Ground Truth Studies Teacher Handbook - Aspen Global Change ...

A Global Change Primer (continued)
Methane
Methane, the second most significant greenhouse gas has as primary natural sources wetlands and termite metabolism.
Anthropogenic sources are coal mining, leaks in natural gas systems, rice paddies, enteric fermentation from the digestion
of grazing animals like cows, from animal wastes, landfills and biomass burning. Methane concentration in the atmosphere
has more than doubled from its pre-industrial levels and until recently was increasing at about 1% per year. Over
the last decade this rate has slowed and is now at about 0.6%. This is still a significant rate, especially when one
considers that each molecule of methane has many times the warming potential as a molecule of carbon dioxide (see
Global Warming Potential (GWP) below).
CFCs
Not all greenhouse gases are naturally occurring like carbon dioxide or methane. Some of the trace gases are artificially
manufactured and are new to this century. They include the chloroflurocarbons (CFCs), invented in the 1930s, and the
recently developed CFC-replacement family of hydrogenated chloroflurocarbons (HCFCs), hydrogenated fluorocarbons
(HFCs), and various compounds with bromine (halons). These trace gases often have long lifetimes in the atmosphere,
and on a per molecule basis, some are equivalent to thousands of CO 2
molecules in their greenhouse heat-rapping effect.
Coincidentally, CFCs and related chemicals are also responsible for the loss of ozone in the stratosphere due to the
release of chlorine and bromine when the compounds are transported by convection to the stratosphere and are exposed to
more intense ultraviolet light. Chlorine and bromine catalyze the destruction of ozone above natural rates. More about
CFCs and ozone later. Recent research has indicated that ozone depleting chemicals such as the CFCs, that have a direct
warming effect as greenhouse gases, also have an indirect cooling effect by removing ozone, which itself is a greenhouse
gas from the lower stratosphere. Because of this development, the net effect of some of the halocarbons as greenhouse
gases is unclear.
Tropospheric ozone
Tropospheric Ozone is a greenhouse gas of
increasing significance. This lower atmosphere
ozone is photochemically (chemical reaction
driven by sunlight) produced when nitrogen
oxides (NO x
) react with carbon monoxide (CO),
CH 4
, non-methane hydrocarbons (NMHCs), and
sunlight. Most of the pollutants which lead to
the formation of tropospheric ozone in cities
come from automobiles, power plants and other
human activities. In addition to its long-term
effects as a greenhouse gas, ozone is a major
component of “smog,” which causes significant
health problems for people in many cities,
notably acute respiratory disorders. Ozone at
Concentration of CO 2
(ppmv)
380
360
340
320
CO 2
Concentration Mauna Loa, Hawaii
Monthly
atmospheric CO 2
concentrations
300
1955 1965 1975 1985 1995
Year
Figure 4 Annual average carbon dioxide at
Mauna Loa Hawaii, 1958 – 1993.
© ASPEN GLOBAL CHANGE INSTITUTE 1995 GROUND TRUTH STUDIES
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