July
2004 HOME
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printer 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. (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.)
Methane (1859-1970s) |
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In 1859 John Tyndall, intrigued by the recently discovered ice ages,
took to studying how gases may block heat radiation. 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 the hydrocarbon
methane (CH4) mixed with more complex gases.
Tyndall found that for heat rays, this gas is 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. |
<=Simple models
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Tyndall immediately went on to study other chemicals, 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" would change 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
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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
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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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OF PAGE |
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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) In the 1950s, a small amount of research
did get underway 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 began 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, 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
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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
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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.(9*) When scientists explained that 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
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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
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
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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 mostly opaque
already, so you had to add a lot more of the gas to make a serious
difference. 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.
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=>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)
Taking up his own challenge, in 1985 Ramanathan and collaborators
looked at some 30 trace gases that absorbed infrared radiation.
These additional "greenhouse gases," the team estimated, put together
could bring as much global warming as CO2 itself.(16) The next year Robert Dickinson and Ralph Cicerone published
a calculation of the possible consequences of this additional influence.
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)
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<=Radiation math
![](images/ram.gif)
V. "Ram" Ramanathan, 1997
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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, as risky 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)
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<=>Public opinion
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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 in the guts of
cud-chewing cows, among other places. And what about accelerated emissions from the soil
bacteria 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.
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<=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 was about 1% a year, bringing an 11% increase of methane
in the past decade alone. (Later studies found the rate varying greatly
from year to year. During the 1990s the rate decreased, for unknown
reasons, while remaining uncomfortably high). Each molecule of methane
had 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. It seemed likely that the
rising methane level was already having a measurable impact.(21) |
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This raised alarming new possibilities for potentially catastrophic
feedbacks. A huge reservoir of carbon is locked up in organic compounds
in the deep permafrost layers of peat that underlie northern tundras.
As Arctic regions warmed up, would biological activity in the endless
expanses of sodden tundra emit enough methane to accelerate global
warming? |
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Equally 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
=>Rapid change
=>Government
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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
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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)
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After 1988
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=>after88 |
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.
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 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. Methane got special attention since it offered some of
the most peculiar and unsettling possibilities, such as increased
emission from wetlands. [Measurements were scanty, but in 2004 for
one well-studied Swedish bog, researchers reported an increase in
methane emissions from 1970 to 2000 of at least 20 percent, and perhaps
much more.](25) Among other threats, it seemed increasingly
possible that at some future time (probably, but not certainly, remote),
clathrates on the seabed could release the gas in outbursts that might
redouble global warming. |
<=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."(26)
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, the Sun's radiation itself, and so on. This subtle but
important shift in approach increasingly took hold over the following
decade. |
=>Aerosols
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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 some long-lingering gases had
a potential for warming, molecule for molecule, thousands of times
stronger than CO2.(27)
For example, 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. And the level was soaring, thanks
to emissions from fertilizers and cow manure. |
<=International |
Experts now agreed that sound policy should take into
accoun all the warming potentialst. To take one surprising example,
leaks of methane from gas pipelines turned out to add significantly
to global warming. Aggressive steps to cut back inefficient releases
of certain gases might be the most cost-effective way to begin reducing
the risk of harm from global warming. Restraining the rapid growth
of smog-producing ozone in the lower atmosphere, to take another example,
would bring great savings for human health and even agriculture, entirely
aside from helping the climate. 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. |
<=>Aerosols
<=>Government
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RELATED:
Home
The Carbon Dioxide Greenhouse Effect
Biosphere: How Life Alters Climate.)
1. Tyndall (1863); Tyndall (1861); Tyndall (1873),
quote p. 40.
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). 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).
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
20. Rasmussen and Khalil
(1981); see Rasmussen and Khalil (1981); Craig and Chou (1982).
BACK
21. Blake and Rowland (1988).
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 (1990).
BACK
24. Ramanathan (1988), quote
p. 293.
BACK
25. Isaksen and Hov (1987);
the greenhouse effect of CO was therefore perhaps greater than that of
N2O. For a summary, see IPCC (2001), p. 256
and passim. Swedish bog: Christensen et al. (2004).
BACK
26. Dickinson and Cicerone
(1986).
BACK
27. IPCC (1990).
BACK
copyright
© 2003-2004 Spencer Weart & American Institute of Physics
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