Other Greenhouse Gases
While all eyes were turned on carbon dioxide, almost by chance a
few researchers discovered that other gases emitted by human activity
have a greenhouse effect strong enough to add to global warming. In the
mid 1970s, they began to realize that these gases could bring as much
damage as carbon dioxide itself. But the sources and interactions of the gases were complex and uncertain, and the research made little impact on policy. (This essay is supplementary to the core
essay on The Carbon Dioxide Greenhouse Effect For the most important
greenhouse gas, water vapor, see the essay on Simple
Models of Climate.)
Keywords: climate change, global warming, CO2, CH4, methane, ozone,
nitrates, CFCs
Methane (1859-1970s) |
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In 1859John Tyndall, intrigued by
the recently discovered ice ages, took to studying how gases may block
heat radiation and thus affect the global climate. Since the work of Joseph Fourier in the 1820s, scientists
had understood that the atmosphere might hold in the Earth's heat.
The conventional view nevertheless was that gases were entirely transparent.
Tyndall tried that out in his laboratory and confirmed it for the
main atmospheric gases, oxygen and nitrogen, as well as hydrogen.
He was ready to quit when he thought to try another gas that happened
to be right at hand in his laboratory: coal-gas. This was a fuel used
for lighting (and Bunsen burners), produced industrially by heating
coal. It consisted of carbon monoxide (CO) mixed with a bit of the
hydrocarbon methane (CH4) and more complex
gases. Tyndall found that for heat rays, the gas was as opaque as
a plank of wood. Thus the industrial revolution, intruding into Tyndall's
laboratory in the form of a gas-jet, declared its significance for
the planet's heat balance. |
Full discussion in
<=Simple models

John Tyndall
= Milestone |
Tyndall immediately went on to study other gases, finding that carbon dioxide gas
(CO2) and water vapor in particular also block
heat radiation. Tyndall figured that besides water and CO2,
"an almost inappreciable mixture of any of the stronger hydrocarbon
vapors" such as methane would affect the climate.(1) But there was far more water vapor circulating, and although
CO2 was only a few parts in ten thousand in the
Earth's atmosphere, that was still much more than other gases. There
is so little methane in the atmosphere that it was not detected there
until 1948.(2) In unraveling
the causes of the ice ages or any other climate change, there seemed
no reason to look further at methane and the like, and for a century
nobody paid the matter much attention. |
=>CO2 greenhouse
|
Largely out of simple curiosity about geochemical
cycles involving minor carbon and hydrogen compounds, in the 1960s
and 1970s scientists cataloged a variety of sources for methane in
the atmosphere. It turned out that emissions from biological sources
outranked mineral sources. Especially important were bacteria, producing
the methane ("swamp gas") that bubbles up in wetlands. That included
humanity's countless rice paddies.(3) |
<=External input
|
These studies, however, gave no reason to think that the gas had
any significance for climate change. Thus an authoritative 1971 study
of climate almost ignored methane. "To the best of our knowledge,"
the review concluded, "most atmospheric CH4 is
produced [and destroyed] by microbiological activity in soil and swamps."
The annual turnover that the experts estimated was so great that any
addition from human sources added only a minor fraction. "For this
reason, and because CH4 has no direct effects
on the climate or the biosphere, it is considered to be of no importance
for this report." The authors recommended monitoring the atmospheric
levels of the gases SO2, H2S,
NH3, and even oxygen, but not methane.(4) There the matter rested through the 1970s. |
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Ozone and CFCs (1970-1980)
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If methane drew little attention, still less went to
other trace chemicals in the atmosphere. They
were seen as curiosities scarcely worth a scientist's effort. Up to the 1970s, the atmosphere, as
one expert later recalled, "was viewed as inert chemically, and for good reason most of
the chemicals known to be present near the surface were essentially inert." The air seemed to be
just a simple, robust fluid "that transported pollution away from cities, factories, and fires."(5) A small amount of research
did get underway in the 1950s on how various atmospheric chemicals behaved, but only because their
interactions were responsible for urban smog. The public had begun to demand action on the
smelly and sometimes lethal pollution. Scientists
were especially puzzled by the rapidly thickening smog of Los Angeles, so different from
familiar coal-smoke hazes. It was a biochemist who finally recognized, by the smog's peculiar
odor, what was going on. When the bright Southern California sunshine irradiated automobile
exhaust it created a witch's brew of interacting compounds, starting with highly reactive
ozone.(6) The scientists who studied
ozone chemistry,
interested in ground-level pollution, gave no thought to possible connections with global
warming.
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<=Public opinion
<=External input
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The history of climate science is full of unexpected linkages, but perhaps
none so odd and tenuous as the events that drew public attention to
ozone in the upper atmosphere. It started with concerns over a fleet
of supersonic transport airplanes that governments envisioned. Beginning
in 1970, a few scientists drew attention to the nitrates (NO, NO2,
and in general NOx)
that the jet planes would emit in the stratosphere, along with sulfates
(SO2) and water vapor. They speculated that
the chemical aerosols could stimulate the formation of water droplets,
altering cloud cover with unknown effects on climate. Moreover, the
chemist Paul Crutzen showed that a single nitrate molecule, reacting
again and again in catalytic cycles, could destroy many molecules
of ozone.(7) That could be serious, for the wispy layer
of stratospheric ozone is all that blocks harmful ultraviolet rays
from reaching the Earth's surface. For the first time, a portion of
the atmosphere was shown to be chemically fragile, easily changed
by a modest addition of industrial emissions. The ozone problem combined
with other, weightier arguments to sink the plans for a supersonic
transport fleet. |
<=>Aerosols
=>Government
|
The new ideas provoked a few scientists to take a look at how the
upper atmosphere might be affected by another ambitious project
the hundreds of space shuttle flights that NASA hoped to launch. They
found that the chlorine that shuttles would discharge as they shot
through the stratosphere might be another menace to the ozone layer.
This concern, discussed at a meeting in Kyoto in 1973, helped inspire
Mario Molina and Sherwood Rowland look into other chemical emissions
from human activities. The result of their calculations seemed fantastic.
The minor industrial gases known as CFCs (chlorofluorocarbons) could
become a grave threat to the ozone layer. |
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Experts had thought
that the CFCs were environmentally sound. Industry produced the gases
in relatively small quantities. And they were very stable, never reacting
with animals and plants — which seemed like a point much in
their favor. James Lovelock had decided to track these gases in the
atmosphere precisely because they were stable markers of industrial
activity. His interest arose from meteorologists' concerns about the
haze that was marring summers in rural England was this actually
smog produced by industry? Measuring CFCs, which had no source but
human industry, seemed a good way to check this. First Lovelock needed
to measure the base-level of the gas, far out at sea. Not without
difficulty he managed to do this (his proposal for government funds
was rejected and he only semi-officially got a spot on a research
vessel). As expected, CFCs were everywhere. Not wishing to stir up
environmentalists, in 1973 Lovelock remarked that "The presence of
these compounds constitutes no conceivable hazard."(8) |
<=Aerosols
=>Biosphere
|
In fact, it was exactly the stability of CFCs that made them a hazard.
They would linger in the air for centuries. Eventually some drifted
up to a high level where, as Molina and Rowland explained, ultraviolet
rays would activate them. They would become catalysts in a process
that would destroy ozone, threatening an increase of skin cancer and
other dangers.(Back in 1961 veteran meteorologist Harry Wexler had recognized that chlorine atoms could act as catalysts to destroy ozone. A heart attack felled him before he could publish the information, and a decade passed before it was rediscovered. It is a striking demonstration of the meager state of research on atmospheric chemistry — like many other topics related to climate — in the 1960s.)(9*) |
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When scientists explained the threat to the ozone layer to the public, an agitated
controversy broke out over the use of CFCs in aerosol spray cans and
the like. The crude but worrisome calculations, and the vehement public
response, drove a major expansion of observational and theoretical
studies of the stratosphere's chemistry. |
= Milestone
<=>Public opinion |
If these peculiar gases could do so much to ozone, could they also
affect climate? Already in 1973, Lovelock remarked at a scientific
conference that CFCs might make a contribution to the greenhouse effect.(10)
He followed up by demonstrating that there were unexpectedly high
levels of the familiar industrial chemical carbon tetrachloride (CCl4)
in the atmosphere, and warned that it was important to unravel the
atmospheric chemistry of all chlorine-bearing carbon compounds.(11) |
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Lovelock's findings, combined with Rowland and Molina's warnings
that CFCs would linger in the atmosphere for centuries, provoked a
closer look into the question by NASA's Veerabhadran Ramanathan (known
to his colleagues as "Ram"). In 1975 he reported that CFCs absorb
infrared radiation prodigiously a single molecule could be
10,000 times as effective as a molecule of CO2.
A calculation suggested that CFCs, at the concentrations they would
reach by the year 2000 if the current industrial expansion continued,
all by themselves might raise global temperature by 1°C (roughly
2°F).(12*) The following year another group made
a more elaborate calculation with a simplified model of the atmosphere,
admittedly "primitive" but good enough to get a general idea of the
main effects. They reported that if there was a doubling in the atmosphere
of two other gases that had previously been little considered, N2O
(nitrous oxide) and methane, these would raise the temperature another
1°C.(13) Meanwhile Ramanathan's group calculated
that ozone too significantly trapped radiation. Keeping its level
in the stratosphere high would add to the greenhouse effect.(14) |
=>CO2 greenhouse
= Milestone
|
All these gases had been overlooked because
their quantities were minuscule compared with CO2.
But there was already so much CO2 in the air
that the spectral bands where it absorbed radiation were already quite
opaque, so you had to add a lot more of the gas to make a serious
difference (for more on this "saturation" see the essay
on Basic Radiation Calculations).
A few moments' thought would have told any scientist that it was otherwise
for trace gases. Each additional wisp of these would help obscure
a "window," a region of the spectrum that otherwise would have let
radiation through unhindered. But the simple is not always obvious
unless someone points it out. Understanding took a while to spread.
Well into the 1980s, the public, government agencies, and even most
scientists thought "global warming" was essentially synonymous with
"increasing CO2." Meanwhile, many thousands of
tons of other greenhouse gases were pouring into the atmosphere. |
=>Biosphere
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Other Gases as a Major Factor (the
1980s) TOP
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In 1980, Ramanathan published a surprising estimate of the
contribution to global warming from miscellaneous gases methane,
N2O, and ozone along with CFCs produced
by industry and also by agricultural sources such as fertilizer.
He found that these gases might contribute as much as 40% of the
total warming due to CO2 and all other gases
of human origin. He warned that his estimate was highly uncertain
and "may become outdated before it appears in print." Scientists
were just beginning to work out the complicated chemical interactions
among the trace gases and between each gas and sunlight. For example,
it had only recently been recognized how much ozone was generated
in the air from other smog chemicals. "The problem," Ramanathan
concluded, "because of its potential importance, should be examined
in more detail."(15) |
<=Radiation math

Veerabhadran "Ram"
Ramanathan wn.com
|
Several years passed without anybody taking up the challenge. It was
hard for scientists to conceive that gases whose presence in the atmosphere
was barely detectible could have a serious impact on climate. Eventually
Ramanathan did the job himself. In 1985 his team published a study
of some 30 trace gases that absorbed infrared radiation. These additional
"greenhouse gases," they estimated, added together could bring as
much global warming as CO2 itself. The announcement
shocked the community of climate scientists (for by now the different
specialties that dealt with climate followed one another’s work
closely enough to form a community).(16) Would the climate changes expected to result from a doubled
CO2 level, a level the world might reach a
century ahead, in fact come upon them twice as fast — perhaps
within their own lifetimes? The next year Robert Dickinson and Ralph
Cicerone addressed the question with a calculation based on the new
estimate of the effects of all greenhouse gases. They figured that
by the year 2050 global temperature could rise several degrees, "and
possibly by more than 5°C," if self-reinforcing feedbacks took
hold. The 22nd century would be even worse.(17) |
= Milestone
=>International
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Ramanathan and
others argued that the potential for global warming
gave reason to restrict production of CFCs. However, most of the scientific
and public concern was turning to a more immediate problem, the "ozone
hole." This seasonal dearth of protective ozone was discovered over
Antarctica in 1985. It seemed likely that CFCs were to blame. Within
two years that was demonstrated, when daring flights over Antarctica
confirmed new theories of how the chemicals could destroy ozone in
very cold air.(18*) The threat of increased skin cancer and other direct harm
to living creatures now seemed imminent, and gave reason enough to
further restrict production of CFCs.(19)
|
<=>Aerosols
<=>Public opinion |
Appeals from scientists and public activists led to a ground-breaking international
agreement, the 1987 Montreal Protocol. It had great success over the
following decade in reducing emissions of CFCs. The consequences for
climate, however, were ambiguous. Since CFCs exerted a considerable
greenhouse effect, the reduction certainly helped restrain global
warming. But some of the chemicals that industry substituted for CFCs
were themselves greenhouse gases. So was ozone, and as it was restored
in the stratosphere, it would add its bit to the warming. |
<=>International |
For other
emissions such as sulfates and nitrates, scientific and public attention again focused on
short-term local harms, the foul smog and acid rain. Some
researchers pointed out, however, that these chemicals could affect climate indirectly by forming
aerosols that would alter cloud cover. The
pollution studies were rapidly building a stock of scientific information about the complex
chemistry of the atmosphere, and it seemed increasingly relevant to climate researchers. So did
the unsettling news that a gas like ozone, which significantly influenced the planet's radiation
balance, could go through large swings. The groups who were constructing complex computer
models of climate began worrying how to incorporate atmospheric chemistry as yet another
factor in their systems. |
<=Public opinion
<=Aerosols
=>Models (GCMs) |
After Ramanathan identified methane as a significant greenhouse
gas, studies of its role in global
carbon cycles accelerated. During the 1980s, scientists came to see that although the methane in
the air comes largely from plants and animals, that did not mean human effects were negligible.
For humanity was transforming the entire global biosphere. Specialists in obscure fields of
research turned up a variety of biological methane sources that were rapidly increasing. The gas
was abundantly emitted by bacteria found in the mud of rice paddies and burped up from the guts of
cud-chewing cows, among other places. Especially intriguing was methane from the guts of termites: an early experiment on one species of termite suggested they might be extremely important. (Later work with other species lowered their significance; it turned out that wetlands were the largest natural source of methane, with termites a distant second producing only one-fifth to one-tenth as much.).(19a) And what about accelerated emissions from the soil
bacteria as well as termites that proliferated following deforestation and the advance of agriculture? Moreover,
natural biological activity could be altered by the rise of
CO2 levels and by global warming itself, making for complicated
and
enigmatic feedbacks.
|
<=Biosphere
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The importance of all this was driven home by a tentative 1981
report that methane in the atmosphere was increasing at an astounding
rate, perhaps 2% a year. The following year, a study of air bubbles
trapped in ice drilled from the Greenland icecap confirmed that methane
was climbing. The climb, radically different from any change that
could be detected in past millennia, had started in the 16th century
and accelerated wildly in recent decades.(20) By 1988, painstaking collection of air samples at many
remote locations gave an accurate measure of the recent rise. The
actual rate of increase was about 1% a year, bringing a shocking 11%
increase of methane in the past decade alone. (Later studies found
the rate of rise varying greatly from year to year.) Since there was still not much methane in the atmosphere, each additional molecule of methane
would have a greenhouse effect more than twenty times that of a molecule
of CO2. In addition, some of the methane was
converted into ozone and water vapor in the stratosphere, where they
would exert their own greenhouse effects. (Taking these and other atmospheric interactions into account, it was later calculated that additional methane would be some thirty times more effective per molecule in producing global warming than additional CO2.) It seemed likely that the
rising methane level was already having a measurable impact.(21*) |
|
This raised alarming new possibilities for potentially catastrophic feedbacks.
Particularly ominous ominous were the enormous quantities of carbon
atoms locked in the strange "clathrates" (methane hydrates)
found in the muck of seabeds around the world. Clathrates are ice-like
substances with methane imprisoned within their structure, kept solid
only by the pressure and cold of the overlying water. A lump of the
stuff brought to the surface will fizz and disintegrate, and meanwhile
a match can set it aflame. When it became apparent how widespread
the clathrates are, they attracted close study as a potentially lucrative
source of energy. In the early 1980s, a few scientists pointed out
that if a slight warming penetrated the sediments, clathrates might
melt and release colossal bursts of methane and CO2
into the atmosphere. That would bring still more warming.(22*) |
<=External input
<=>Biosphere
PHOTO of clathrate
=>Rapid change
=>Government |
The importance of methane became clearer as more cores
were drilled from the ice of Greenland and Antarctica, revealing changes
in the levels of gases in the atmosphere back through previous glacial
periods. Measurements published in 1988 showed that over hundreds
of thousands of years, methane had risen and fallen in step with temperature.
The level had been a factor of two higher in warm periods than in
glacial periods. Perhaps this was due to variations in how much gas
was generated by bacteria in wetlands? Or by abrupt releases from
undersea clathrates? For whatever reason, there was evidently some
kind of feedback between temperature and the level of methane in the
atmosphere, a feedback that might gravely accelerate any global warming.(23) |
=>Climate cycles
= Milestone

Methane in ice cores
CLICK FOR FULL IMAGE
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In 1988 Ramanathan remarked dryly, "the greenhouse theory of climate change has reached the crucial
stage of verification." If the predictions were valid, he said, the rise in trace gases together with
CO2 would bring a warming unprecedented in human history. He
expected it would become apparent before the year 2010.(24)
| |
Struggling toward Policies
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Attention to gases other than CO2 continued
to grow. Ozone holes in the stratosphere over the poles each winter
drove home the idea that even small concentrations of some industrial
emissions could have powerful effects. Out of public view, experts
delved into the chemical interactions among ozone, nitrates, water
vapor, and so forth in every level of the atmosphere from the
ground up. Ingenious and difficult computer modeling showed that
the concentration of one type of chemical altered the concentration
of others, so that the indirect action of a gas could be even greater
than its direct greenhouse effect. For example, carbon monoxide
(CO) does not intercept much heat radiation by itself, but the massive
amounts of the gas that humanity was emitting did alter the levels
of methane and ozone. The community of climate scientists could
reach no consensus on how serious these complex indirect effects
were, and from this point on. the question drove extensive research.(25)
|
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Methane got special attention, for it offered some of the most peculiar
and unsettling possibilities, such as increased emission from wetlands
as the climate warmed. An especially huge reservoir of carbon is locked
up in organic compounds in the permanently frozen peat (permafrost),
often many meters deep, that underlies Arctic tundras. Around 1990,
scientists began to wonder what would happen if a warming climate
turned more of the upper layers to marsh. Would biological activity
explode in the endless expanses of sodden tundra, with microbes emitting
enough methane to accelerate global warming? One of the scientists,
Richard Harriss, argued that monitoring methane emissions from tundra
could give an early warning of enormous changes.(26) |
|
Measurements
were scanty. But in one especially well-studied Swedish bog, researchers
reported an increase in methane emissions from 1970 to 2000 of at
least 20 percent, and perhaps much more. By 2006 the thawing of large
areas of permafrost was visibly underway in many Arctic regions, presumably
emitting ever more methane (and equally significant amounts of carbon
dioxide). There was good reason to expect that much more would thaw
by the end of the century. Meanwhile, a 2005 study of the complex
chemical interactions in the atmosphere calculated that adding methane
was even more powerful in bringing greenhouse warming than previous
studies had estimated. It also seemed increasingly likely that clathrates
in the warming seabed would release massive amounts of the gas, although
(good news for once) that would probably take thousands of years.(27) Any of these processes
might leave the planet stuck more or less permanently with a climate
unlike any that had been seen for many millions of years. |
<=Rapid change
=>International
<=>Rapid change
|
Back in 1986, Dickinson and Cicerone had carefully
separated the temperature changes that gases might ultimately cause
from their immediate and direct physical influence on radiation. They
called these direct influences "thermal trappings" what later
came to be called "radiative forcings."(28)
Unlike the ultimate global temperature with its complex feedbacks,
the physical forcings could be calculated in a straightforward and
reliable way. That made it easier to compare the consequences of changes
in the different agents not only different gases but also aerosols,
cloud cover, changes in land vegetation, the Sun's radiation itself,
and so on. This subtle but important shift in approach increasingly
took hold over the following decade. |
=>Aerosols
|
In 1990, a report by an international panel of scientists put the idea
in a revised form more useful for policy decisions: the "Global
Warming Potential." This included not only the effects of a gas,
but also how long it would stay in the atmosphere. That pushed into
the very center of policymaking the fact that additions of some long-lingering
trace gases had a potential for warming, molecule for molecule, hundreds or thousands
of times stronger than additional CO2.(29)
In particular, although the current greenhouse effect from N2O
was not very large, studies found that the gas would remain in the
atmosphere for a century or more, with some 300 times the global warming potential per molecule compared with CO2. And the level was soaring, thanks
to emissions from fertilizers and cow manure. Climate scientists
had never given this gas as much attention as they gave to methane,
with its fascinating biological feedbacks. But by the early 21st
century, N2O had become nearly as important
a greenhouse gas as methane. |
<=International |
By 2009 many scientists believed the effects of nitrates had been
seriously underestimated. Indeed, replacing fossil fuels with "biofuel"
manufactured from corn might increase global warming, thanks
to the emissions from soil bacteria stimulated by the fertilizers
used to grow the corn. The more scientists studied the emissions of
this and other nitrogen compounds such as nitrates, the more confused
they got. Not only was it hard to measure how much was emitted, but
the compounds reacted in complicated ways with smog chemicals, ozone,
methane and CO2. Meanwhile nitrogen compounds
fertilized plants and ocean plankton. Some of the interactions resulted
in more greenhouse warming, while others removed greenhouse gases
and would have a net cooling effect. |
|
Computer models offered only limited
help. A survey found many differences among models in how they handled
the chemical interactions among trace gases and between the gases
and aerosols. Some models were afflicted with elementary errors of
chemistry or computer coding. The best that could be said was that
about half the models agreed reasonably well with observations and
with one another, so that "some confidence can be placed in their
predictions." The uncertainties made it hard to come up with
defensible policies.(30) |
=>Aerosols
|
Experts now agreed that
sound policy should take into account all the potential causes of
warming. To take one surprising example, leaks of methane from gas
pipelines turned out to add significantly to global warming. Meanwhile
the headlong rise of methane in the atmosphere seen in the 1970s and
1980s had slowed to a more sedate pace. The reasons were unclear (perhaps
the collapse of the Soviet Union's economy, or greater efficiencies
in production and distribution of the gas, or the draining of wetlands?).
After 2000 the methane level did not rise at all... until 2008, when
it again began to climb ominously. It appeared that the world's changing climate, growing warmer and wetter, was stimulating increased emissions from tropical and arctic wetlands.(31)
|
<=>Aerosols |
That drove home
the uncertainty of any prediction of future methane levels. Aggressive
steps to further cut back inefficient releases of such gases might
be the most cost-effective way to begin reducing the risk of harm
from global warming. Another example: restraining the rapid increase
of "black carbon" smoke and soot, an aerosol pollutant that
interacted with chemical gases, would bring immediate savings for
human health as well as reducing climate change. Yet another example:
the gases known as HFCs (hydrofluorocarbons), developed to replace
ozone-destroying CFCs, would add significantly to greenhouse warming
unless they too were restricted by an extension of the Montreal Protocol.
|
<=>Government |
Nevertheless, CO2 continued to hog the spotlight.
Other gases (and aerosols) were often overlooked in public arguments,
and even in much of the expert policy discussion. As one policy expert
sighed, in negotiations "CO2 sucks all
the oxygen out of the room."(32) |
|
|
RELATED:
Home
The Carbon Dioxide Greenhouse Effect
Biosphere: How Life Alters Climate.)
1. Tyndall (1863); Tyndall (1861); Tyndall (1873),
quote p. 40. Tyndall measured what he called "carbonic acid" gas, a common term for what is now called carbon dioxide.
BACK
2. Migeotte (1948).
BACK
3. A pioneer especially for rice paddies was Koyama (1963); wetlands: Ehhalt
(1974).
BACK
4. Wilson and Matthews (1971),
p. 242.
BACK
5. Cicerone (1999), p. 19, see
also H. Schiff's comments, p. 115.
BACK
6. Brimblecombe (1995).
BACK
7. Crutzen (1970) calculated that
even small amounts of nitrates could be important as catalysts; this was independently and
explicitly linked to supersonic transports and ozone damage by Johnston (1971).
BACK
8. Lovelock et al. (1973); wryly
quoted by Lovelock himself, Lovelock (1974), p. 293; on
motives and funding Lovelock (2000), ch. 8.
BACK
9. At this point the compounds were called, more precisely,
chlorofluoromethanes. Molina and Rowland (1974)
(submitted in June); that "the oxides of chlorine... may constitute an
important sink for stratospheric ozone" was independently worked out in
Stolarski and Cicerone (1974) (submitted in
January) but the consequences were not grasped the first journal
to which the paper was submitted rejected it when a reviewer declared
the idea was "of no conceivable geophysical consequence"; Cicerone
(2003); see also Cicerone et al. (1974) (submitted in September); for discussion,
Gribbin (1988). Wexler: Fleming (2010), pp. 219-21. BACK
10. Gribbin (1988).
BACK
11. Lovelock (1974).
BACK
12. Ice-albedo feedback, he added, could give considerably
greater warming in arctic regions. Ramanathan (1975).
BACK
13. Their best guess was 0.7°C for N2O,
0.3°C for methane, and 0.1°C for ammonia. Wang
et al. (1976). BACK
14. Ramanathan et al. (1976).
BACK
15. Ramanathan (1980), quote
p. 269.
BACK
16. Ramanathan et al. (1985);
for a comment see Bolin (2007), p. 37.
BACK
17. Dickinson and Cicerone
(1986), quote p. 109.
BACK
18. Farman et al. (1985);
Susan Solomon and, independently, Michael McElroy and Steven Wofsky explained
that the unexpected factor destroying ozone was catalysis on the surface
of ice crystals in high clouds. For history and scientific references,
see Roan (1989); Christie (2000), and reporting by Richard Kerr in Science
magazine from 1987. BACK
19. Roan (1989), see pp. 92,
195.
BACK
19a. Fraser et al. (1986). BACK
20. Rasmussen and Khalil
(1981); see Rasmussen and Khalil (1981); Craig and Chou (1982).
BACK
21. Blake and Rowland (1988).
The levelling off in the 1990s was probably due to the collapse of the
Soviet economy, while droughts reduced natural wetland emissions and temporarily
held back further rise in the early 2000s, according to Bousquet
(2006). BACK
22. To be precise, the sediments would release methane,
some of which would convert to CO2. "A potential
does exist for significant positive feedback" from Arctic Ocean clathrates,
warned Bell (1982), who was stimulated by a
1980 paper presented by Gordon J. MacDonald, see MacDonald
(1980). BACK
23. For the last glacial period, Stauffer et al. (1988); Raynaud et
al. (1988); for a 160,000 year record Chappellaz
et al. (1990); Nisbet (1990a).
BACK
24. Ramanathan (1988),
quote p. 293. BACK
25. Isaksen and
Hov (1987); the greenhouse effect of carbon monoxide was therefore
perhaps greater than that of N2O. Derwent
(1990) is cited as a pioneer by Le Treut et
al. (2007), see pp. 108-9. For a summary, see IPCC
(2001), p. 256 and passim. BACK
26. Kvenvolden
(1988); Harriss et al. (1992); Harriss
(1993). BACK
27. Swedish bog: Christensen
et al. (2004). See also Walter et al. (2006).
Methane: Shindell et al. (2005), Keppler
et al. (2006). Clathrates: Archer and Buffet
(2005). BACK
28. Dickinson and
Cicerone (1986). BACK
29. IPCC (1990).
BACK
30. Rodhe
(1990) calculated that the contribution of an N2O
molecule to global warming is 300 times that of a CO2
molecule. Underestimate, biofuels: Crutzen
et al. (2008). Models survey: Eyring et al.
(2006), see also Doherty (2009). .BACK
31. Dlugokencky et al. (2009); Bloom et al. (2010). BACK
32. HFCs: Velders
et al. (2009). David Doniger, quoted by Andrew C. Revkin, "Ozone
Solution Poses a Growing Climate Threat," June 22, 2009, online here.
BACK
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© 2003-2011 Spencer Weart & American Institute of Physics
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