simply
remarked that the physical sciences habitually ignored the physical
effects of life "to the point of blindness." Long afterward, he reflected
that "Conventional biology and planetary science held the false assumption
that organisms merely adapt to their environment. My ideas for life
detection acknowledged that organisms change their environment...
Neither my critics nor I were aware of this fundamental difference
of viewpoint." Lovelock’s difficulties illustrated how hard
it was to grasp that living creatures could play a huge role in the
geochemistry of their planet.(47*) |
<=>Venus & Mars
=>CO2 greenhouse
|
In 1974, Lovelock put together a grand generalization in collaboration
with Lynn Margulis, who had a deep understanding of microbiology (and
shared a taste for planet-sized speculation with her former husband,
Carl Sagan). Their article was entitled, "Atmospheric homeostasis
by and for the biosphere: The Gaia hypothesis." Lovelock and Margulis
proposed that the ensemble of living creatures had taken "control
of the planetary environment" in a way that would maintain conditions
favorable for life itself. This pushed to the limit the new way of
seeing the atmosphere as something susceptible to biological influence.
Under the new hypothesis the atmosphere was altogether "a component
part of the biosphere," in fact a "contrivance." The rhetoric and
the name, after the Greek Earth-goddess, carried an implication of
purposeful and indeed supernatural guidance, which disgusted many
scientists. But if you stripped away any implication of conscious
purpose, the idea that biology controlled atmospheric content was
rationally defensible.(48) |
|
For more than a decade the Gaia hypothesis
led nowhere scientifically. Most scientists considered it visionary
at best. Then in 1987 Lovelock, working with Robert Charlson and others,
argued plausibly that the DMS that ocean plankton emitted could influence
climate, much like the smoggy sulfur aerosols produced by human industry.
In the clean air over the oceans, particles of DMS were a major source
of nuclei for the condensation of the water droplets that would form
clouds. This suggested a Gaia-like self-regulation. Perhaps if the
oceans got warmer, the plankton would produce more DMS... which would
make more clouds and more reflection of sunlight from the atmosphere...
which would bring a compensatory cooling back toward normal. Perhaps
this biological regulation "has already counteracted the influence
of the recent increase in CO2 and other
'greenhouse' gases." (Studies in later years found that there might indeed be a slight cooling effect, but not enough to counteract the greenhouse warming.)(49) On the other hand, one could also imagine scenarios where global warming killed off plankton,
bringing a vicious circle of increasing
warmth. |
=>Aerosols
|
Some people hoped
the Gaia hypothesis could put a scientific foundation under the traditional
belief in ecological self-regulation, the beneficent "balance of nature."
(see above) Over the long run, species that
damaged their ecosystem were automatically laid low (a troubling thought,
given that humankind was such a species). To others the hypothesis
was misleading, not science but mysticism. If the Earth's atmosphere
had remained favorable for life over the past billion years, most
scientists saw no logic or evidence compelling them to think that
the stability was due to anything but sheer good luck. Lovelock himself admitted
that the hypothesis might never be proved definitively. In any case,
he later added, human interference might be large enough to force
the global system beyond the point where nature could maintain a balance.
What the Gaia hypothesis did accomplish was to encourage scientists
to investigate how biology could show up in every corner of atmospheric
chemistry — which in turn affected everything from clouds to
the weathering of rocks. "The science of Gaia is now part of
conventional wisdom," boasted Lovelock in 1991 with only partial
exaggeration, "and is called Earth system science; only the name
Gaia is controversial." (49a)
For both scientists and the public the debate promoted an understanding
that life interacts with climate in ways unforeseeable and disturbingly
powerful. |
=>Public opinion
=>Climatology |
Everything is connected to everything else:
from a high-minded but nebulous philosophy, this viewpoint had evolved
into a scientific requirement for analyzing the planet. The final
answer to the question of climate change would be a set of predictions
for the levels of gases, temperatures and precipitation, and their
impacts on ecosystems and human society. That could only come through
calculations with a model that incorporated all the significant
factors and their interactions. A start at mapping
such a model was made at a workshop held in Jackson Hole, Wyoming
in 1985. The panelists projected sketches on a wall and scribbled
over it until they got a consensus on what the most important subsystems
of the model would be. The result, which became known in the modeling
community as the "wiring diagram," had more than three dozen arrows
connecting an even larger number of boxes. Similar diagrams sketched
a decade earlier had ignored biology, but here it was at the center.
The boxes were highly simplified ("cloudiness," "nutrient recycling,"
"human land use," "marine biological production," and the like), and
the community was a long way from knowing how to calculate what happened
in most of them. Even if scientists had known all that, computers
that could handle the calculations were decades in the future.(50) |
"wiring
diagram"
=>Simple models |
From the 1980s forward there was extensive research on how emisions from human agriculture and animal husbandry might add to global warming. Some results are described in the essay on Other Greenhouse Gases. |
|
Global Warming Feedbacks TOP
OF PAGE |
|
Like clouds drifting in from the horizon heralding the possibility
of a storm, the prospect of global warming increasingly caught the
attention of scientists far afield from traditional meteorology. They
began work to organize big, long-term field studies in dozens of specialized
topics of agriculture, forestry, and so forth, to see how climate
might interact with the planet's many ecosystems. There was far too
little money to support all those studies, but some important questions
were at least partly answered. |
|
The oldest question was whether a change
in vegetation, especially a change caused by humans, could alter
regional climates? The answer was now certain: Yes. At several locations,
overgrazed grasslands with dried-out soils had become demonstrably
hotter than less-used pastures. And the heating would make it all
the harder for grass to return. Some rainforests that had been
cut down showed a measurable decrease in rainfall, since moisture
was no longer evaporated back into the air from the leaves of trees
— in Brazil, rain fled from the plough.On the other hand,
work published in 2004 gave a more complex picture: under some circumstances,
deforestation could bring more rain storms when air rose from the
hotter ground. Such regional studies were too few to paint a clear
picture of how the many types of vegetation in total could affect
global climate. The studies did show that wherever vegetation was
altered there could be serious feedbacks with a potential for a
lasting, self-sustained regional change. Deforestation and other
deliberate changes in land use, however,seemed less likely to make a great
difference outside the region that people were altering.(51) |
|
That left open the question of inadvertent changes, as vegetation
everywhere reacted to greenhouse gases. For example, some scientists
pointed out that if climate change encouraged forests to grow farther
north, the dark pines would absorb more sunlight than snowy tundra
and heat the air, adding to global warming.(51a) |
<=Simple models
|
A 1989 review of computer climate studies
concluded that the next generation of models would have to include
detailed representations of vegetation. By the mid 1990s, biologists
and modelers were discussing all sorts of details — for example, the way increased levels
of CO2 would affect the evaporation of moisture from leaves. Small holes (stomata) in every leaf admit the CO2 that photosynthesis uses as feedstock. As the level of the gas in the atmosphere rose the stomata would close up a bit, and therefore less water would escape. That could have a surprisingly large impact on the amount of moisture in the air, affecting rainfall thousands of miles away. But nothing influenced the world's vegetation as much as humans, so the models would also have to include social and economic forces.(52)
|
|
Some scientists
stuck by the old view that natural systems were self-stabilizing,
and found biological feedbacks reassuring rather than alarming. They
held that fertilization from the increased CO2 in the atmosphere would benefit agriculture and forestry so
much that it would make up for any possible damage from climate change.(53) The fertilization effect was confirmed by field measurements
of the exchange of carbon in various forests, and by studies of the
consequences of blowing extra CO2 across crops,
grasslands, and so forth. For the planet as a whole, biomass did seem
to be absorbing more CO2 than in earlier decades.
(That was pinned down in 2017 by ingenious measurements in Antarctic ice cores. Terrestrial uptake of carbon, had risen some 30% during the 20th century.) However, the same studies turned up some unsettling results. The numbers
were often very different from what the handful of earlier, more primitive
studies had suggested. And the consequences of fertilization were
not straightforward. For example, under some circumstances the extra
CO2 might benefit weeds and insect pests more
than desirable crops, and the crops themselves would become less nutritious. |
=>Public opinion
=>Impacts |
In any case, as the level of the gas continued to rise, plants
would reach a point (nobody could predict how soon) where they would
be unable benefit from more carbon fertilizer; the increase in plant growth would
level off. In the late 1980s some experts predicted that warming would eventually
foster decay, with a net emission of greenhouse gases,
bringing yet more warming.
Could there be a positive feedback that would run away exponentially? Others attacked the question, narrowing the estimates. They quelled fears of a horrid runaway, but they did find that biological activity in tundra and other warming soils would add to the greenhouse gases in the atmosphere. For example, a 1995 survey of laboratory data confirmed that soil decomposition in general might "provide a positive feedback in the global carbon cycle."(53a) |
|
By now, most specialists in paleobiology, geochemistry and the
like were coming around to the view that natural systems were not
always self-stabilizing — biological and physical systems
alike were susceptible to positive feedbacks. Studies of
fossil pollen confirmed a growing suspicion that as climate changed,
entire assemblages of species could be driven into configurations
that had never been seen before.Some began to foresee extinctions of species and the
impoverishment, perhaps the utter failure, of vital ecosystems. In one authoritative study, an international group of 19 experts estimated that by 2050, somewhere between 15 and 37% of all the species in a large sample of regions would be "committed to extinction."(53b) |
|
A few people suggested solving the greenhouse
problem by using biology deliberately. Perhaps we could manipulate
the "biological pump" of dead plankton that snowed down upon the
ocean floor, taking carbon with them? The plankton did not flourish
without trace minerals, which are scarce in mid-ocean. For decades
there had been talk about improving the biological productivity
of barren ocean regions by adding nutrients, something like the
traditional nitrate and phosphate fertilizers used by farmers. Studies
in the late 1980s and 1990s suggested that iron was the keystone
fertilizer. By dumping iron compounds where the element was lacking,
we might be able to stimulate plankton to bloom. Could the biological
pump bury carbon as quickly as our industries emitted it? The pioneer
of the theory, John Martin, joked in a Strangelove accent, "Give
me a half tanker of iron and I will give you an ice age!"(54)
|
=>Climate mod |
Scientists began planning experiments to see just how much carbon
they could send to the sea floor with a shot of fertilizer. Quite
a lot, under the right circumstances, according to studies completed
after 2001. But the details of these circumstances were as obscure
and complex as everything else in the oceans. Many people warned
that in view of how little we knew about ocean ecosystems, this
sort of meddling might just make things worse. For example, what
if fertilizing plankton made them emit extra methane or other potent
greenhouse gases? |
|
Meanwhile Broecker and a few other dedicated
specialists tried to unravel the tangled biological and chemical
history of the oceans through glacial periods, by following tracer
minerals such as cadmium. Broecker's initial ideas were in error,
as he realized "almost before the ink had dried on the publication."
That was the story of much that followed. As he admitted in 2000,
"The prize has yet to be grasped." Oceanographers were just
starting to realize that the drifting plankton formed communities
as complex as a rainforest. Only a tiny fraction of the marine
species had even been identified. Climate change would profoundly
affect these communities, but nobody could say just
how.(55*)
|
|
Moreover, as increasing amounts of CO2
dissolved into the oceans the surface waters were growing more acidic.
Many creatures would find it increasingly difficult to make their
shells. Unless humanity restricted its emissions, in future centuries
the oceans would inevitably become more acidic than they had been
for hundreds of millions of years. Combined with warming, it could reduce some of the planet's
grandest and most productive ecosystems to ugly ruin. Already within the
next few decades the increasing acidity seemed likely to severely
deplete key species — which would not only damage coral reefs and fisheries
but might reduce the capacity of the biological "pump"
to sequester CO2 in the ocean depths. Besides changing ecosystems, acidity would directly affect how plankton shells took up carbon and later dissolved, or did not dissolve, as empty shells sank to the sea floor. Exploring
how seawater chemistry and temperature affected each important
species, and the interactions among the myriad creatures, and the
consequences for the movement of carbon, was a project that would
take many decades.(55a*) |
<=>The oceans
<=Impacts |
Attempts to balance the current carbon budget
continued to hold center stage through the 1990s. Debate persisted
over such issues as whether tropical forests were a net source or
sink for carbon. Meanwhile some continued to present arguments that
excess CO2 was mostly sinking into the oceans,
opposed by others who came up with equally persuasive arguments that
the gas was mostly going into plants. Only more data could resolve
these questions. Particularly helpful were regular measurements of
CO2 levels at many locations, made by the U.S.
government (to be precise, NOAA, with analysis chiefly under Keeling
at Scripps). Flasks of air were gathered at a string of stations running
from the South Pole up to an ice floe in the Arctic Ocean. The variations
from season to season said much about the movements of the gas. Another
powerful way to interpret these numbers came from new and precise
data on oxygen in the atmosphere. The oxygen level is fractionally
altered wherever burning fuel emits CO2 and
wherever plants emit or take up the gas, but the oxygen level is unaffected
when CO2 is taken up in the oceans. The ingenious
and painstaking measurements were the work of Ralph Keeling, Charles
David's son.(56) Over the course of the 1990s,
the various numbers tended to converge, suggesting that none of the
debaters was entirely right or entirely wrong. |
|
The reasons for the long-standing confusion were explained in part
by new studies, which showed that the uptake of carbon by forests
and soils was varying erratically and massively. A region that had
absorbed carbon overall during one decade might be a major source
of carbon in the next. In particular, it seemed that much of the "missing
carbon" had been absorbed by Northern Hemisphere forests in some decades,
but not in others. The uptake might depend on various things, such
as the global weather fluctuations brought on by El Niño events in the southern Pacific Ocean.(57) |
|
By the start of the 21st century it was
established that overall, humanity was emitting seven billion metric
tonnes of carbon each year by burning fossil fuels and another one
or two by clearing tropical forests (which turned out to be a net source of CO2 in spite of fertilization).The emissions were increasing, indeed accelerating, by roughly an additional one percent
a year. About half of this stayed in the atmosphere, and the oceans
absorbed a quarter, which left roughly two billion tonnes per year
that terrestrial ecosystems must somehow absorb. Some studies pointed
to rapidly growing Northern Hemisphere forests, others located the
main uptake in tropical forests. One study might turn up evidence
of carbon taken up by peat bogs, another might point to the world’s
desert soils as "the long-sought missing carbon sink," or
it might be something else entirely. In 2011 an international team largely settled the issue with a major study which found that forests (mainly in the temperate zones) could account for all the missing carbon.(58*) |
<=The oceans |
Looking over
a much longer term, maverick paleoclimatoligist William Ruddiman boldly argued
that humanity had been altering climate for thousands of years as
the spread of agriculture produced ever more CO2
and methane. Measurements in ice cores showed that these gases had not declined over the last several millennia, as they had at a comparable point in all previous interglacial periods. A variety of studies pointed to the emissions from rice paddies, deforestation, and so forth as the reason
why the world had not been cooling as it normally did at this stage
of the glacial cycle. After a decade of controversy, scientific opinion converged on accepting Ruddiman's theory. In any case nobody now disputed
that human activity, and its interactions with the rest of the biosphere,
was currently responsible for massive changes in the global carbon cycle. Nor
did any scientist doubt that the future was likely to see even greater
changes, as emissions mounted and biological systems responded.(59) |
|
Looking to the future,
experts still had not resolved such basic questions as whether tropical
forests, by absorbing or releasing carbon dioxide, were more likely
to retard global warming or hasten it. In every ecosystem, the carbon
balance would depend heavily on what humans did. Alongside deforestation
and reforestation it was important to account for the effects of fertilization
— including global fertilization not only by CO2
but also by our rising emissions of nitrate gases. Experts also began
to debate how agricultural practices and other land use by humans
affected the storage of carbon in soil, and indeed could directly
change things like rainfall, as observed since the days of Columbus.
These changes, one scientist remarked, "may be at least as important
in altering the weather as changes in climate patterns associated
with greenhouse gases" (if not globally than regionally, which
is what most people worry about).(60) All these uncertainties raised severe problems for international
negotiators, when they tried to assign responsibility to particular
nations for how much they added to the greenhouse effect or subtracted
from it. |
<=>CO2 greenhouse
=>International |
In the late 1990s, models
based largely on speculation and hand-waving began to give way to
quantitative models based on solid data. A key result appeared in
2000, published by a team of researchers who had managed to couple
computer models for the atmosphere, oceans, vegetation and soils
all together. Their preliminary results were ominous. It appeared
that warming would make it harder for the planet to take up carbon,
and might even trigger increased emissions. In particular, their
simulated tropical forests dried up and began to emit massive amounts
of CO2. The team's best guess was that around
mid-century the planet's biosphere as a whole would turn from a
net absorber to a major emitter of carbon, speeding up climate change. The importance of the fertilization mechanism was driven home by a study that showed that if there had been no fertilization in the past, there would now be much more CO2 in the atmosphere and a corresponding increase in warming.(61)
|
= Milestone |
As research proceeded, the results continued to be
discouraging. A 2004 model run estimated that during the 21st century there would be roughly half a degree more warming than would happen if there were no soil feedback. But there were many unknowns in how warming might change the global carbon cycle; if we were unlucky, it might lift future warming from, say, three degrees to four or even more.(61a) An increasing number of groups worked up models that
coupled climate changes with changes in soils, vegetation and the oceans. Up to this point, models had mostly treated plant life as an element of physics, something with a surface roughness and reflectivity, through which water and gases flowed depending on temperature and so forth. But now the modelers understood that carbon cycling was a key issue, and they began to incorporate the physiology of plants and much other detail. Typical was a French model that traced water through 11 levels of soil (of several different soil types) including transfer to plants, evaporation or runoff, taking into account the growth and death of leaves and roots including carbon uptake or release, and even competition between different species as the climate changed. An example of the modelers' complex struggles was a 2010 experiment in which the world's arid regions were not assigned enough vegetation to hold the soil in place. The model atmosphere filled with dust, and that fertilized the oceans and caused huge blooms of plankton — a disaster presumably unlike our actual planet's future.(61b)
|
<=Simple models
=>Models (GCMs) |
The most prominent worry was the Amazon forest, sustained
by rains that were largely water evaporated from the jungle itself.
Modelers found that warming combined with the rampant deforestation
might flip the entire system to a parched scrubby forest and grasslands ravaged by fires. Worries redoubled when prodigious droughts struck the Amazon in 2005 and 2010, releasing large amounts of carbon into the atmosphere. However, scientists did not have enough data to say with any confidence what would happen in the Amazon basin, nor in the many other regions with special characteristics. Some scientists warned that the models had not yet incorporated factors such as forest fires and insect infestations, which were already noticeably on the rise, reducing the ability of plants and soils to take up carbon. Reports by official panels conservatively played down the extreme scenarios. On land or in the sea, there were many other biological feedbacks not yet taken into account, which could be either favorable or unfavorable. But overall, everyone agreed that an ever higher fraction of the CO2 that humanity emitted would stay in the air, adding substantially to global warming. A large international collaboration that compared a variety of models found many differences, but every team predicted positive feedbacks.(62) |
=>CO2 greenhouse
|
Field studies were
also ominous. In 2015 a massive study of the Amazon, with three decades of observations in 321 plots, reported that the forests had been taking up carbon markedly more slowly than in the past. The Brazilian basin, degraded by rampant deforestation and fires, was already emitting more carbon than it absorbed, and there were indications the entire region might be approaching a tipping point where it would transform irreversibly into dry grasslands. A 2022 study reported that in the worst case, the collapse could begin when global temperature rose as little as 1°C above the present and could be completed wihtin another half-century. Looking more widely, a gglobal survey finally nailed down, as a general rule, what had increasingly turned up in regional studies, neatly summarized in the paper's title: "Tropical deforestation causes large reductions in observed precipitation." Brazil was not the only place where replacing trees with crops or cattle was drying the land. |
Ecosystems
at risk
|
Meanwhile a long-term study of plants confirmed the worry that fertilization by CO2 was limited by the availability of other nutrients. Advances in soil science had overturned the traditional belief that soils would always provide reliable long-term carbon storage. Apparently the standard “Earth System” models for climate change overestimated the value of forests for absorbing carbon as the world got warmer. By 2020, global surveys from satellites and many kinds of studies on the ground in dozens of countries had confirmed that since the 1980s the fertilization benefit had been fading away. As warming continued, tropical forests in particular would stop taking up carbon altogether and become net emitters. This "saturation," two experts declared, was "a terrible omen for the future pace of climate change." |
|
The good news was that while tropical forests were taking up less carbon, the immense boreal forests of Canada and Siberia were expanding northward and taking up more. The net effect on carbon uptake was currently positive, holding back the rise of CO2. Nobody could say how long that would continue (among other things, warming was bringing boreal forests more fires and pest infestations). Tundra was even more perplexing. Already
in 2003, a measurement of an Arctic bog showed a sharp rise in
dangerous methane emissions since 1970, and later studies in Siberia confirmed
this was happening all around the Arctic. There were signs that as warming continued, by mid-century the thawing tundra would be making a substantial addition to the atmosphere's burden of greenhouse gases. |
|
The scientists offered their conclusions tentatively, for they had only a scattering of observations of the tortuously complex wetlands systems. For example, studies published in 2020 concluded that methane emissions would be lower than expected because methane-eating microbes would oxidize the gas before it could escape. Meanwhile, studies of actual permafrost reported that as soil thawed it slumped and formed ponds, so that the movement of water and the exposure of long-buried carbon made for much greater emissions than expected.(63) |
|
In the oceans, there was new evidence that while climate had important effects on plankton, plankton acted on climate in return. The creatures not only took CO2 out of the atmosphere and sequestered it in the deeps but might, for example, directly affect cloudiness through the aerosol particles they emitted. Studies had to factor in not only temperature changes but the increasing acidity of seawater. Into the 2020s biologists argued over huge differences in their estimates of such factors in the climate equation. In this case, as with some of the complex interactions in soils and so forth, it was not clear whether the sum of the feedbacks would help or hurt the climate.(63a) |
|
People who did not want to worry about global warming were encouraged by studies that found that vegetation was "greening" around the world. Whatever might happen later in the century, at present plants were growing more abundantly due to increased CO2 fertilization (along with warmer weather in some regions). The extra growth not only improved crop yields but also stored extra carbon in living plants and their decay products, offsetting part of the rise of industrial emissions. But fertilization was not an unalloyed benefit, for it helped weeds as much as crops, and the fast-growing crops themselves were less nutritious. Also, when fertilization enhanced the growth of dark vegetation around the Arctic that made for more absorption of sunlight, enhancing the Arctic warming feedback loop. |
|
There seemed no end to the unexpected but significant effects that biologists turned up as they worked through the staggering complexities of ecosystems. The biggest concern was enhanced greenhouse gas emissions. For example, studies found that the emission of carbon by microbes in Arctic soils would increase as global temperatures rose, and likewise for tropical forests in night-time respiration. As one team put it, "rising temperatures will stimulate the net loss of soil carbon to the atmosphere, driving a positive land carbon-climate feedback that could accelerate climate change." The prediction was confirmed in 2018, when observations around the globe detected a sustained trend of soil carbon loss. Many other studies had disturbing results, while continuing to demonstrate that we had a lot to learn about the way increasing temperatures and CO2 levels might affect biological systems. |
=>International
=>CO2 greenhouse
=>Rapid change |
"The last decade has seen revolutionary advances in understanding," a marine biologist remarked in 2021. The traditional relatively stable picture, in which different effects of climate change added up separately, was giving way to ideas about effects interacting and multiplying one another. That raised "the possibility of sudden and unexpected shifts" as systems passed tipping points. A new research specialty emerged as ecologists tried to identify signals that would warn when a particular system was approaching a fatal tipping point. The problem was so difficult, however, that a breakdown might not be discovered until it was underway.."Ecosystem dynamics are complex and nonlinear," another expert explained, "and unexpected phenomena may arise as we push the planet into this unknown climate state." Any surprises would probably be unpleasant ones, given that natural ecosystems and human agriculture were well adapted to the traditional.climate.(64*) |
|
These concerns were strengthened by detailed studies of ice cores that revealed how the levels of CO2
and methane in the atmosphere had lurched up and down as ice ages
came and went. Both gases moved almost exactly in tandem with temperature. Once something caused a bit of warming or cooling, the planet had responded with a strong rise or fall of the levels of greenhouse gases, which led to further warming or cooling, and so forth. It was a strong confirmation that the gases played a potent role in climate change
through feedbacks — one of whose main engines, it was now clear, was located
in the biosphere.
This essay only touches on the extensive scientific work on expected impacts on living creatures (including us). See the summary of expected Impacts
of Global Warming. |
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The Carbon Dioxide Greenhouse Effect
Simple Models of Climate
General Circulation Models of the Atmosphere
1. Vernadsky (1945), p.
4. BACK
2. Colón (1960),
p. 147. BACK
3. Fleming (1990); Fleming (1998), ch.s 2-4; Stehr
and von Storch (2000), introduction and chapter 4; the latter is a
translation of Brückner (1890), chapter 1. BACK
4. Vernadsky's Geochemistry Vernadsky
(1924) was published in France (and in Russia in 1927), likewise his
Biosphere, Vernadsky (1929); see Vernadsky
(1945); Bailes (1990). At least one earlier
review of geochemistry, Clarke (1920), ch. 2, included plants along with mineral
chemistry as possibly important sources of some gases.
BACK
5. Hutchinson (1954), 389-90;
see also Hutchinson (1948). BACK
6. A pioneer carbon cycle diagram, remarking that
"a quantitative statement is rarely attempted," was Hutchinson
(1948), pp. 222-23; the idea may have been drawn from "compartment"
models of biological systems familiar in the 1940s to people who worked
with radioactive tracers, see Atkins (1969). A typical example is in Fonselius
et al. (1956). BACK
7. Craig (1957a). BACK
8. Eriksson and Welander (1956).
BACK
9. Especially important was Oeschger
et al. (1975), see p. 191 for applications, and see for references
to other models. BACK
10. Eriksson and Welander (1956).
BACK
11. Rossby (1959), p.
16. BACK
12. Eriksson and Welander (1956),
quote p. 171; the model most used in the next couple of decades was Craig (1957a). BACK
13. Models were reviewed by Keeling (1973); the only one that went so far as to use
stepwise computer integration was Eriksson and Welander
(1956). BACK
14. "Looking back, the papers published in the 1960s...
are astonishing to read... The biochemist G.E. Hutchinson was almost alone
when he wrote that methane, nitrous oxide, and other gases probably came
from bacterial sources." Lovelock (1999), p. 204; for an estimate that most of
the air's methane comes from bacteria in animal guts, see Hutchinson
(1954), pp. 392-93; carbon dioxide balance: Berner
et al. (1983). BACK
15. Conservation Foundation, 1963, p. 5; National Academy of Sciences
(1966), p. 11. BACK
16. Keeling (1960); tonnage:
e.g., Bolin and Keeling (1963).
BACK
17. Pearce (2002); Stebbing (1935), "seized the opportunity of man's stupidity,"
Arthur Hill, p. 523. BACK
18. Charney (1975); see
also Lamb (1977), pp. 14, 671.
BACK
19. E.g., human-caused albedo variations from desertification,
and to some extent tropical deforestation, were connected with past global
climate changes by Sagan et al. (1979); a pioneering
model confirming "the long-held idea that the surface vegetation... is
an important factor in the Earth's climate" was Shukla and Mintz (1982); Amazon Basin: Salati
and Vose (1984); more recently, see Kutzbach
et al. (1996). "It is very likely that sea surface temperature
change, natural vegetation [feedback] processes, and land use change have
acted synergistically to produce the unusual [Sahel] drought," concluded
Zeng (2003). In particular, warming of the Indian
Ocean influenced monsoon rains, Giannini et al.
(2003). Effects of haze (sulfate aerosols): Hegerl
et al. (2007), p. 715. BACK
20. For precipitation change one early suggestion
was Newell (1971), quote p. 459. Computer simulations "suggest
that the relatively cool climate in the second half of the 19th century
is largely attributable to cooling from deforestation" according
to Bauer et al., (2003).
BACK
21. Broecker et al. (1971),
p. 292-93. BACK
22. Keeling (1973), p.
320; similarly, "a surprisingly large fraction of the fossil-fuel CO2"
went into the biosphere in a model of Machta (1973),
p. 26. BACK
22a. Odum (1969); "Odum's paradigm:" Popkin (2015). McMahon et al. (2010). BACK
23. Keeling (1973), p.
279; Anderson and Malahoff (1977), see p. 22 for overview. BACK
24. Reiners (1973); Hutchinson (1954). BACK
25. Stumm (1977); Bolin's
new estimate was 10-35% from biota. Bolin (1977),
p. 615; his earlier view of plants in equilibrium or a net sink is explained
e.g. in Bolin (1970). BACK
26. Woodwell and Houghton
(1977); also Woodwell et al. (1978); Woodwell (1978). BACK
27. Woodwell (1978), p.
40. BACK
28. A 1977 workshop thrashed out the issues once
again without conclusion: Bolin et al. (1979),
see p. xxvii. BACK
29. The net fluxes "appear to have been negligible
over recent decades." Stuiver (1978), p. 258;
Broecker et al. (1979), quote p. 417; in writing this section
I have benefitted from Elliott (1977-89).
BACK
30. "threat:" Woodwell (1978),
p. 43; see Woodwell et al. (1983).
BACK
31. Broecker et al. (1979),
pp. 409, 417, "missing sink" p. 415. BACK
32. Broecker (1982a), crediting
G. Brass and N. Niitsuma for preliminary ideas; Broecker
(1982b); for a precursor, see Hutchinson (1954),
p. 384; the phrase "carbon pump" was defined and three types analyzed
in Volk and Hoffert (1985).
BACK
33. "holy grail": Sigman
and Boyle (2000), p. 859. BACK
34. Broecker (1982a); also
Broecker (1982b); other papers with discussions and a variety
of ideas include: Anderson and Malahoff (1977);
McElroy (1983); Siegenthaler and
Wenk (1984); Sarmiento and Toggweiler (1984);
Knox and McElroy (1984); for a later example, Boyle (1988a); a review: Sigman and
Boyle (2000). BACK
34a. Houghton
et al. (1983). BACK
35. Seiler and Crutzen (1980),
p. 1980, note the large bibliography. BACK
36. For a review, see Detweiler
and Hall (1988); Woodwell (1991), p. 246.
BACK
37. "Biomass burning has previously been considered
unimportant as a global source of atmospheric trace gases — our analysis
shows that this is not the case." Crutzen et al.
(1979). BACK
38. Ehhalt (1974). BACK
39. For references, see Mooney
et al. (1987). More than a tenth (with nitrous oxide about as important as methane), according to the Food and Agriculture Organization of the United Nations: Gerber et al. (2013). BACK 40. Zimmerman et al. (1982);
more recent estimates are summarized in IPCC (2001a),
p. 250. BACK
41. Schneider and Londer (1984),
p. 312n (citing a 1980 paper by Gordon MacDonald); for more recent figures,
which could be lower or higher depending on how wetlands are classified,
see IPCC (2001a), pp. 192, 194. BACK
42. Harriss et al. (1985);
for other references, see Mooney et al. (1987).
BACK
43. E.g., one pioneer paper suggested "marine biospheric
activity," Berner et al. (1980), p. 234-35.
BACK
44. Lovelock et al. (1972). Lovelock's suggestion that dimethyl sulfide governed cloud production over the oceans has turned out incorrect: Quinn and Bates (2011). BACK
45. "products:" Lovelock and
Margulis (1974), p. 9. BACK
46. Oxygen: Berkner and Marshall (1965). Hitchcock and Lovelock
(1967), "disequilibrium" p. 150, "blindness" p. 158.
BACK
47. "Conventional" Lovelock
(2000), p. 235, for Gaia, see ch. 9. BACK
48. Lovelock and Margulis
(1974), p. 5. The more usual spelling was Gaea.
BACK
49. Charlson et al. (1987),
p. 661, known as the "CLAW hypothesis" after the initials of the authors. The central idea was first proposed by Shaw
(1983). Later work is reviewed by Ayers and Cainey (2007). BACK
49a. Lovelock
(1991), p. 1 BACK
50. Fisher (1988); for
the 1974 diagram, Kellogg and Schneider (1974),
p. 1164. The diagram is also known as the "Bretherton Diagram" after Francis Bretherton, who chaired the committee that developed it. See Berrien Moore III Interview by Rebecca Wright, Earth System Science at 20 Oral History Project, NASA, 2011, Johnson Space Center, online here. BACK
51. The pioneering demonstration that the Amazon
Basin generated much of its own rainfall was Salati
and Vose (1984);more rain
(in dry season only): Negri (2004). BACK
51a. Bonan et al. (1992); Couzin (1999). BACK
52. Rowntree (1989),
p. 174; IPCC (2001a), pp. 440-43.
BACK
53. Especially Idso (1989).
BACK
53a. Less nutritious: Myers et al. (2014), Broberg et al. (2018). Early fertilization estimates: Houghton & Woodwell (1989), Jenkinson et al. (1991), Raich & Schlesinger (1992). Uptake ("primary production"): Campbell et al. (2017). Tundra emissions: Oechel et al. (1993); quote: Kirschbaum (1995), p. 753. BACK
53b. Pollen: Webb (1986); committed to extinction: Thomas et al. (2004). BACK
54. The importance of iron was demonstrated by Martin
and Fitzwater (1988); Martin (1990); Martin made his famous "half tanker" remark
at a Woods Hole 1988 conference, see US Joint Global Ocean Flux Study Newsletter 1(2), (US JGOFS Planning Office, Woods Hole Oceanographic Institution, 1990).
For the history see Stoll (2020). Examples of papers confirming that fertilization of the oceans by iron
could have played a role in ice ages: Moore et
al. (2000); Kohfeld et al. (2005); Abelmann
et al. (2006); Martínez-Garcia et al. (2011). BACK
55. The Cd pioneer was Edward Boyle, e.g., Boyle (1988b); Coate et al. (1996)
was a successful fertilization experiment; for an important 2004 experiment see Smetacek et al. (2012); "ink had dried...prize:" Broecker (2000). For more on this topic
see news reports in the journals Nature and Science,
e.g., Chisholm (2000). BACK
55a "Within a few centuries the
ocean pH may reach a level not seen for hundreds of millions of years,
and within the present century many organisms are likely to be affected”
is the authoritative conclusion of Denman et al.
(2007). A popularized summary is Kolbert (2006a).
BACK
56. Keeling et al. (1989)
(this is C.D.); Tans et al. (1990); oxygen work
by Ralph K.: Keeling and Shertz (1992); Keeling
et al. (1993); and see Broecker and Kunzig
(2008), pp. 85-87 BACK
57. Among the many publications: Battle et al. (2000),Bousquet
et al. (2000), Schimel et al. (2001).
BACK
58. Among the many publications: Prentice and Lloyd (1998), Schindler
(1999), Stephens et al. (2007), Baccini et al. (2017). "Long-sought:"
Stone (2008). Pan et al. (2011). BACK
59. Ruddiman and Thomson (2001), see Kerr (2004a); Ruddiman (2005, 2010); Ruddiman
(2006); evidence against the hypothesis was advanced, e.g., by Elsig et al. (2009); on acceptance see Stanley (2016), Ruddiman et al. (2016a), Ruddiman (2016b), Koch et al. (2019). BACK
60. Pielke (2005).
BACK 61. Coupled models (emphasizing soil emission):
Cox et al. (2000), confirmed by a study with multiple
models, Friedlingstein et al. (2001). Historical fertilization: Shevliakova et al. (2013). BACK
61a. Models: Cox et al. (2000), Zeng et al. (2004). Unlucky: e.g., Matthews and Keith (2007). BACK
61b. Dahan (2010); 2010 (Hadley) model: Heffernan (2010). BACK
62. Comparison of 11 models: Friedlingstein
et al. (2006) for the 2007 IPCC conclusions see Meehl
et al. (2007) pp. 789-93. Amazon: Oyama and
Nobre (2003); reviews: Malhi et al. (2008), Malhi et al. (2009); droughts: Phillips et al. (2009), Lewis et al. (2011). Attention fixated on the Amazon basin but the smaller Central African rainforests were also vulnerable, Réjou-Méchain et al. (2021).
"Fires, storms, insects, and disease... have been largely ignored,"
Houghton (2007), p. 338 (a review); another
review: Heimann and Reichstein (2008). "[R]apid forest collapse as a result of drought could convert the world's tropical forests from a net carbon sink into a large carbon source during this century:" Choat et al. (2012). BACK
63. Amazon studies: Brienen et al. (2015), Qin et al. (2021), Gatti et al. (2021), Covey et al. (2021); tipping point: Boulton et al. (2022), Armstrong McKay et al. (2022); precipitation survey: Smith et al. (2023). Fertilization: Reich and Hobbie (2012). Soil carbon storage: Terrer et al. (2021),Soong et al. (2021), Gabriel Popkin, "A Soil-Science Revolution Upends Plans to Fight Climate Change," Quanta (July 27, 2021), online here. Surveys: Wang et al. (2020), many studies: including Sullivan et al. (2020), Hubau et al. (2020), Duffy et al. (2021); Nottingham et al. (2020), Koch et al. (2021), "omen:" Rammig and Lapola (2021). BACK
63a. Net positive effect: Tagesson et al. (2020). Methane: Christensen et al.
(2004), surprising results of Keppler et al.
(2006), Walter et al. (2006), etc.; for
examples and discussion see Flannery (2006),
pp. 196-98. Substantial addition from tundra: Schuur et al. (2009); Dorrepaal et al. (2009); Schaefer et al. (2011). Microbes: Oh et al. (2020), Dyonisius et al. (2020); slumping: Rodenhizer et al. (2020), similarly for thermokarst lakes, Zandt et al. (2020). Plankton, some examples: emissions affecting cloudiness, Meskhidze and Nenes
(2006); climate affecting plankton blooms, Behrenfeld
et al. (2006); carbon sequestration, Schuster and Watson (2007), Le Quéré et al. (2007). BACK
64. Greening: Zhu et al. (2016), quantified by Keenan et al. (2021). Darker vegetation: Jeong et al. (2014); Arctic soils: Karhu et al. (2014); tropical forests: Anderegg et al. (2015); accelerate: Crowther et al. (2016), see also He et al. (2016); detected: Bond-Lamberty et al. (2018). A later study reported that a majority of microbe communities emitted CO2 on warming at a higher rate than models anticipated, Smith et al. (2019). For a summary report on surprises,
mostly unpleasant, in the short period 2005-2009 see Richardson
et al (2009). Other examples: a multi-year study of grass found carbon uptake sharply decreased
in hotter summers, Arnone et al. (2008); since leaves function more efficiently in
diffuse light than in dappled bright-or-dark direct light, clearer skies
will reduce carbon uptake, Mercado et al. (2009);
a controversial study reported detecting severe loss in abundance of ocean phytoplankton, Boyce et al. (2010); warming kills plankton, resulting in less emission of DMS and thus less cooling clouds, Six et al. (2013); changes in Arctic rivers and coastlines could bring more carbon loss than models anticipated, Abbott et al. (2016); a 7-year laboratory study found warmed wet tundra produces more methane than expected in relation to CO2, Knoblauch et al. (2018); carbon emissions from peatlands drained and converted to croplands had been overlooked, Qiu et al. (2021); on the other hand, warming and adaptation of ocean systems may bring more carbon uptake than models expect, Lomas et al. (2022) and models could show less warming if they included the daily vertical migration of ocean plankton, Dunne (2022). "Revolutionary advances:" Gary Griffith, "Coming to Recognize Marine Ecosystems as Complex Adaptive Systems," in Nature Climate Change (2021); "Ecosystem dynamics:" Doney (2006), p. 695.
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