Emission Rates and Concentrations
of Greenhouse Gases
Several atmospheric trace gases are relatively transparent to incoming solar radiation but absorb, or trap, outbound infrared radiation emitted from the earth's surface. Water vapor, one of several naturally occurring greenhouse gases (including CO2, methane (CH4), and nitrous oxide (N2O)), also traps heat by mechanisms associated with mechanical transfer and convection. The trapping of heat by these gases is called the "greenhouse effect." This label emphasizes the similarity of the phenomenon to the warming that occurs in a greenhouse, although that warming is mostly associated with blocking convective heat transfer. Without the naturally occurring greenhouse effect, the earth's average temperature would be about 33°C (59°F) colder than it is now, and the planet would be much less suitable for human life. This report is not concerned with the natural greenhouse effect, but rather with additional global warming due to human-induced increases in the atmospheric concentration of greenhouse gases. This enhancement of the greenhouse effect is called "greenhouse warming." Table 11.1 presents key attributes of the principal greenhouse gases deriving, in part or entirely, from human activities.
The increase from 280 ppmv (parts per million by volume) to 354 ppmv in the atmospheric concentration of CO2 since 1800 is well established from ice core studies (Neftel et al., 1985; Friedli et al., 1986) and direct atmospheric measurements (Keeling et al., 1989a). The error in the ice core record is probably not more (and perhaps less) than ± 10 ppmv,1 and the atmospheric record since 1957 is known to within ±1 ppmv. The emission of CO2 into the atmosphere via the burning of fossil fuels is also well
TABLE 11.1 Key Greenhouse Gases Influenced by Human Activity
quantified over much of the last century (Marland, 1990). Other anthropogenic input rates of CO2 are not as well known, but it is unlikely that the total of such input since 1700 has exceeded one-half of the fossil fuel inputs. The 1990 total of emissions resulting from other human activities, even with the present rate of tropical deforestation, is about one-quarter the rate from burning fossil fuels alone.
Calculating the effective atmospheric lifetime of the actual emissions is complex because CO2 molecules are constantly exchanged between the atmosphere, the oceans, and the biosphere. Although a typical molecule stays in the atmosphere about 4 years, atmospheric CO2 concentrations vary seasonally and between the northern and southern hemispheres. However, because of the exchange with other "sinks," the "lifetime" associated with an individual CO2 anomaly, or surge in atmospheric concentration, is 50 to 200 years. In addition, the total natural flow of CO2 into and out of the atmosphere is about 30 times greater than that caused by human activities. This natural carbon cycle, however, was approximately constant over the 10,000 years prior to the Industrial Revolution, even though there were changes in the biospheredeserts emerged and forests migrated, for example. What matters for greenhouse warming is the increased concentration of CO2 (and other greenhouse gases) in the atmosphere rather than the total annual flow into and out of the atmosphere.
The increase in atmospheric CO2 concentration since 1860, which is approximately the beginning of the fossil fuel era, has been slightly more than 60 percent of the concentration that would be in the atmosphere if all the fossil fuel emissions had remained in the atmosphere. The results of most models of the ocean-atmosphere carbon system are consistent with this approximate 60-40 split of the total fossil fuel emissions. However, the ocean-atmosphere models cannot reproduce the observed (from direct atmospheric measurements and the derived concentrations from ice cores) CO2 increase in this century if one adds to the fossil fuel CO2 emissions the estimated net flux of CO2 that results from land cover change. On the other hand, to match the ice core CO2 record for the period 1700 to 1900, it is necessary to invoke a terrestrial source of CO2 in addition to the fossil fuel emissions. Improved understanding of various aspects of relevant phenomena is reported from time to time (e.g., Keeling et al., 1989a,b; Kirchman et al., 1991). At present, however, there are no data indicating that much of the missing carbon is in the deep ocean (to which the transfer of CO2 would be expected to be very slow), nor are there as yet any observations to suggest that it might have become incorporated into the soil and/or biomass of the continents.
Although there are substantial uncertainties in our understanding of the components of the carbon cycle, the atmospheric concentration of CO2 has been steadily increasing because of human activities2 in a manner that can be described by using a simple empirical relationship; e.g.,
increment in atmospheric CO2 at time t = 60% of accumulated fossil fuel emissions.
However, there is no real assurance that the nature of the unknown sink is consistent with such an extrapolation, which essentially assumes the terrestrial
flux from land cover change. This uncertainty is especially troublesome in discussions of the atmospheric condition at times when CO2 (or the equivalent in other greenhouse gases) may have become twice or more the present value.
In short, there is a clear record of recent increases in atmospheric CO2, a reasonably good estimate of the anthropogenic emissions, and a range of CO2 uptake by the oceans with which the scientific community seems comfortable. However, the panel concludes that there are no data or compelling hypotheses that identify sinks and transfer mechanisms that could redress the observed very large imbalance in this century between the emissions and the known sinks.
The atmospheric concentration of CH4 in 1990, about 1.71 ppmv, is more than double the preindustrial value of about 0.8 ppmv (Craig and Chou, 1982; Blake and Rowland, 1988). Concentrations currently are increasing at about 0.9 percent per year (Blake and Rowland, 1988). Two main pathways for CH4 generation have been identified: (1) reduction of CO2 with hydrogen, fatty acids, or alcohols as hydrogen donors and (2) transmethylation of acetic acid or methyl alcohol by CH4-producing bacteria. The principal phenomena involved include biological processes in natural wetlands and rice paddies, burning of plant material in tropical and subtropical regions, consumption of biomass by termites, anaerobic decay of organic waste in landfills, and ventilation of coal mines. Additional sources include enteric fermentation by ruminant animals, leakage from natural gas pipelines, and venting from oil and gas wells. The amount of CH4 currently released from these various sources, however, is not known with precision. The Intergovernmental Panel on Climate Change (IPCC) estimated 1990 total CH4 releases to be between 400 and 600 teragrams (Tg) per year (Intergovernmental Panel on Climate Change, 1990).
Global measurements suggest that CH4 concentrations are greatest at latitudes poleward of 30°N. The approximately 0.14 ppmv greater concentration of CH4 in the northern hemisphere over the southern hemisphere is not surprising given that most CH4 is produced over land (Wuebbles and Edmonds, 1991).
The major sink for CH4 is reaction with the hydroxyl radical (OH) in the atmosphere. In addition, soils may be a sink for CH4, but this has not yet been determined quantitatively. CH4 has a relatively short lifetime in the atmosphere of 10 ± 2 years (Prinn et al., 1987). Recent work on the reaction of OH with CH4 (Vaghjiani and Ravishankara, 1991) indicates a longer CH4 lifetime (by about 25 percent) and a smaller inferred flux (about 100 Tg CH4 per year) than was previously estimated. Reduction in the amount
of global atmospheric OH may provide an explanation for 20 to 50 percent of the increase in atmospheric CH4 concentration, but this is highly uncertain given the unavailability of reliable direct measurements of tropospheric OH concentrations (Wuebbles and Edmonds, 1991).
Methane emissions from wetlands are sensitive to temperature and soil moisture (Crutzen, 1989). Future climatic changes could thus significantly affect the fluxes of CH4 from both natural wetlands and rice paddies. A highly uncertain but potentially large source of CH4 is the methane hydrates stored in permafrost sediments, CH4 trapped in permafrost, and decomposable organic matter frozen in the permafrost (Cicerone and Oremland, 1988). Future climate warming could release these sources of CH4, thereby further adding to the warming. However, the lowering of the water table in tundra as a result of a warmer, drier climate could decrease CH4 fluxes into the atmosphere owing to increased microbial oxidation. This might provide a negative feedback for atmospheric CH4 (Whalen and Reeburgh, 1990). The net effect of climatic changes on atmospheric concentrations of CH4 is highly uncertain.
In short, the atmospheric concentration of CH4 has more than doubled over preindustrial levels. In addition to many natural sources, there are a number of anthropogenic sources of atmospheric CH4. The extent of relevant human activities has increased over the last 100 years, and an inferred decrease in atmospheric OH during this period also could contribute to the CH4 increase. However, the quantitative importance of each of the factors contributing to the observed increase in CH4 is not known at present.
With the exception of methyl chloride, most halocarbons come entirely from human activities. Atmospheric concentrations of the major anthropogenic halocarbons in 1990 are 280 parts per trillion by volume (pptv) for CFC-11; 484 pptv for CFC-12; 60 pptv for CFC-113; and 146 pptv for carbon tetrachloride (CCl4). Their annual rates of increase over the past few years are 4 percent for CFC-11; 4 percent for CFC-12; 10 percent for CFC-113; and 1.5 percent for CCl4 (Fraser and Derek, 1989; Intergovernmental Panel on Climate Change, 1990; Wuebbles and Edmonds, 1991). Concentrations of the fully halogenated chlorofluorocarbons (CFCs) are slightly greater in the northern hemisphere than in the southern hemisphere. This fact is consistent with the geographic distribution of releases (>90 percent from industrialized nations), a 45°N to 45°S mixing time of about 1 year, and the very long atmospheric lifetimes of these CFCs.
Halocarbons are used as aerosol propellants (CFCs 11, 12, and 114), foam blowing agents (CFCs 11 and 12), solvents (CFC 113, methyl chloroform (CH3CCl3), and CCl4), and fire retardants (halons 1211 and 1301).
Methyl chloride is primarily released from the oceans and during biomass burning.
There is no significant tropospheric removal mechanism for the fully halogenated halocarbons such as CFCs 11, 12, 113, 114, and 115 or for halon-1301. They are primarily removed by photodissociation in the middle to upper stratosphere. The fully halogenated halocarbons therefore have long atmospheric lifetimes, ranging from about 60 years (CFC-11) to about 400 years (CFC-115).
Emissions of many halocarbons are governed by the Montreal Protocol on Substances That Deplete the Ozone Layer. The Protocol specifies that the production and consumption of CFCs 11, 12, 113, 114, and 115 be limited to 50 percent of their 1986 levels by 1998 and of halons to their 1986 levels after 1994. The London Protocol, adopted in June 1990, requires a total phaseout of CFCs, halons, and CCl4 by 2000 in industrialized countries and by 2010 in developing countries. Emissions of these gases will continue after their production has ceased, however, because of their presence in refrigerants, foams, fire retardants, and so on.
The 1990 average atmospheric concentration of nitrous oxide (N2O) is about 310 ppbv and is increasing at a rate of about 0.2 to 0.3 percent per year (Khalil and Rasmussen, 1988). Ice core data suggest that preindustrial atmospheric concentrations of N2O were about 288 ppbv (Khalil and Rasmussen, 1988). The atmospheric concentration of N2O is about 1 ppbv higher in the northern hemisphere than in the southern hemisphere (Rasmussen and Khalil, 1986).
Denitrification in aerobic soils is thought to be a dominant source of atmospheric N2O (Keller et al., 1986). There is, however, considerable uncertainty in the source budget for N2O and in the causes for its increasing concentration in the atmosphere. The two possible sources that most likely account for the observed increase are (1) nitrification and denitrification of nitrogen from industrially produced fertilizers and (2) high-temperature combustion, such as from coal-burning power plants (Wuebbles and Edmonds, 1991). Some results are contradictory, however, as to whether combustion is the major source or totally unimportant (Hao et al., 1987; Muzio and Kramlich, 1988). The oceans also are a significant, although probably not dominant, source of N2O (McElroy and Wofsy, 1986). An accurate determination of the global annual ocean flux is difficult because of uncertainties associated with quantifying the gas exchange coefficient and because the partial pressure of N2O in the surface waters varies considerably, ranging from being supersaturated by up to 40 percent in upwelling regions to being undersaturated by a few percent in areas around Antarctica and in upwelling
currents (Cline et al., 1987). It is still unclear whether N2O is primarily produced from nitrification in near-surface waters or denitrification in oxygen-deficient deep waters. Quantification of global N2O emissions from soils is difficult because of the heterogeneity of terrestrial ecosystems and the variability in environmental conditions. N2O emissions from the use of nitrate and ammonium fertilizers are difficult to quantify because the N2O fluxes depend on numerous factors, including type of fertilizer, soil type, soil temperature, weather, and farming practices. Conversion of nitrogen to N2O ranges from 0.01 to 2 percent (Conrad et al., 1983). Leaching of nitrogen fertilizers into groundwater may result in additional fluxes of N2O into the atmosphere.
The major atmospheric process removing N2O is photochemical decomposition in the stratosphere. N2O is not chemically active in the troposphere. N2O has a lifetime in the atmosphere of about 150 years.
In short, we know that the atmospheric concentration of N2O is now about 8 percent higher than in the preindustrial period. N2O has a relatively long atmospheric lifetime. It is difficult to account for the source of the observed increase in atmospheric N2O, but it is thought to be anthropogenic. The observed rate of growth indicates a 30 percent imbalance between sources and sinks (Hao et al., 1987).
1. Some believe the error for concentrations derived from ice core data may be as large as ±20 ppmv.
2. That human activities are causing the increasing atmospheric concentration of CO2 is known from carbon isotope studies.
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