Discussion And Commentary

The state of knowledge about the methane cycle is that we have a clear understanding of the global distributions and trends in recent decades and over the last century for which ice core data are used. This is a directly measurable component of the global balance. There are no substantive differences among the various groups who have measured methane in the atmosphere. Field studies of emissions from the various sources are also in broad agreement, and the differences that have been Observed are explained by environmental variables. Extrapolation of the field data to global emission rates remains a major source of uncertainty, leaving a sizable uncertainty in the estimates of global emissions from each source. The main features of the trends, both the increases over the last century and the slowdown of the trend in recent times, are consistent with what we know about the change of emissions from anthropogenic sources. There is enough uncertainty that trends caused by changes of OH concentrations can be accommodated.

Although the current understanding of the methane distribution and trends can be explained by the known sources and sinks, the very nature of these explanations clouds our ability to predict future concentrations. We see that the major anthropogenic sources—rice fields, cattle and also biomass burning—are all stabilizing not because of legislated controls, but because there are natural limitations to the growth of these sources. These sources will not keep pace with increasing population as new technologies make it unnecessary to do so. For instance, new high yielding varieties of rice do not require as much land or time in the growing season to produce the same amount of rice as before, thus reducing the emissions of methane per bushel of rice grown. If the anthropogenic sources could be related to population in the future, it would then be easier to predict future emissions under various assumptions of population growth—but this is not possible as we have discussed. Various scenarios that had been hypothesized are no longer likely (Alcamo et al., 1995). Past estimates of the doubling of methane to 3 to 4 ppmv are now unlikely with no known sources that could increase sufficiently to cause such high concentrations. Perhaps the only prediction that can be made is that it is quite unlikely that the concentrations of methane will increase substantially or double in the next decade or two. This is good news for global warming since it is not likely to be as much as previously expected from the increase of methane. As such, methane will continue to play an important role in the global environment, but this role is not likely to increase for years to come.

REFERENCES

Alcamo, J., A. Bouwman, J. Edmonds, A. Grübler, T. Morita, and A. Sugandhy, An evaluation of the IPCC IS92 emission scenarios, in J. T. Houghton, L. G. Meira Filho, J. Bruce, H. Lee, B. A. Callander, E. Haites, N. Harris, and K. Maskell (Eds.), Climate Change 1994, Radiative Forcing of Climate Change and an Evaluation of the IPCC IS92 Emission Scenarios, Intergovernmental Panel on Climate Change, Cambridge University Press, Great Britain, 1995.

Bergamaschi, P., and G. W. Harris, Measurement of stable isotope ratios (13CH4/12CH4; 12CH3D/12CH4) in landfill methane using a tunable diode laser absorption spectrometer, Global Biogeochem. Cycles, 9, 439 447, 1995.

Blake, D. R., and F. S. Rowland, Continuing worldwide increase in tropospheric methane, 1978 to 1987, Science, 239, 1129-1131, 1988.

Brown, M., Deduction of emissions of source gases using an objective inversion algorithm and a chemical transport model, J. Geophys. Res., 98, 12639-12660, 1993.

Chappellaz, J., J. M. Barnola, D. Raynaud, Y. S. Korotkevich, and C. Lorius, Ice-core record of atmospheric methane over the past 160,000 years, Nature, 345, 127-131, 1990.

Crutzen, P. J., and P. H. Zimmermann, The changing photochemistry of the troposphere, Tellus, 43AB, 136-151, 1991.

DeMore, W. B., S. P. Sander, C. J. Howard, A. R. Ravishankara, D. M. Golden, C. E. Kolb, R. F. Hampson, M. J. Kurylo, and M. J. Molina, Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling, JPL Publication 97-4, National Aeronautics and Space Administration Jet Propulsion Laboratory, Pasadena, CA, 1997.

Dlugokencky, E. J., L. P. Steele, P. M. Lang, and K. A. Masarie, The growth rate and distribution of atmospheric methane, J. Geophys. Res., 99, 17021-17043, 1994.

Etheridge, D. M., G. I. Pearman, and P. J. Fraser, Changes in tropospheric methane between 1841 and 1978 from a high accumulation-rate Antarctic ice core, Tellus, 44B, 282-294, 1992.

Fabian, P., R. Borchers, G. Glentje, W. A. Matthews, W. Seiler, H. Giehl, K. Bunse, F. Müller, U. Schmidt, A. Volz, A. Khedim, and F. J. Johnen, The vertical distribution of stable trace gases at mid-latitudes, J. Geophys. Res., 86, 5179-5184, 1981.

Fung, I., J. John, J. Lerner, E. Matthews, M. Prather, L. P. Steele, and P. J. Fraser, Three-dimensional model synthesis of the global methane cycle, J. Geophys. Res., 96, 1303313065, 1991.

Graedel, T. E., and W. C. Keene, Tropospheric budget of reactive chlorine, Global Biogeochem. Cycles, 9, 47-77, 1995.

Hein, R., P. J. Crutzen, and M. Heimann, An inverse modeling approach to investigate the global atmospheric methane cycle, Global Biogeochem. Cycles, 11, 43-76, 1997.

Keene, W. C., A. A. P. Pszenny, D. J. Jacob, R. A. Duce, J. N. Galloway, J. J. Schultz-Tokos, H. Sievering, and J. F. Boatman, The geochemical cycling of reactive chlorine through the marine troposphere, Global Biogeochem. Cycles, 4, 407—430, 1990.

Khalil, M. A. K., and R. A. Rasmussen, Sources, sinks, and seasonal cycles of atmospheric methane, J. Geophys. Res., 88, 5131-5144, 1983.

Khalil, M. A. K., and R. A. Rasmussen, Atmospheric methane: Recent global trends, Environ. Sei. Technol., 24, 549-553, 1990a.

Khalil, M. A. K., and R. A. Rasmussen, Constraints on the global sources of methane and an analysis of recent budgets, Tellus, 42B, 229-236, 1990b.

Khalil, M. A. K., and R. A. Rasmussen, Decreasing trend of methane: Unpredictability of future concentrations, Chemosphere, 26, 595-608, 1993.

Khalil, M. A. K., and M. J. Shearer, Sources of methane: An overview, in M. A. K. Khalil (Ed.), Atmospheric Methane: Sources, Sinks, and Role in Global Change, NATO ASI Series I: Global Environmental Change, Vol. 13, Springer-Verlag, Berlin, 1993.

Khalil, M. A. K., and R. A. Rasmussen, Global emissions of methane during the last several centuries, Chemosphere, 29, 833-842, 1994.

Khalil, M. A. K., R. A. Rasmussen, and F. Moraes, Atmospheric methane at Cape Meares: Analysis of a high resolution data base and its environmental implications, J. Geophys. Res., 98, 14753-14770, 1993.

Khalil, M. A. K., R. A. Rasmussen, and M. J. Shearer, Trends of atmospheric methane during the 1960s and 1970s, J. Geophys. Res., 94, 18279-18288, 1989.

Khalil, M. A. K., M. J. Shearer, and R. A. Rasmussen, Atmospheric methane over the last century, World Resource Rev., 8, 481^492, 1996.

Krol, M., P. J. van Leeuwen, and J. Lelieveld, Global OH trend inferred from methylchloroform measurements, J. Geophys. Res., 103, 10697-10711, 1998.

Lassey, K. R., D. C. Lowe, C. A. M. Brenninkmeijer, and A. J. Gomez, Atmospheric methane and its carbon isotopes in the Southern Hemisphere: Their time series and an instructive model, Chemosphere, 26, 95-109, 1993.

Lu, Y., and M. A. K. Khalil, Tropospheric OH: Model calculations of spatial, temporal, and secular variations, Chemosphere, 23, 397^144, 1991.

Madronich, S., and C. Granier, Impact of recent total ozone changes on tropospheric ozone photodissociation, hydroxyl radicals, and methane trends, Geophys Res. Lett., 19,465-461, 1992.

Pinto, J. P., and M. A. K. Khalil, The stability of tropospheric OH during ice ages, inter-glacial epochs and modern times, Tellus, 43B, 347-352, 1991.

Prather, M., R. Derwent, D. Ehhalt, P. Fraser, E. Sanhueza, and X. Zhou, Other trace gases and atmospheric chemistry, in J. T. Houghton, L. G. Meira Filho, J. Bruce, H. Lee, B. A. Callander, E. Haites, N. Harris, and K. Maskell (Eds.), Climate Change 1994, Radiative Forcing of Climate Change and an Evaluation of the IPCC IS92 Emission Scenarios, Intergovernmental Panel on Climate Change, Cambridge University Press, Great Britain, 1995.

Quay, P. D., S. L. King, J. Stutsman, D. O. Wilbur, L. P. Steele, I. Fung, R. H. Gammon, T. A. Brown, G. W. Farwell, P. M Grootes, and F. H. Schmidt, Carbon isotopic composition of atmospheric CH4: Fossil and biomass burning source strengths, Global Biogeochem. Cycles, 5, 25^7, 1991.

Rasmussen, R. A., and M. A. K. Khalil, Increase in the concentration of atmospheric methane, Atmos. Environ., 15, 883-886, 1981.

Rasmussen, R. A., and M. A. K. Khalil, Atmospheric methane in the recent and ancient atmospheres: Concentrations, trends, and interhemispheric gradient, J. Geophys. Res., 89, 11599-11605, 1984.

Schmidt, U., A. Khedim, D. Knapsa, G. Kulessa, and F. J. Johnen, Stratospheric trace gas distributions observed in different seasons, Adv. Space Res., 4, 131-134, 1984.

Schmidt, U., G. Kulessa, E. Klein, E.-P. Roth, P. Fabian, and R. Borchers, Intercomparison of balloon-borne cryogenic whole air samplers during the MAP/GLOBUS 1983 campaign, Planet. Space Sci., 35, 647-656, 1987.

Singh, H. B., and J. F. Kasting, Chlorine-hydrocarbon photochemistry in the marine troposphere and lower stratosphere, J. Atmos. Chem., 7, 261-285, 1988.

Steele, L. P., E. J. Dlugokencky, P. M. Lang, P. P. Tans, R. C. Martin, and K. A. Masarie, Slowing down of the global accumulation of atmospheric methane during the 1980's, Nature, 358, 313-316, 1992.

Stevens, C. M., and A. Engelkemeir, Stable carbon isotope composition of methane from some natural and anthropogenic sources, J. Geophys. Res., 93, 725-733, 1988.

Taylor, F. W., A. Dudhia, and C. D. Rogers, Proposed reference models for nitrous oxide and methane in the middle atmosphere, in G. M. Keating (Ed.), Handbook for MAP, Vol. 31, 1989, pp. 67-79. Middle Atmosphere Program 1SCU SCOTEP, U. of Illinois, Urbana , IL, USA.

Thompson, A. M., The oxidizing capacity of the Earth's atmosphere: Probable past and future changes, Science, 256, 1157-1165, 1992.

Thompson, A. M., J. A. Chappellaz, and I. Y. Fung, The atmospheric CH4 increase since the Last Glacial Maximum (2) Interactions with oxidants, Tellus, 45B, 242-257, 1993.

106 ATMOSPHERIC METHANE

Tyler, S. C., Stable carbon isotope ratios in atmospheric methane and some of its sources, J.

Geophys. Res., 91, 13232-13238, 1986. Wahlen, M., N. Tanaka, R. Henry, B. Deck, J. Zeglen, J. S. Vogel, J. Southon, A. Shemesh, R. Fairbanks, and W. Broecker, Carbon-14 in methane sources and in atmospheric methane: The contribution from fossil carbon, Science, 245, 286-290, 1989.

0 0

Post a comment