Geoscience Reference
In-Depth Information
HOW RISING CO 2 CONCENTRATIONS CHANGE THE CLIMATE
Once we have projections of CO 2 concentrations along with other
GHGs and other important input data, climatologists put these into cli-
mate models. Climate models are mathematical representations of the
circulation of the atmosphere and oceans. These models start with
some fundamental laws of physics and details of the earth's geography
that are written into computer programs, but you can think of them as
equations representing the dynamics of the atmosphere and oceans. So
to understand climate models, we need to understand the basic science
underlying the equations.
The heat that we feel from the sun is radiant energy, or radiation. If
you turn your face to the sun, you will feel the warmth of the radiation
hitting your skin. Radiation comes in waves of different lengths or fre-
quencies. Most of the energy from the sun is visible as light, which is
“hot” and has short wavelengths. About 30 percent of the hot radiation
is refl ected back to space. The atmosphere and surface of the earth ab-
sorb the rest of the energy, and this warms the earth. The energy com-
ing in and going out are balanced, so the earth emits radiation back to
space. But because the earth is warm rather than hot, the outgoing
earth radiation has a longer wavelength than incoming solar radiation.
Here is the interesting part. Some gases in the atmosphere, such as
CO 2 and methane, as well as water vapor, absorb more outgoing warm
radiation than incoming hot radiation. This selective absorption acts
like a blanket on a cold winter's night, which captures some of our
body heat and keeps us warm. This is why the atmosphere is described
as a natural “greenhouse”—because gases such as water and CO 2 trap
the heat. Because radiation is retained near the earth, the equilibrium
temperature of the earth rises. This is called the “natural greenhouse
effect.” Scientists have calculated that the natural greenhouse effect—
that is, the result of gases that were in the atmosphere before humans
started adding more—warms the earth about 33°C (60°F) above what
its temperature would be with no atmosphere. In other words, if there
were no GHGs, the earth's surface temperature would be −19°C, whereas
the actual average temperature on earth is 14°C. Using this relationship,
 
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