Geoscience Reference
In-Depth Information
Sea level and atmospheric temperature
are often related. When global tempera-
tures cool, continental scale glaciers can
form, and they can contain so much of
what was once the ocean's water that they
cause drops in sea level as large as four hun-
dred feet. There have been at least seven
major fluctuations of the sea over the last
two million years.
At present the Earth's huge and ever
growing human population is affecting the
climate through (1) the emission of gases
that cause the Earth to act like a green-
house; (2) the discharge of high concentra-
tions of atmospheric particles called aero-
sols; and (3) changes in land use. All three
are interrelated and work in tandem to
change the Earth's temperature.
A real greenhouse works differently from
the Earth's atmosphere. Glass-enclosed
greenhouses prevent winds from dispers-
ing the heat created by the Sun's rays. Thus
the interior space retains heat. The atmo-
spheric greenhouse works because the
greenhouse gases allow short-wavelength
sunlight to pass through the atmosphere.
This radiation is converted to heat, which
warms the Earth's atmosphere, causing it to
emit longer-wavelength infrared radiation
back into space. The greenhouse gases ab-
sorb some of this infrared radiation head-
ing out into space, which in turn heats up
the Earth's atmosphere even more. Eventu-
ally the infrared rays escape the Earth, but
the more greenhouse gases there are in the
atmosphere, the more radiation is trapped,
resulting in increased temperatures.
While the greenhouse effect is natural
and necessary for human life, the funda-
mental problems are that human activities
have created an excess of greenhouse gases
in the atmosphere, and that the enhanced
greenhouse effect has brought about global
warming at a rapid and accelerating rate.
The Greenhouse Gases
So, what exactly are the major greenhouse
gases? The principal greenhouse gases
whose presence in the atmosphere causes
warming are water vapor (H 2 O), carbon
dioxide (CO 2 ), methane (CH 4 ), and nitrous
oxide (N 2 O). All the greenhouse gases are
“trace gases”: that is, they all make up a
very small part of the atmosphere. Carbon
dioxide is usually measured in parts per
million (ppm). Present measurements in-
dicate that there are around 390 molecules
per million molecules of air. Methane and
nitrous oxide are measured as parts per
billion (ppb). Even water vapor is a minor
constituent of the atmosphere—at most a
few percent.
Yet water vapor is responsible for most
of the greenhouse effect, accounting for be-
tween 36% and 66% of the warming (aver-
age of 60% globally), but it is more or less
a long-term constant in the greenhouse
equation. That is, it is not directly respon-
sible for the global climate change that the
Earth is experiencing. Unlike the other
greenhouse gases, which are uniformly
distributed in the atmosphere, water vapor
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