April 1998

NF-222
The Earth Science Enterprise Series
These articles consider Earth’s many dynamic processes and their interactions.

Global Warming
Global Change /Climate Change cooler atmosphere above. Because, on average, the
outgoing infrared radiation balances the incoming solar
he prediction of climate change due to radiation, both the atmosphere and the surface will be

T human activities began with a prediction

made by the Swedish chemist, Svante
Arrhenius, in 1896. Arrhenius took note of
warmer than they would be without the greenhouse
gases. One should distinguish between the “natural”
and a possible “enhanced” greenhouse effect. The
the industrial revolution then getting underway and natural greenhouse effect causes the mean temperature
realized that the amount of carbon dioxide being of the Earth’s surface to be about 33°C warmer than it
released into the atmosphere was increasing.
Moreover, he believed carbon dioxide
concentrations would continue to increase as
the world’s consumption of fossil fuels, SUN
particularly coal, increased ever more rapidly. Some solar radiation
is reflected by the
His understanding of the role of carbon Earth and the
atmosphere.
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 ATMOSPHERE
that time.

Solar Some of the infrared

Arrhenius was referring to a potential modifi- radiation is absorbed
radiation
cation of what we now call the greenhouse passes and re-emitted by the
through the greenhouse gases. The
effect. A simplified explanation of this is as clear effect of this is to warm
atmosphere. the surface and the
follows (see the diagram). Shortwave solar lower atmosphere.
radiation can pass through the clear atmo-
sphere relatively unimpeded, but longwave
infrared radiation emitted by the warm surface
of the Earth is absorbed partially and then re- Most radiation is EARTH Infrared radiation is
absorbed by the Earth’s emitted from the
emitted by a number of trace gases—particu- surface and warms it. Earth’s surface.
larly water vapor and carbon dioxide—in the A simplified diagram illustrating the greenhouse effect
 • 2•

would be if natural greenhouse gases were not present. changes in climate will affect the patterns of vegetation
This is fortunate, for the natural greenhouse effect growth. For instance, forest stands that require rela-
creates a climate in which life can thrive and human- tively cool conditions may not be able to adjust to the
kind can live under relatively benign conditions. relatively rapid warming that is being predicted for the
Otherwise, the Earth would be a very frigid and inhos- interiors of continents. With slow warming, scientists
pitable place. On the other hand, an enhanced green- expect that the northern edges of North American
house effect refers to the possible raising of the mean forests would creep slowly forward to more-favorable
temperature of the Earth’s surface above that occurring conditions, while the southern edges would give way to
due to the natural greenhouse effect because of an grasslands that are better suited to the warmer condi-
increase in the concentrations of greenhouse gases due tions. With overly rapid warming rates, however, the
to human activities. Such a global warming would loss at the southern edge would be more extreme, and
probably bring other, sometimes deleterious, changes in the migration at the northern edges would not be able
climate; for example, changes in precipitation, storm to make up for the loss at the southern edge.
patterns, and the level of the oceans. The word “en-
hanced” is usually omitted, but it should not be forgot- Other feedback effects at work also must be consid-
ten in discussions of the greenhouse effect. ered. In normal conditions, plant leaves take in carbon
dioxide from the air and release moisture to the air as
Nearly 100 years after the Arrhenius prediction, we are part of the photosynthesis process. The release of
now aware that carbon dioxide in the atmosphere is moisture through evapotranspiration causes the air to
increasing, with the possibility that it will double by the cool. With increasing atmospheric carbon dioxide, one
middle of the next century from the levels at the time can expect to see a change in plant carbon exchange
of Arrhenius. Post-World War II industrialization has rates and water relations. This may result in reduced
caused a dramatic jump in the amount of carbon evaporation rates, thus amplifying the summer conti-
dioxide in the atmosphere. As the prospect of consider- nental warming. Without plants, the ground and air
able change in the atmosphere becomes more real and would become warmer, exacerbating the problem.
threatening, new computer models are being applied to
the problem. These models take into account the Greenhouse Gases
natural processes that must be part of the whole picture
to understand what could happen to Earth’s climate as To predict climate change, one must model the climate.
carbon dioxide increases. An important aspect of the One test of the validity of predictions is the ability of
newer models is their treatment of the “amplifier” or the climate models to reproduce the climate as we see
feedback effect, in which further changes in the atmo- it today. Elements of the models such as the physics
sphere occur in response to the warming initiated by and chemistry of the processes that we know—or think
the change in carbon dioxide. we know—are essential to represent in the models.
Therefore, the models have to embody the characteris-
In addition to moisture and cloud processes, the newer tics of the land and the oceans that serve as boundaries
models are beginning to take into account the role of of the atmosphere represented in the models. Models
vegetation, forests, grasslands, and crops in controlling also have to take into account the radiative characteris-
the amount of carbon dioxide that actually will be in tics of the gases that make up the atmosphere, includ-
the atmosphere. Along with their role as “sinks” for ing the key radiative gas, water vapor, that is so vari-
carbon dioxide, the various types of vegetation in the able throughout the atmosphere.
biosphere have further effects on climate. Plants heat or
cool the air around them (through the reflection and Global records of surface temperature over the last 100
absorption of solar radiation and the evaporation years show a rise in global temperatures (about 0.5° C
process), remove momentum from surface winds, and overall), but the rise is marked by periods when the
take up and release moisture into the air (thus contrib- temperature has dropped as well. If the models cannot
uting to alterations in the hydrologic cycle). In turn, explain these marked variations from the trend, then
 • 3 •

we cannot be completely certain that we can believe in reliance on energy and chemical-based products.
their predictions of changes to come. For example, in
the early 1970’s, because temperatures had been • Chemical-physical-biophysical models of the Earth
decreasing for about 25 to 30 years, people began System that tell us what happens to gases released
predicting the approach of an ice age! For the last 15 to into the atmosphere, e.g., how much carbon
20 years, we have been seeing a fairly steady rise in dioxide is taken up by the oceans and the bio-
temperatures, giving some assurance that we are now sphere, and how industrial and agricultural uses of
in a global warming phase. chemicals and natural processes on Earth’s surface
affect the release of methane, nitrogen oxides, and
The major gases in the atmosphere, nitrogen and other greenhouse gases into the atmosphere.
oxygen, are transparent to both the radiation incoming
from the sun and the radiation outgoing from the Earth, • Coupled ocean-atmosphere models that tell us how
so they have little or no effect on the greenhouse the climate system, e.g., temperatures, humidity,
warming. The gases that are not transparent are water clouds, and rainfall, responds to changes in the
vapor, ozone, carbon dioxide, methane, nitrous oxide, chemical composition of the atmosphere.
and the chlorofluorocarbons (CFCs). These are the
greenhouse gases. Getting reliable predictions from models is difficult
because many of the secondary processes are not
There has been about a 25% increase in carbon dioxide understood. For example, when temperatures start to
in the atmosphere from 270 or 280 parts per million 250 warm because of the direct radiative effect of increasing
years ago, to approximately 360 parts per million today carbon dioxide, will clouds increase or decrease? Will
(see Figure 1 in NASA Facts, NF-223, titled Biosphere). they let in less radiation from the sun, or more? These
The record of carbon dioxide in the atmosphere shows secondary processes are important.
a variation as seasons change. This variation is more
pronounced in the northern hemisphere, with its The direct radiative effect of doubling carbon dioxide is
greater land area, than in the southern hemisphere relatively small, and there is not much disagreement on
because of interactions in the atmosphere caused by this point among models. Where models conflict is in
vegetation. In the growing season, during daylight, regard to the secondary, or feedback effects. Models
vegetation takes in carbon dioxide; at night and in the that predict a very large warming from carbon dioxide
senescent season, vegetation releases carbon dioxide show cloud cover changes that greatly amplify the
(see Figures 2a & 2b in NASA Facts, NF-223, titled warming effects, while models that predict more-
Biosphere). The effect is more pronounced in the modest warming show that clouds have a small or even
northern hemisphere because most of the land on Earth damping effect on the warming.
is located there.
Can we match the observation of temperature trends
Modeling with the model predictions? The temperature record of
the past hundred years does show a warming trend, by
To understand and predict climate change, the follow- approximately 0.5°C. However, the observed warming
ing types of models are needed: trend is not entirely consistent with the carbon dioxide
change. Most of the temperature increase occurred
• Socio-economic models that predict future fossil before 1940, after which Earth started to cool until the
fuel consumption and utilization of alternative early seventies, when warming resumed. Carbon
fuels. These models depend upon technology, e.g., dioxide, on the other hand, has been increasing steadily
industrial production methods, energy efficiency, throughout the past century. Other factors that could
new materials, etc.; public policy and social atti- have affected climate during this period include the
tudes, e.g., concern for the environment; and possible change in the solar energy reaching Earth, the
economic development, standard of living and cooling effects of volcanic aerosols, and the possibility
 • 4•

that sulfur dioxide and other pollutants might be predicted that the annual global temperature would
affecting the amount of solar radiation that is reflected reach a new record high sometime during the first three
back to space. Some of these effects can cause a years of the 1990’s. Indeed, that record was reached in
cooling that could counteract the warming due to 1990. However, in June 1991, the Mount Pinatubo
carbon dioxide and other greenhouse gases. All of volcano erupted and sent 25 to 30 million tons of sulfur
these effects would have to be taken into account and dioxide into the stratosphere. There, the sulfur dioxide
appropriately modeled in order to predict the changes reacted with water vapor to produce a long-lasting haze
that one might expect in the next century. of sulfuric acid droplets.

NASA Investigations of the Greenhouse Effect The GISS group then inserted the new information into
the model, estimated how much sunlight the Pinatubo
Over the past 30 years, a number of satellite missions aerosol cloud would block, and predicted that the
have been launched to obtain the data about Earth’s global temperature would drop about 0.3°C. Again, the
radiation budget that are critical to understanding the predicted change actually occurred. These successful
greenhouse effect. Some of these missions are listed in climate predictions have been encouraging, and scien-
the accompanying table. tists are continuing their research to further increase the
credibility of their model predictions.
Another very important aspect of greenhouse investiga-
tions has been the development of models. A number An important need in the further development and
of climate models have been developed by NASA, and verification of climate models is the acquisition, assem-
one of the most detailed is a General Circulation Model bly, and analysis of reliable climate data. The highly-
(GCM) developed by the Goddard Institute for Space accurate, self-consistent, and long-term data sets that
Studies (GISS) in New York City. A GCM uses extremely will be acquired by the Earth Observing System (EOS),
high-speed computers to solve the basic equations as part of NASA’s Mission to Planet Earth, with a series
governing atmospheric motions and processes by of satellite launches beginning in 1998, are designed to
numerical techniques. The GISS group, using its model, fulfill that need.

Earth Science Enterprise

Selected Missions Leading to an Improved Understanding of the Greenhouse Effect

Mission Launch Scientific Objective

Explorer-7 First satellite radiation budget experiment (spinning black and white
1959
hemispheres)
Nimbus-2, -3 Global radiation budget measurements from the Medium Resolution
1966, 1969
Infrared Radiometer
Earth Radiation Budget Coordinated radiation budget measurements from the Earth Radiation
Experiment (ERBE)/Earth Budget Experiment sensors in three different orbits
Radiation Budget Satellite
(ERBS) 1984
NOAA-9 1984
NOAA-10 1986
Tropical Rainfall Radiation budget data from the Clouds and Earth's Radiant Energy
Measuring Mission 1997 System (CERES) experiments. This mission will complement CERES
(TRMM) experiments on EOS satellites
Earth Observing System Radiation budget data from CERES to complement experiments on
(EOS) 1998- TRMM. A broad spectrum of physical and chemical measurements of
atmosphere, ocean, and land characteristics from different EOS missions