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9 Historical Trends in Atmospheric Methane Concentration and the Temperature Sensitivity of Methane Outgassing from Boreal and Polar Regions ROBERT C. HARRISS Langley Research Center National Aeronautics and Space Administration Recent studies have documented two trends in atmospheric methane (CH4) concentrations. First, a modern trend of increas- ing global atmospheric CH4 has been documented in the trapped gas in polar ice cores (Craig and Chou, 1982; Khali! and Rasmussen, 1987) and with regular monitoring of ambient CH4 at remote lo- cations around the world (Steele et al., 1987; Blake and Rowland, 1988~. These data indicate that CH4 has increased from a concen- tration of approximately 650 parts per billion by volume (ppbv) 200 years ago to 1,690 ppbv in 1988 (Figure ~1~. This recent increase in atmospheric CH4 over the past several hundred years correlates with the growth of the human population and industrial society and is hypothesized to be a result of increased CH4 emissions related primarily to the expansion of rice agriculture, domestication of rumi- nant animals, landfi~ling of organic wastes, and the mining and use of fossil fuels (Ehhalt, 1985; Pearman and Eraser, 1988~. A second trend from low concentrations of CH4 (350 ppbv) at times of glacial magnum (approximately 20,000 years B.P.), in- creasing to 650 ppbv during interglacial times (Figure 9-2) has been observed in ice core samples from Antarctica and Greenland (Stauf- fer et al., 1988~. This variability in prehistoric CH4 is hypothesized to be due to the expansion of arctic and boreal peatiands following glacial retreat (Harries et al., 1985~. These high-latitude ecosystems are commonly wetlands with oxygen-deficient, organic-rich soils that 79

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l 80 1 ,600 > Q Q Q A_ z ~ 1,200 z z 800 ROBERT a. HARRISS . #~' +~~ - art; 1600 1700 1800 1900 YEAR FIGURE 9-1 Atmospheric methane (CH4) variations (in ppbv) over the past few centuries. Each point represents measurements or (in the earlier centuries) estimates with error bars. (Adapted from Khalil and Rasmussen, 1987.) 800 > 700 Q ~ 600 c o ._ c 500 a) o In, 400 - 300 200 0 Dye 3 Byrd Station :~l Ti* . . , . , . , . 1 00 80 60 40 20 0 Age (103 years) FIGURE 9-2 Atmospheric methane (CH4) variations in glacial and interglacial times, as determined from ice corings. Abscissa is thousands of years before present. (Reprinted, by permission, from Stouffer et al., 1988. Copyright (~)1988 by Macmillan Magazines Ltd.)

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ATMOSPHERIC METHANE 81 emit CH4 as a byproduct of microbial decomposition processes. It has been estimated that these boreal and arctic peatIands may currently produce about 60 percent of the global CH4 flux to the atmosphere from natural wetlands (Matthews and F ung, 1987~. Methane is also present in large quantities as "frozen" CH4 cIathrates in subsurface sediments of the polar regions, and as trapped gas in permafrost (Bell, 1982; Revelle, 1983~. If substan- tial warming of the polar oceans and landscape were to occur, these "frozen" sources of CH4 would be released to migrate through the soil and overlying sediments into the atmosphere. However, at present this issue is highly speculative. There is little doubt that increasing concentrations of CH4 and other trace gases can have a profound influence on the earth's at- mospheric chemistry and on climate (Thompson and Cicerone, 1986; Ramanathan, 1988~. The question of what influence climate change has on emissions of trace gases from the global biosphere is less certain. If emissions of CH4 are enhanced by global warming, a positive feedback can result, with the increasing concentrations of atmospheric CH4 further enhancing the tendency for a greenhouse warming. Such positive feedback could contribute to abrupt climate changes, resulting in considerable ecological and societal disruption. At present much less is known about negative feedbacks on climate warming such as increased evaporation, which would lead to more clouds and a higher albedo, with subsequent cooling of the earth's surface. Increased atmospheric water vapor could also lead to higher concentrations of atmospheric hydroxy} (OH), which destroys CH4, consequently mitigating the increase in source emissions due to cli- mate warming. Very little quantitative information is currently available on the long-term, integrated response of sources or sinks of atmospheric CH4 to climate change. The data that are available on the temperature sensitivity of CH4 sources from organic soils and sediments clearly indicate that CH4 emission rates increase with increasing surface temperature (Baker-Blocker et al., 1977; Crill et al., 1988~. Fig- ure ~3 illustrates the response of CH4 emissions to seasonal warming of boreal peatiand soib in northern Minnesota. From these data it is reasonable to hypothesize that the initial response to warming of both natural and anthropogenic organic soils and sediments, which are the dominant sources of atmospheric CH4, will be an increasing flux of CH4 to the atmosphere. A large fraction of the worId's old carbon is in boreal and arctic regions, so these ecosystems are of

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82 1 000 - C~ ~100 - X I ROBERT C. HARRISS 10 1 OPEN BOG .` .. S / me,. ~ 5 10 15 SOILTEMPERATURE (C) FIGURE 9-3 Effect of soil temperature on methane (CH4) emissions to the atmosphere from an open bog in the Marcell Experimental Forest, Minnesota. particular importance to understanding the response of CH4 sources to climate change. Thus, if the greenhouse warming preclicted by models (e.g., see Ramanathan, 1988) is realized, it is likely that the rate of increase in atmospheric CH4 will increase further over the next few decades clue to enhanced flux from boreal and arctic wetlands, rice paddy soils, landfills, and other soil and sediment sources. The mid-term (10- to 100-year) response of CH4 sources to a greenhouse warming of the earth's surface is impossible to predict at present. Major scientific issues related to negative feedback mecha- nisms on the CH4 increase (e.g., increases in atmospheric OH and drying of wetland soils in major source regions) must be resolved. Recent advances in environmental measurement technologies and techniques make possible, for the first time, regional- to global-scale quantification of biosphere-atmosphere interactions. The uncertain- ties in how CH4 sources and sinks will respond to future climate change or to socioeconomic developments that influence CH4 sources can be reduced by vigorous research programs in global tropospheric chemistry (NRC, 1984) and earth system science (NRC, 1985~. Spe- cific research to better understand the importance of increasing at- mospheric CH4 as a cause and/or consequence of climate change must include the following components:

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ATMOSPHERIC METHANE 83 1. The quantification of both natural and anthropogenic CH4 sources must be improved. Such improvements will require integrated ground, aircraft, and satellite measurements, which will provide ac- curate estimates of CH4 flux to the atmosphere at regional scales. Isotope studies using newly developed accelerator mass spectromet- ric techniques (Lowe et al., 1988), combined with more detailed temporal resolution from ice core sampling, could resolve the issue of what contribution fossil sources of CH4 make to the variability in atmospheric CH4 over long and short time scales. 2. The boreal and arctic regions store much of the earth's soil carbon in wetlands, which are important sources of atmospheric CH4. It will be especially important to quantify the temperature sen- sitivity of the physical and biological processes responsible for CH4 outgassing from these ecosystems, since they are in regions where climate models predict an enhanced greenhouse warming effect. 3. The impact of increasing concentrations of tropospheric car- bon monoxide, ozone, and CH4 on global OH distributions can be resolved with the availability of quantitative source data and the de- velopment of advanced three-dimensional photochem~cal models for prediction of OH. 4. The flux of CH4 from the troposphere to the stratosphere can be quantified with a comprehensive program of aircraft and satellite measurements. Methane decomposes in the stratosphere to products such as water vapor that can alter chemical and radiative transfer processes. In summary, during the past decade it has become clear that current changes in the earth's atmospheric composition are global in scale. Studies on atmospheric CH4 will provide critical information on how the earth's biosphere and atmosphere will respond to the global warming that is forecast by climate models to occur in the next few decades. Such research will also provide the necessary scientific data to make sound regulatory decisions if policymakers decide to arrest or reverse the growth of sources of CH4 produced by human activities. REFERENCES Baker-Blocker, A., T.M. Donahue, and K.H. Mancy. 1977. Methane flux from wetland areas. Tellus 29:245-250. Bell, P.R. 1982. Methane hydrate and the carbon dioxide question. In Carbon Dioxide Review 1982, W.C. Clark (ed.~. Oxford University Press, New York, pp. 401-406.

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84 ROBERT a. HARRISS Blake, D.R., and F.$. Rowland. 1988. Continuing worldwide increase in tropospheric methane, 1978 to 1987. Science 239:1125-1131. Craig, H., and G.C. Chou. 1982. Methane: Record in polar ice cores. Geophys. Res. Lett. 9:1221-1224. Grill, PdM., K.B. Bartlett, R.~. Harries, E. Gorham, E.S. Verry, D.I. Sebacher, L. Madear, and W. Banner. 1988. Methane flux from Minnesota peatlands. (global Biogeochem. Cycles (in press). Ehhalt' E).~. 1985. Methane in the global atmosphere. Environment 27:6-12. Harries, R.C., E. Gorham, D.I. Sebacher, K.B. Bartlett, and P.A. Flebbe. 1985. Methane flux from northern peatlands. Nature 315:652-653. Khalii, M.A.K., and R.A. Rasmussen. 1987. Atmospheric methane: Trends over the last 10~000 years. Atmos. Environ. 21:2445-2452. Lowe, ma, G.~.M. Brenninkmeijer, M.R. Manning, R. Sparks, and G. Wallace. 1588. Radiocarbon determination clef atmospheric methane at Baring Head, New Zealand. Nature 332:522~524. Matthews, E., and Id Fung. 1987. Methane emission from natural wetlands: Global distribution, area, and environmental characteristics of sources. Global Eliogeochem. Cycles 1:61-86. National Research Council (NRC). 1984. Global Tropospheric Chemistry: A Plan for Action, National Academy Press, Washington, D.C., 194 pp. National Research Council (NRC). 1985. A Strategy for Earth Science from Space in the 1980's and 1990's, Part II: Atmosphere and Interactions with the Solid Earth, Oceans, and Biota, National Academy Press, Washington, O.C., 14g pp. Pearman, (:.I., and P.J. Fraser. 1988. Sources of increased methane. Nature 332:489~490. Ramanathan, V. 1988. The greenhouse theory of climate change: A test by an inadvertent global experiment. Science 240:293-299. Revelle, R.R. 1983. Methane hydrates in continental slope sediments and increasing atmospheric carbon dioxide. Pp. 252-261 in Changing Climate, National Academy Press, Washington, D.C. Stouffer, B., E. Lochbronner, H. Oeschger, and J. Schwander. 1988. Methane concentration in the glacial atmosphere was only half that of the preindus- trial Holocene. Nature 332:812-814. Steele, L.P., P.J. Fraser, R.A. Rasmussen, M.A.K. Khalil, T.J. Conway, A.J. Crawford, RQ Gammon, K.A. Masarie, and K.W. Thoning. 1987. The global distribution of methane in the troposphere. J. Atmos. Chem. 5:125- 171. Thompson, A.M., and R.J. Cicerone. 1986. Possible perturbations to atmo- spheric CO, CH4, and OH. J. Geophys. Res. 91:108S3-10864.