Environmental Engineering Reference
In-Depth Information
to the times when dinosaurs lived. Gas bubbles found in ancient ice in
Greenland and Antarctica indicate that during the end of the last great ice
age, 10,000 to 20,000 years ago, CO 2 levels were about 2/3 of what they are
After the last ice age (5,000-9,000 years ago) the summers were about
2°F warmer than today and the CO 2 concentrations grew to preindustrial
levels. Since then, there has been a 25% increase in CO 2 . The burning of
organic matter may be a large part of this increase. As CO 2 goes into the
atmosphere at a much higher rate than it can be withdrawn or absorbed
by the oceans or living plants, there is a CO 2 buildup and this could be one
of the controls moderating the climate.
Carbon dioxide is not the only greenhouse gas that humans have
been changing. Methane is another important greenhouse gas. It has
increased in the atmosphere by almost 100% since 1800 but has been stable
or even seen a slight decrease since 1990. See Table 2-2.
Methane is produced by biological processes where bacteria have
access to organic matter such as marshlands, garbage dumps, landfills
and rice fields. Some methane is also released in the process of extracting
coal or transporting natural gas.
Methane is 20-30 times as effective at absorbing infrared radiation
as CO 2 . But, it is not as important in the greenhouse effect since the CO 2
percentage is much greater in the earth's atmosphere. Chlorofluorocarbons
(CFCs) are even more effective greenhouse gases, but are only a small
part of the CO 2 greenhouse gases. CFCs are involved in the depletion of
stratospheric ozone.
Ozone is a form of oxygen (O 3 ), where three oxygen atoms combine
into one molecule. Ozone has the property of absorbing most of the sun's
ultraviolet radiation. It does this in the upper part of the atmosphere, called
the stratosphere, which is about 6 to 30 miles (10-50 kilometers) above the
earth. This absorption of ultraviolet energy causes the stratosphere to heat
up. Life on earth has been dependent on the ozone layer shielding us from
harmful solar ultraviolet radiation. Ozone is part of the greenhouse effect,
although it is not as important as CO 2 or methane. Ozone in the lower
atmosphere can cause damage to plant or lung tissues and is a pollutant
in photochemical smog.
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