Part 2. Exploring Scientific Evidence and
Evaluating Choices
Section 5: Aedvanced information on why greenhouse gas concentrations
have risen
In addition to the major greenhouse gases, there are two others, halocarbons
and nitrous oxide, which contribute to global warming. Are the increased
concentrations of these gases also due to human activity?
Sources of halocarbons
As discussed under the description of greenhouse gases, the sources of halocarbons are almost exclusively anthropogenic. Halocarbons CFC-11, 12, and 114 are used as propellants in aerosol spray cans (although this use has been discontinued in the US); CFC 12 and 114 and HCFC-22 are used as refrigerants; CFC-11 and 12 are used in making foams (e.g., styrofoam); and other halocarbons are used as solvents and fire retardants in fire extinguishers.These chemicals are broken down only very slowly in the atmosphere and so have lifetimes of decades to centuries. For example, the lifetimes of CFC-11, 12, and 114 are 65, 130, and 200 years, respectively.
Sources of nitrous oxide
We know the sources of nitrous oxide as shown in the figure below.
IPCC, 364pp (1990)
Natural sources include oceans, tropical soils, wet forests, dry Savannah, and extra-tropical forests. Total emissions are about 4-10 teragrams. Anthropogenic sources include cultivated soils (including use of nitrogen fertilizers), biomass burning and other combustion processes, and acid production processes. The largest known process for destruction of nitrous oxide is stratospheric photolysis (breakdown by solar energy, principally ultraviolet radiation). From these estimates we can see that in spite of large uncertainty, human contributions to nitrous oxide in the atmosphere are comparable with natural sources and are likely the cause of the 3-4.5 teragrams per year increase in the amount of nitrous oxide in the atmosphere. Because of their long lifetime (stability) in the troposphere, natural removal processes are incapable stemming this increase.
Agricultural use of nitrogen fertilizer is a significant
anthropogenic source of nitrous oxide.
EPA
As shown on the pie chart
, China is a big user, followed (according to these data)
by the former Soviet Union, the United States, and India.
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