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Global Warming
Global Change/Climate Change
The prediction of climate change due to human activities began with a prediction made by the
Swedish chemist, Svante Arrhenius, in 1896. Arrhenius took note of the industrial revolution then
getting underway and realized that the amount of carbon dioxide being released into the atmosphere
was increasing. Moreover, he believed carbon dioxide concentrations would continue to increase as the
world's consumption of fossil fuels, particularly coal, increased ever more rapidly. His understanding of the
role of carbon dioxide in heating Earth, even at that early date, led him to predict that if atmospheric
carbon dioxide doubled, Earth would become several degrees warmer. However, little attention was paid to
what must have been seen to be a rather far-out prediction that had no apparent consequence for
people living at that time.
Arrhenius was referring to a potential modification of
what we now call the greenhouse effect. A simplified explanation of this is as follows (see the
diagram). Shortwave solar radiation can pass through the clear atmosphere relatively unimpeded, but longwave
infrared radiation emitted by the warm surface of the Earth is absorbed partially and then re-emitted by a
number of trace gases--particularly water vapor and carbon dioxide--in the cooler
atmosphere above. Because, on average, the outgoing infrared radiation
balances the incoming solar radiation, both the atmosphere and the surface will
be warmer than they would be without the greenhouse gases. One should
distinguish between the "natural" and a possible "enhanced" greenhouse
effect. The natural greenhouse effect causes the mean temperature of
the Earth's surface to be about 33 degrees C warmer than it would be if natural
greenhouse gases were not present. This is fortunate for the natural greenhouse
effect creates a climate in which life can thrive and man can live under
relatively benign conditions. Otherwise, the Earth would be a very frigid and
inhospitable place. On the other hand, an enhanced greenhouse effect refers
to the possible raising of the mean temperature of the Earth's surface above that
occurring due to the natural greenhouse effect because of an increase
in the concentrations of greenhouse gases due to human activities. Such a
global warming would probably bring other, sometimes deleterious, changes
in climate; for example, changes in precipitation, storm patterns, and
the level of the oceans. The word "enhanced" is usually omitted, but it should not be
forgotten in discussions of the greenhouse effect.
Nearly 100 years after the Arrhenius prediction, we
are now aware that carbon dioxide in the atmosphere is increasing, with the likelihood that it will
double by the middle of the next century from the levels at the time of Arrhenius. Post-World War II industrialization
has caused a dramatic jump in the amount of carbon dioxide in the atmosphere. As the prospect of
considerable change in the atmosphere becomes more real and threatening, new
computer models are being applied to the problem. These models take into
account the natural processes that must be part of the whole picture to
understand what could happen to Earth's climate as carbon dioxide increases. An
important aspect of the newer models is their treatment of the "amplifier" or feedback effect, in
which further changes in the atmosphere occur in response to the warming initiated by the change in
carbon dioxide.
In addition to moisture and cloud processes, the
newer models are beginning to take into account the role of vegetation, forests, grasslands, and crops in
controlling the amount of carbon dioxide that actually will be in the atmosphere. Along with their role as
"sinks" for carbon dioxide, the various types of vegetation in the biosphere have further effects on
climate. Plants heat or cool the air around them (through the reflection and absorption of solar
radiation and the evaporation process), remove momentum from surface winds, and take up and
release moisture into the air (thus contributing to alterations in the hydrologic cycle). In turn, changes in
climate will affect the patterns of vegetation growth. For instance, forest stands that require relatively cool
conditions may not be able to adjust to the relatively rapid warming that is being predicted for the interiors
of climates. With slow warming, scientists expect that the northern edges of North American forests would
creep slowly forward to more-favorable conditions, while the southern edges would give
way to grasslands that are better suited to the warmer conditions. With overly rapid warming rates, however,
the loss at the southern edge would be more extreme, and the migration at the northern edges would not be
able to make up for the loss at the southern edge.
Other feedback effects at work also must be
considered. In normal conditions, plant leaves take in carbon dioxide from the air and release moisture to
the air as part of the photosynthesis process. The release of moisture through evapotranspiration
causes the air to cool. With increasing atmospheric carbon dioxide, one can expect to see a change in
plant carbon exchange rates and water relations. This may result in reduced evaporation rates, thus
amplifying the summer continental warming. Without plants, the ground and air would become warmer,
exacerbating the problem.
Greenhouse Gases
To predict climate change, one must model the climate. One test of the validity of predictions is the
ability of the climate models to reproduce the climate as we see it today. Elements of the models such as
the physics and chemistry of the processes that we know--or think we know--are essential to
represent in the models. Therefore, the models have to embody the characteristics of the land and the oceans that serve
as boundaries of the atmosphere represented in the models. Models also have to take into account the
radioactive characteristics of the gases that make up the atmosphere,
including the key radioactive gas, water vapor, that is so variable throughout the atmosphere.
Global records of surface temperature over the last
100 years show a rise in global temperatures (about 0.5 degrees C overall), but the rise is marked by
periods when the temperature has dropped as well. If the models cannot explain these marked variations
from the trend, then we cannot be completely certain that we can believe in their predictions of changes to
come. For example, in the early 1970's, because temperatures had been decreasing for
about 25 to 30 years, people began predicting the approach of an ice
age! For the last 15 to 20 years, we have been seeing a fairly steady rise in
temperatures, giving some assurance that we are now in a global warming phase.
The major gases in the atmosphere, nitrogen and
oxygen, are transparent to both the radiation incoming from the sun and the radiation outgoing from the
Earth, so they have little or no effect on the greenhouse warming. The gases that are not transparent are water
vapor, ozone, carbon dioxide, methane, nitrous oxide, and the chlorofluorocarbons (CFCs). These are the
greenhouse gases. There has been about a 25% increase in carbon dioxide in the
atmosphere from 270 or 280 parts per million 250 years ago, to approximately 350 parts per million today.
The record of carbon dioxide in the atmosphere
shows a variation as seasons change. This variation is more pronounced in the northern hemisphere,
with its greater land area, than in the southern hemisphere because of interactions in the atmosphere caused by
vegetation. In the growing season, during daylight vegetation takes in carbon dioxide; at night and in the
senescent season, vegetation releases carbon dioxide. The effect is more
pronounced in the northern hemisphere because most of the land on Earth is
located there.
Modeling
To understand and predict climate change, the following types of models are needed:
- Socio-economic models that predict future fossil fuel
consumption and utilization of alternative fuels. These models depend upon technology, e.g.,
industrial production methods, energy efficiency, new materials,
etc.; public policy and social attitudes, e.g., concern for the environment; and
economic development, standard of living and reliance on energy and
chemical-based products.
- Chemical-physical-biophysical models of the Earth
System that tell us what happens to gases released into the atmosphere, e.g., how much
carbon dioxide is taken up by the oceans and the biosphere, and how
industrial and agricultural uses of chemicals and natural processes on
Earth's surface affect the release of methane, nitrogen oxides, and other greenhouse
gases into the atmosphere.
- Coupled ocean-atmosphere models to tell us how the
climate system, e.g., temperatures, humidity, clouds, and rainfall, responds to changes in
the chemical composition of the atmosphere.
Getting reliable predictions from models is difficult
because many of the secondary processes are not understood. For example, when temperatures start
to warm because of the direct radioactive effect of increasing carbon dioxide, will clouds increase or
decrease?. Will they let in less radiation from the sun, or more? These secondary processes are important.
The direct radioactive effect of doubling carbon dioxide
is relatively small, and there is not much disagreement on this point among models. Where models
conflict is in regard to the secondary, or feedback effects. Models that predict a very large warming from carbon
dioxide show cloud cover changes that greatly amplify the warming effects, while models that predict
more-modest warming show that clouds have a small or even damping effect on the
warming.
Can we match the observation of temperature trends
with the model predictions? The temperature record of the past hundred years does show a
warming trend, by approximately 0.5 degrees C. However, the observed warming trend is not entirely consistent with
the carbon dioxide change. Most of the temperature increase occurred before 1940, after which Earth
started to cool until the early seventies, when warming resumed. Carbon dioxide, on
the other hand, has been increasing steadily throughout the past century.
Other factors that could have affected climate during this period include the
possible change in the solar energy reaching Earth, the cooling effects of volcanic
aerosols, and the possibility that sulfur dioxide and other pollutants
might be affecting the amount of solar radiation that is reflected back to space. Some of
these effects can cause a cooling that could counteract the warming due to carbon dioxide and
other greenhouse gases. All of these effects would have to be taken into account and appropriately
modeled in order to predict the changes that one might expect in the next century.
NASA Investigations of the Greenhouse Effect
Over the past 30 years, a number of satellite missions have been launched to obtain the data about Earth's
radiation budget that are critical to understanding the greenhouse effect. Some of these missions are listed
in the accompanying table.
Another very important aspect of greenhouse
investigations has been the development of models. A number of climate models have been developed by
NASA, and one of the most detailed is a General Circulation Model (GCM) developed by the Goddard
Institute for Space Studies (GISS) in New York City. A GCM uses extremely high-speed computers to solve
the basic equations governing atmospheric motions and processes by numerical techniques. The
GISS group, using its model, predicted that the annual global temperature would reach a new record high
sometime during the first three years of the 1990's. Indeed, that record was reached in 1990. However, in
June 1991, the Mount Pinatubo volcano erupted and sent 25 to 30 million tons of sulfur
dioxide into the stratosphere. There, the sulfur dioxide reacted with
water vapor to produce a long-lasting haze of sulfuric acid droplets.
The GISS group then inserted the new information into
the model, estimated how much sunlight the Pinatubo aerosol cloud would block, and predicted
that the global temperature would drop about 0.3 degree C. Again, the predicted change actually occurred.
Although these successful climate predictions are encouraging, most scientists agree that much remains
to be done to improve climate models before we will be able to predict future climate in a
credible manner.
An important need in the further development and
verification of climate models is the acquisition, assembly, and analysis of reliable climate data. The
highly-accurate, self-consistent, and long-term data sets that will be acquired by the Earth Observing
System (EOS), as part of NASA's Mission to Planet Earth with a series of satellite launches beginning in
1998, are designed to fulfill that need.
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