Greenhouse Gases

Greenhouse gases are gases
in the atmosphere that can absorb infrared radiation, the
energy that radiates from the Earth and from the lowest level of
the atmosphere, the troposphere. Let’s review the major
anthropogenic greenhouse gases. Here are the important greenhouse
gases, their anthropogenic source, and their global warming potential. Global warming potential refers to
the amount of infrared radiation that a given quantity of each
gas will absorb or retain. It’s a measure of how
potent a greenhouse gas is. Rather than give you the
actual value of absorption, we compare each greenhouse
gas to carbon dioxide. This means carbon dioxide is given
a global warming potential of one. Other gases will have a global warming
potential for more or less than one. Carbon dioxide– CO2. The two major anthropogenic sources
are the combustion of fossil fuels and the conversion of land resulting
in the net loss of vegetation. And this includes deforestation. The global warming potential is one. Methane– CH4. The sources are anaerobic
decomposition– that is, decomposition without oxygen– which
occurs in waterlogged irrigated fields, such as rice paddies; the digestive
tract of ruminant animals; and created wetlands; and landfills. The global warming
potential of methane is 25. That is, it’s 25 times more
potent than carbon dioxide. Nitrous oxide– N2O. Sources are wet nitrate-rich
soils, such as agricultural fields, and other flooded areas of land. The global warming potential for nitrous
oxide is 125 to 300 times that of CO2. CFCs– chlorofluorocarbons. These are made by humans for
refrigerators and air conditioners. The global warming
potential varies widely, depending on the particular CFC, from
roughly 1,500 to over 10,000 times the global warming
potential of carbon dioxide. And water– water vapor. And it’s a little strange to talk
about water vapor as a greenhouse gas. It occurs naturally in
evapotranspiration, of course. But if humans are responsible for
increasing the amount of evaporation, we can say then that humans are
responsible for putting this greenhouse gas into the atmosphere
in greater quantities. The global warming potential is
less than one, and it varies. Remember, we’ve already discussed
how carbon dioxide concentrations in the atmosphere have risen
steadily since measurements began by Dave Keeling, in 1958,
at the Mauna Loa Observatory. Current atmospheric carbon dioxide
concentration is about 400 PPM. CO2 measurements from ice cores show
that for the past 400,000 years, CO2 concentrations had
not been above 300 PPM. A sharp increase in global
carbon dioxide concentrations occurred after 1950. This increase in carbon
dioxide concentration coincides with increased burning of
fossil fuels and net changes in land. Production of carbon dioxide has
been greatest in the developed world. The 20% of the global population
living in the developed world has produced roughly 3/4
of the additional carbon dioxide released to the atmosphere. Global temperatures have increased by
0.85 degrees C– roughly 1.5 degrees Fahrenheit– from 1880 through 2012. There is a close correspondence
between historic temperatures and CO2 concentrations. However, it’s not clear if
temperature is driving higher CO2 or if higher CO2 concentrations
are driving temperature. This increase may seem insignificant–
1.5 degrees Fahrenheit– but the warming is not evenly
distributed around the globe. Northern regions of the globe,
particularly the Arctic, have experienced more substantial
increases in temperature since 1880. We also need to remember that
even small amounts of warming could lead to feedback loops that would
then hasten the rate of global warming.

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