Part 2. Exploring Scientific Evidence and
Evaluating Choices
Advanced information on greenhouse
gases
Carbon Dioxide
We now have the capability of looking at an even longer record by examining ice cores taken from a 2-km deep hole drilled in the Antarctic ice sheet. A team of scientists from France and the former Soviet Union analyzed tiny bubbles of air trapped in this core at various levels below the present ice surface. Deeper layers correspond to times in the more distant past. These bubbles can be analyzed for the relative abundance of carbon dioxide to estimate atmospheric carbon dioxide levels at times extending back 160,000 years.
U.S. Global Change Research
Program
Concentrations of isotopes of oxygen trapped in these bubbles are also measured, and their ratio, being temperature dependent, gives an estimate of the surface temperature on the Antarctic continent at the time the ice was formed. The accompanying graph shows that carbon dioxide concentrations have gone from about 180 ppm up to about 300 ppm over the last 160,000 years. And you can see from our previous diagram also plotted here, that present concentrations exceed 350 parts per million, higher than it's been in the last 160,000 years.
Halocarbons
Halocarbons are molecules containing carbon along with chlorine
or bromine. Chlorinated fluorocarbons (CFCs), the most notable
halocarbons are manmade chemicals that were first produced in the
late 1930s. They have been used in a variety of industrial
processes and consumer products, such as freezers, refrigerators,
air conditioners, aerosol spray cans, foam insulation, and
cleaners for electronic parts, to name a few. These gases are
very harmless, but they have a very long lifetime in the
atmosphere and have a cumulative effect on global warming.
The present atmospheric concentrations and trends of CFC-11 and CFC-12 are shown in the next figure. Note that concentrations are given in parts per trillion (ppt).
Dutton,
E., NOAA (published in Nature by Elkins et al, 1992).
Chlorinated fluorocarbons haveing an extra hydrogen atom are called HCFCs.
HCFCs are being used to replace CFCs because they contribute far less to both
global warming and stratospheric ozone depletion.
Nitrous Oxide
Nitrous oxide, N2O, is a colorless, odorless, non-reactive gas
that is very stable in the lower atmosphere (lifetime of 110-168
years). It should not be confused with NO2, NO, or other oxides
of nitrogen.
IPCC, 364pp (1990)
Nitrous oxide concentrations have been steadily increasing with time, as shown on the accompanying plot which reports measurements at several locations since 1977. It is important to note that the units of these measurements are given in ppb. Nitrous oxide concentrations then are about one fifth of methane concentrations and about a thousand times smaller than carbon dioxide.
Data over a longer time scale as derived from ice cores show that over the last 2,000 years nitrous oxide concentrations were nearly constant at about 280 ppb until about the beginning of the Industrial Revolution, at which time there began a fairly dramatic increase which continues today at about 0.2 to 0.3% per year.
Ozone
The contribution of ozone to the earth's temperature is more
complicated since its relatively high concentration in the
stratosphere warms the upper atmosphere and contributes to cooling at
the surface.
Depletion of ozone in the stratosphere will tend to cool the
stratosphere and allow more solar energy to reach the surface,
contributing to warming. However, a depleted stratospheric ozone
layer will absorb and re-radiate less infrared radiation from
earth, leading to cooling at the surface. Stratospheric ozone concentrations
are of more concern for their role in ultraviolet radiation, and
ozone concentrations near the surface are of more concern as air
pollutants than greenhouse gases.
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