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OCR for page 63
6
Global Warming
As the weeks of 198S's summer drought and stifling heat
dragged by, a momentous shift took place in the public's attitude
toward the global environment. Suddenly, it seemed, everyone
knew anct cared about a scientific principle long of deep concern
to scientists studying the earth system. This principle, known as
the greenhouse effect, explains why gases produced by human
activity will probably cause the earth's average temperature to
increase within the lifetimes of most people living today.
The 1980s were the warmest decade recorded on a global
basis, but scientists are still uncertain, and will be for years,
whether the warm spell was a normal climatic fluctuation or
a response to the billions of tons of carbon injected into the
atmosphere each year by human activities. Scientists working
in climatology and related fields say that the insulating effects
of the greenhouse gases should be clear to all of us within a few
decades, and possibly by the end of the 1990s.
One cannot infer from a specific summer that global warm-
ing has begun, though a warmer climate would change the
probabilities for heat waves and possibly for strong hurricanes.
The weather events of 1988 did, however, convey an idea of the
63
OCR for page 64
64
THE FACES OF GLOBAL ENVIRONMENTAL CHANGE
kinds of environmental and commercial effects we could expect
if the current predictions about global warming come to pass.
The North American corn crop was stunted by drought in the
grain belt, and productivity fell below consumption (probably
for the first time in U.S. history), so that no grain was added to
the nation's reserves. Water levels in thoroughfares like the Mis-
sissippi River dropped so low that barges and their cargoes were
stranded for weeks. Forest fires burned uncontrollably in Amer-
ica's great natural parks, a superhurricane threatened America's
Gulf Coast, and floods in Bangladesh led to the deaths of 2000
people and drove millions of others from their homes. These
and other extreme weather events over the course of a single
summer highlighted for billions of people that human society is
highly vulnerable to extremes in the weather.
GREENHOUSE GASES
Although there may be questions about the causes of a
specific drought or flood, there is no controversy about some
basic facts about our atmosphere. Trace gases such as water va-
por, carbon dioxide, methane, chlorofluorocarbons, tropospheric
ozone, and nitrous oxide create a greenhouse effect by trapping
heat near the earth's surface, and the concentrations of many of
these gases are increasing in the atmosphere. Because of these
increases, the gases are expected to trap more energy at the
earth's surface and in the lower atmosphere, in turn causing
increases in temperature, changes in precipitation patterns, and
other as yet unpredictable changes in the global climate.
The principle of the greenhouse effect explains the cold cli-
mate of Mars (where water vapor, a highly efficient greenhouse
gas, is virtually absent), the hot climate of Venus (where the
atmosphere is thick with carbon dioxide and conditions are so
hot that life as we know it could not survive), and the mod-
erate climate here on earth. Scientists have known for decades
that a buildup of carbon dioxide in the atmosphere could warm
the earth's climate. They have also known that atmospheric con
OCR for page 65
GLOBAL WARMING
65
centrations of carbon dioxide alone have increased by about
25 percent since coal, oil, and gas became the primary sources
of energy to fuel the industrial Revolution. Carbon dioxide
concentrations are currently increasing by about 0.4 percent each
year.
After water vapor, carbon dioxide is the most plentiful and
effective greenhouse gas. it occurs naturally but is also produced
in great quantity during the combustion of fossil fuels, partic-
ularly coal. When the fuel is burned, its carbon is oxidized to
carbon dioxide and released. Carbon dioxide also is released as
forests are cleared and the organic matter is burned or allowed
to decay. These human activities are injecting almost 6 billion
tons of carbon into the atmosphere each year. By comparing
this figure with the actual increases in concentrations of carbon
dioxide (about 3 billion tons annually), scientists presume that
about half of the carbon injected into the atmosphere is being
absorbed by oceans anct plant life ant! about half remains in the
atmosphere.
Only in the last decade have scientists become aware that
other, trace greenhouse gases can also be important contributors
to global warming. Concentrations of many of these trace gases
are known to vary naturally, but there is widespread agreement
that human activities are contributing to the current increases.
Molecule for molecule, the following trace gases absorb
infrared radiation much more effectively than carbon dioxide
does. Because their concentrations in the atmosphere are much
Tower than that of carbon dioxide, their indiviclual effect is much
smaller. Their combined effect, however, is likely to be equal to
or greater than that of carbon dioxicle alone.
Methane (CHIN. Methane, also known as natural gas, is pro-
duced through bacterial activity in bogs and rice pacidies, and
in the digestive tracts of ruminative animals and insects such
as termites. Most atmospheric methane comes from biological
sources. It is present today at roughly 1.7 parts per million
and is increasing at a rate of about I.! percent each year. Analy
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66
THE FACES OF GLOBAL ENVIRONMENTAL CHANGE
sis of gas bubbles trapped in glacial ice shows that the rise in
methane levels parallels the growth of the human population.
Per molecule it is 25 times as effective as carbon dioxide at
trapping heat.
ChZorofluorocarbons (CFCs). ChIorofluorocarbons are a group
of synthetic compounds used in refrigeration, insulation, foams,
and other industrial purposes. Apart from their role as green-
house gases, when CFCs rise to the upper atmosphere, or
stratosphere, they release free chlorine, which then catalyzes
the breakdown of ozone, the protective layer that shields the
earth from ultraviolet radiation. The two most prevalent CFCs
are CFC-12, which per molecule has 20,000 times the capacity
of carbon dioxicle to trap heat, and CFC-~l, which has 17,500
times the capacity. Both of these compounds are Tong-lived and
are increasing in the atmosphere at a rate of about 5 percent
per year. The Montreal Protocol, an international agreement
adopted in 1987 to limit the production of CFCs, will slow but
not eliminate the rate of increase.
Nitrous oxide (N2O). Nitrous oxide is produced naturally,
through microbial action in the soil, and in response to the
spread of agriculture, the burning of timber' the decay of crop
residues, and the combustion of fossil fuels. Agricultural use of
mineral fertilizers containing nitrogen presumably accelerates
its rate of release. Atmospheric concentrations of nitrous oxide
are increasing by about 0.25 percent per year. it has a long
residence time in the atmosphere, and so concentrations would
increase for more than 200 years even if emission rates were to
freeze at current levels. Scientists believe nitrous oxide levels in
the year 2030 will be about 34 percent more than preindustrial
levels. Per molecule, this trace gas has 250 times the capacity of
carbon dioxide to trap heat.
Tropospheric ozone (03~. in the stratosphere, ozone shields the
planet from ultraviolet radiation; nearer the ground in the tro
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GLOBAL WARMING
67
posphere, the moisture-rich atmospheric layer below the strato-
sphere, it is an effective greenhouse gas. It is produced through
reactions involving hydrocarbons and nitrogen oxides, all re-
leased through the combustion of fossil fuels used by motor
vehicles and in industry. Concentrations of tropospheric ozone
appear to be increasing at many locations in the Northern Hemi-
sphere. Results from studies of the Amazon River basin indicate
that tropical forests act as a sink for ozone; if so, their continued
destruction could have a significant effect on regional ozone
balances.
Although scientists have considerable confidence in the
principle of the greenhouse effect and the measurements of
increasing greenhouse gases in the atmosphere, two key ques-
tions remain surrounded by uncertainties: How quickly will the
climate change, and by how much?
THE CLIMATE'S RESPONSE TO
GREENHOUSE GASES
Using three-dimensional mathematical models of the cli-
mate system, scientists draw a number of inferences about what
conditions might be like in the future. The most likely con-
ditions Include significant cooling of the stratosphere, warmer
surface temperature (which would be felt disproportionately at
high latitudes), and changes such as rising sea level, reductions
in sea ice, and increases in total global precipitation (which
again would be nonuniformly distributed around the gIobe).
They also speculate that summers in the mid-continents would
be much dryer than they are today.
These responses to increased greenhouse gas concentration,
as well as their scientific uncertainties, were described by ferry
Mahlman, director of the Geophysical Fluid Dynamics Labo-
ratory of the National Oceanic and Atmospheric Adm~n~stra-
tion (NOAA) In Princeton, New Jersey, at the 1989 Forum on
Global Change and Our Common Future. MahIman outlined
a list of responses, an earlier version of which appeared in the
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68
THE FACES OF GLOBAL ENVIRONMENTAL CHANGE
1987 National Research Council report, Current Issues in Atmo-
spheric Change (see box). The estimates shown reflect assump-
tions about future concentrations of trace gases based on current
trends.
The results of literally millions of measurements and cal-
culations over the past century indicate that the earth is likely
to experience a significant climate change during the next few
decades. The models predict that because of carbon dioxide and
other gases that have built up in the atmosphere since 1860, the
earth is probably already committed to a 0.5° to 1.5°C increase
in average global temperature. If current emissions trends con-
tinue, the combined greenhouse effect of all trace gases would
commit us to an "effective carbon dioxide doubling"-the point
where carbon dioxide and other trace greenhouse gases com-
bined trap the same amount of energy that carbon dioxide would
trap alone if its concentration doubled from the preindustrial
level possibly as early as 2030.
Although the climate models are intricate and require mas-
sive amounts of computer time, they are stark, simplistic repre-
sentations of the complex realities of the real climate system. It
is difficult, for instance, to include cloud cover in the models,
even though clouds may amplify or moderate the greenhouse
effect. Most of the models do not adequately include the dy-
namics of ocean circulation, an essential determinant of carbon
dioxide concentrations in the atmosphere. Nor can the models
incorporate the entire range of uncertainties about potential re-
sponses of the earth system the possibility, for instance, that
an increase in temperature could alter cloud cover or increase
the rate at which soil bacteria break down dead organic matter
and consequently accelerate the biological contribution of car-
bon dioxide to the atmosphere, or the possibility that climate
change could trigger a dramatic shift in ocean circulation that
would completely alter temperature and precipitation patterns.
In spite of these uncertainties imposed by both the practical
computational limits of the models and the incomplete under-
standing of the earth system, scientists cautiously predict how
much global average temperatures would rise with an effective
OCR for page 69
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70 THE FACES OF GLOBAL ENVIRONMENTAL CHANGE
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GLOBAL WARMING
71
carbon dioxide doubling. Many assessments indicate a range of
estimates between 1° and 5°C.
To appreciate what the projected warming really means,
consider the numbers involved. When scientists say that on av-
erage the global temperature could increase by a few degrees
centigrade, they are talking about a very large increase and a
tremendous amount of heat. The current average global temper-
ature is about 14°C (57°F). A 3°C rise would create conditions
that some organisms have not had to contend with in the last
100,000 years. If the temperature rises 4°C, the earth would be
warmer than at any time since the Eocene period, 40 million
years ago. in the midst of the last glaciation, when much of
North America was covered by ice, the average temperature of
the earth was only about 5°C colder than it is now. Thus, what
seems to be a very small average temperature change can have
a very dramatic effect. Moreover, the projected rate of warming
is 15 to 40 times faster than the natural warmings after the major
ice ages and much faster than what most species living on the
earth today have had to face.
The climate mode! results shown in the box are based mainly
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72
THE FACES OF GLOBAL ENVIRONMENTAL CHANGE
on hypothetical and mostly instantaneous and large changes
In concentrations of greenhouse gases. In fact, concentrations
of the gases are increasing gradually. Initially, much of the
excess heat is absorbed into the oceans, but understanding of
the complex interactions between the atmosphere and ocean is
incomplete. We can expect that natural, decadal-scale climatic
fluctuations due to interactions between the atmosphere and
oceans will continue to occur. Mahlman points out that the
midwestern drought in the 1930s and the high water levels of
the Great Lakes in the 1980s are good examples of the results
of such fluctuations. Until such fluctuations can be understood
and predicted, it will be difficult to discern the specific signals
of more long-lasting climate change as they evolve. Detecting
the signals of clunate change becomes even more difficult when
smaller regions and/or shorter periods of time are considered.
The enormous consequences of the various effects of global
warming and the rising clamor for clarification continue to spur
the scientific community to refine their mathematical models.
Despite scientific uncertainties, these computer models are the
only tools available to researchers as they struggle to estimate
to what extent economic and social actions to reduce future
emissions of greenhouse gases can limit the predicted changes
in climate. Stephen H. Schneider, a climatologist at the National
Center for Atmospheric Research in Boulder, Colorado, and
Norman Rosenberg, director of the Climate Resources Program
for Resources for the Future, note that another decade or so of
observations will enable scientists to assess how well present
estimates predicted the sensitivity of climate to increasing trace
gases. But, they add, "While scientists debate, the real climate
system continues to perform the experiment for us."
All of the predictions about climate change are based on
only five models (although there are many attempts to model
portions of the earth system on more limited scales of time
and space). The five models are the NASA/Goddard Institute
for Space Studies (GISS) model, the National Center for Atmo-
spheric Research (NCAR) model, the NOAA Geophysical Fluid
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GLOBAL WARMING
73
Dynamics Laboratory (GFDL) model, the mode! developed at
the Oregon State University (OSU), and the mode! developed
by the United Kingdom Meteorological Office (UKMO).
These are general circulation models (GCMs) that predict
the ways in which temperature, humidity, wind speed and di-
rection, soil moisture, sea ice, and other climate variables evolve
through three dimensions and over time. They use mathematical
equations to express the basic physical, chemical, and biological
processes that govern the global climate system.
The general circulation models agree that change is in the
works and that weather systems worldwide are sensitive to
increases in greenhouse gases. Their calculations reveal that
disruption is all but inevitable and that a wide range of con-
sequences is possible. There are differences between them,
however. Some versions of the GISS and GFDL models now
include scenarios of gradual addition of greenhouse gases into
the atmosphere, whereas the others assume a massive, one-time
doubling of the gases. The GFDL and OSU models attempt to
include ocean processes. The GFDL mode! indicates that some
remarkable effects can occur when an active circulation is in-
cluded. For example, the presence of upwelling circulation in
the circum-AtIantic Ocean acts to delay surface warming there
for extended periods, perhaps centuries.
The models do not necessarily agree on specifics. All project
that average precipitation over the globe will increase signifi-
cantly but differ on what the regional effects would be. The cli-
mate system is so complex and so vast that it is a mincl-boggling
proposition to decipher the interactions and balances among its
myriad components. So far, even the most sophisticated cou-
pled atmosphere-ocean mode! omits important features such as
the biological interactions. Also, current computer power is in-
sufficient to resolve many climatically significant phenomena.
Most modelers believe reliable predictions from this crucial too!
are 10 or 20 years away and that until (and even after) it exists
surprises are likely.
No computer can handle all of the calculations required
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74
THE FACES OF GLOBAL ENVIRONMENTAL CHANGE
to simulate the complexity of the atmosphere. To compensate,
scientists make calculations for areas encompassed by widely
spaced points that form a three-dimensional grid at and above
the earth's surface. in the current models, spacing, or resolution,
of the grid is 300 miles, or 5° latitude. This kind of spatial
resolution means that for purposes of regional analysis, Panama,
for instance, does not exist, and neither does Japan. Nor does
it accurately reflect the influence of factors like clouds, because
they occur over a much smaller area.
If resolution were increased to 2.5° latitude, the cost of run-
ning the computer models would be more than 10 times greater.
At a resolution of 1° latitude by I° longitude, modelers could
calculate effects over an area 60 miles on a side a useful size
for studying regional effects on natural ecosystems, agriculture,
and water supplies. This would require 500 times as much com-
puter time, at great expense. Thus the demands of policymakers
will outstrip the ability of climatologists to deliver answers for
probably the next two decades.
How well can the models simulate climate? As Schneider
explains, "Perhaps the most perplexing question about climate
models is whether they can ever be trusted enough to pro-
vide grounds for altering social policies, such as those govern-
ing carbon dioxide emissions." How can models so fraught
with uncertainties be verified? Schneider explains that there
are three main tests that together can provide evidence about
a model's credibility: whether the mode! can simulate today's
climate, especially the large temperature swings of the seasonal
cycle; whether the mode! can realistically simulate an individual
physical component of the climate system, such as cloudiness;
and whether the mode! can simulate tong-term climate changes
by reproducing the varied climates of the ancient earth, or of
other planets. How the models perform against such known
standards is constantly being reappraised by their users.
The success of the models in passing these tests and the
ability of different models to have similar results show that the
models are getting better at predicting climate change, though
there is much room for improvement in coming decades.
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GLOBAL WARMING
1 200
1100
-
z 1 000
z
LO
of
o
Cal
o
900
800
700
600
500
400
300
200
75
Annual
Growth
Rate
4%/ 13% / 2% /1%
/
Constant
Emission
Lovins et al.
Negative Growth
Scenario
1980 2020 2060 2100
YEAR
2140 2180 2220
The extent to which carbon-dioxide-induced climatic change will prove significant in
the future depends, of course, on the rate of injection of carbon dioxide into the
atmosphere. This depends, in turn, on behavioral assumptions as to how much fossil
fuel burning will take place. (This graph neglects biospheric effects such as carbon
dioxide emissions due to deforestation.) Since the end of World War II, a world
energy growth rate of about 5.3 percent per year occurred until the mid-1970s, the
time of the OPEC price hikes. Rates have come down substantially since then and
hardly grew at all in the early 1980s. The figure shows projected carbon dioxide
concentrations for different annual growth rates in fossil energy use, including one for
the assumption that no increase in fossil energy use occurs (constant 1975 emission)
and even a "negative growth scenario" (Amory B. Lovins et al.) in which energy
growth after 1985 is assumed to be reduced by a fixed amount (0.2 terawatts [1~W]
per year, which is about 2 percent of present demand) each year. (Reprinted, by
permission, from Amory B. Lovins et al. 1989. Least-Cost Energy: Solving the CO2
Problem, Figure 1.1, p. 10. Copyright ~ 1989, Rocky Mountain Institute. As adapted
from Stephen H. Schneider. 1989. Global Warming: Are We Entering the Greenhouse
Century?, Figure 6, p. 100.)
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76
THE FACES OF GLOBAL ENVIRONMENTAL CHANGE
DO WE KNOW ENOUGH TO ACT?
The rising concentrations of greenhouse gases in the atmo-
sphere are a direct response to our actions as we conduct our
lives, drive our vehicles, grow our food, and run our industries.
We are transforming the environment that sustains us.
How much warmer the climate becomes, and how quickly
the warming occurs, depend on whether societies decide to
act to slow emissions of carbon dioxide and other trace gases.
Scientists can provide raw material that can be analyzed before
such decisions are made, but whether to act is a social judgment,
not a scientific one.
The question is then, do we, and those who set the worId's
environmental, economic, and social policies, know enough to
decide whether to slow the rate of greenhouse gas emissions
and, thereby, the rate of global warming? While acknowledging
the many uncertainties, many members of the scientific commu-
nity believe the answer is a guarded "yes," particularly because
the more rapid the change in climate, the more difficult it will
be for societies and ecosystems to adapt.
Many effects of global warming, such as those on agricul-
ture, will be felt unequally around the globe. Researchers can
predict with a fair degree of confidence that changes in tem-
perature, precipitation, and soil wetness will affect agriculture,
improving the competitive advantages of some crops and re-
gions and lessening others, but they cannot say with certainty
which ones. They can pinpoint which coastal areas would be
most affected by a rise in sea level as glaciers melt and the oceans
expand in response to the extra heat. But the faster change oc-
curs, the greater the likelihood of unforeseen consequences. As
Schneider notes, "Quite simply, the 'bottom line' of the evolving
greenhouse gas build-up is that we insult the environment at a
faster rate than we can predict the consequences, and that under
these conditions, surprises are virtually certain."
The following chapters describe the sweeping changes un-
der way or predicted in the global environment, changes caused
by humans as we attempt to satisfy the needs of the worId's
growing population. Some of the direct consequences of global
warming for society the effects on food supply and the impacts
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GLOBAL WARMING
77
on coastal areas are discussed in the following two chapters.
Other issues discussed in subsequent chapters global environ-
mental issues in their own right are also intricately tied to
global warming: Acid deposition is caused by fossil fuel com-
bustion, as is the major share of the increase in greenhouse
gases; destruction of the ozone layer that shields us from the
sun's harmful radiation is caused by industrially produced chIo-
rofluorocarbons, also a powerful greenhouse gas; and the large-
scale felling of the worId's tropical forests contributes to the
increase in atmospheric carbon dioxide, in addition to eracticat-
ing the habitat for millions of plant and animal species. Many
researchers fear that global warming will accelerate the pace
of species extinction as plant and animal communities are torn
apart by the stresses of adapting to a quickly changing climate.
Each of these problems demands at least attention and pos-
sibly action even if the projected global warming never occurs.
Schneider is a vocal proponent of what he has dubbed the "tie-
in" strategy, in which individuals, firms, and nations would take
steps to slow down the rate of buildup of greenhouse gases and
at the same time tackle other societal problems. As insurance
against the surprises that would be more likely the faster the
cInnate change occurs, he urges accelerated testing of alterna-
tive non-fossil fuels, development of strains adapted to wider
climate ranges, adding flexibility to the management of water
systems, and coastal planning to deal with rising sea level and
storm surges. Just one initiative energy conservation-could
reduce the impact of many immediate problems. More efficient
fuel use would cause air pollution to decline, cut acid rain,
lessen the dependence of many nations on unreliable sources
of of} (thereby increasing security), anct improve the competi-
tiveness of manufactured goods as the cost of Producing them
1 - _=
drops along with energy use.
Failure to take steps may force us and other living things to
adapt to a much larger dose of change than if we act today to
slow down the change or to invest to make future adaptations
easier. Says Schneider, "Choosing to wait until the greenhouse
effect signal has clearly been cletected in the climatic record is
not a cost-free delay. It is a basic gamble with our environmental
future."
Representative terms from entire chapter:
global environmental
