Global concentrations of
methane in the troposphere. Adapted from Blake and Rowland (1987).Methane is another atmospheric constituent whose concentration has increased in recent years. Methane is also a greenhouse gas that is about twenty times as effective on a molecule for molecule basis as is CO2. One methane molecule will absorb 20 times as much infrared radiation as CO2. Its lifetime is much shorter than carbon dioxide, however, so this partially compensates for its higher absorption.
Global concentrations of
methane in the troposphere. Adapted from Blake and Rowland (1987).
Actually, methane is the most rapidly increasing greenhouse gas. The plot above shows the present concentration of methane, in parts per million by volume (ppm), to be about 1.7 ppm. Sometimes methane concentration is given in parts per billion by volume (ppb), and then would have a value of 1700 ppb. The numerical values show that methane is much less abundant than carbon dioxide which has a present concentration of about 360 ppm. However, the curve shows that the concentration is increasing at about 1% per year. If we look at a longer term, as shown in the second graph below, we see that concentrations have increased substantially since the Industrial Revolution. Estimate of atmospheric methane from a thousand years ago suggest values around 0.7 ppm (700 ppb) which were constant until about the late 1700s. Since that time, concentrations have more than doubled. If we examine the Antarctic ice core data going back 160,000 years, we see that methane levels fluctuated between about 300 parts per billion and 700 parts per billion until the Industrial Revolution when it began its climb to near 1700 parts per billion.
Average atmospheric methane
concentrations. Adapted from Khalil and Rasmussen, C and E News,
64 (47), 23.
Methane, CO2 and
temperature profiles. Adapted from Woodwell et al, Scientific
American, April 1989
What are the sources of methane? The 1992 IPCC report lists the largest natural source of methane to be wetlands, which produce 115 teragrams (1012 grams) of carbon annually. The uncertainty in these numbers, however, is very large. Termites are very significant producers of methane in that they eat wood and release methane in the digestion process. The ocean produces about 10 teragrams per year of methane, and fresh water and methane hydrate contribute smaller amounts.
Estimated sources and sinks
of methane. Adapted from the IPCC supplemental Report,
1992.
Anthropogenic sources include coal mining, natural gas and petroleum industry at about 100 teragrams, which is almost as much as natural wetlands. Rice paddies produce on the order of 60 teragrams by means of a process where methane produced in the soil is able to travel up to the hollow stem of the rice plant and be released into the atmosphere without passing through the water which would tend to suppress the evolution of methane gas. As shown in the pie chart from EPA, China is the world's lead producer of rice, followed by India and Indonesia.
Enteric fermentation, the digestion process in ruminant animals such as cattle, sheep and goats, produces very large amounts of methane. Animal wastes produce about 25 teragrams; domestic sewage, 25 teragrams; landfills about 30 teragrams; and biomass burning, about 40 teragrams. Some landfills are now being tapped for their methane as a source of power production. This makes good sense on the basis of global warming in addition to getting a "free" source of combustion gas. Burning one methane molecule produces one CO2 molecule, but the global warming potential is reduced by a factor of 20 because the carbon dioxide molecule is only about one-twentieth as effective as the methane molecule in absorbing infrared radiation.
Increases in animal populations are contributing to the increase in atmospheric methane. The next plot shows recent increases in several different classes of livestock. If humans continue to have an appetite for meat, the upward trend in animal production and resulting production of methane will likely continue. A particular situation to watch is the development and possible dietary changes in China. If we examine the eating habits of Japan, South Korea, and other Asian nations that have developed very rapidly, one of the significant changes that occurs during economic development is that people's eating habits change from eating primarily grains, mainly rice in these cases, to substantial increases in meat. The big question on the horizon right now is what's going to happen in China? China has an enormous population and it is developing extremely rapidly. If China follows the pattern of other Asian nations, the demand for meat will increase dramatically. I estimated that if you gave every Chinese person 4 Big Macs per year, it would take all of the corn raised in Iowa in a year.
Trends in domestic animal
population (1890-1985). EPA, 1989: Policy options for stabilizing global
climate.
Sinks for methane include atmospheric removal of about 470 units, removal by soil of about 30 teragrams, leaving an atmospheric increase of about 32 units. By taking subtotals of natural and anthropogenic sources, it is easy to see that humans contribute at least as much as natural sources and possible much more. Such numbers as these are consistent with the observed build-up of methane in the atmosphere in the last 200 years. (See the sources and sinks table above).
Carbon monoxide is another carbonaceous gas in the earth's atmosphere that participates in the global carbon cycle. The accompanying table gives the sources and sinks of carbon monoxide in teragrams of carbon per year. Fossil fuel combustion is a significant source, as is biomass burning. Carbon monoxide also can be produced in secondary oxidation reactions with methane or non-methane hydrocarbons (NMHC). Major sinks include reaction with the hydroxyl radical and soil uptake. These estimates also carry large uncertainties, but again it is very likely that anthropogenic sources dominate natural sources. Carbon monoxide is much more reactive that carbon dioxide, so its lifetime in the atmosphere is comparably shorter.
Estimated sources and sinks
of carbon monoxide. EPA, 1989: Policy options for stabilizing global climate.
Transcription by Theresa M. Nichols
