Introduction

Methane has been increasing in the atmosphere for about two centuries, resulting in current concentrations that are more than twice the natural levels. Because of these trends, methane is considered to be a potentially important contributor to global warming and other man-made environmental changes that may occur in the future. As a greenhouse gas, every gram of methane released to the atmosphere is about 20 to 60 times as effective as a gram of C02, when considered over periods of 20 to 50 years. Moreover, methane has other critical roles in atmospheric chemistry that are also affected by its trends. It exercises a strong influence on the abundance of hydroxyl radicals (OH). These radicals in turn are responsible for removing many man-made and natural gases from the atmosphere. Increasing levels of methane can lead to a lowering of OH levels, that could in turn lead to increases of other gases that may be undesirable. Methane has a complex role in stratospheric chemistry where it is a source of water vapor that tends to deplete the ozone layer but, on the other hand, methane can scavenge chlorine atoms thus protecting the ozone layer from destructive effects of man-made chlorofluorocarbons. In this role high levels of methane are considered desirable. Methane is, therefore, integrally involved in the stability of Earth's environment and, as such, is regarded as one of the important trace gases that are significantly affected by human activities.

We will start by examining the observational data consisting of global distributions and trends. The atmospheric observations are the foundation for most of our current knowledge of the global cycle of methane and our interest in its possible environmental effects. Next we will see how these observations are explained in terms of the processes that produce and destroy methane. Based on the understand-

Handbook of Weather, Climate, and Water: Atmospheric Chemistry, Hydrology, and Societal Impacts, Edited by Thomas D. Potter and Bradley R. Colman. ISBN 0-471 -21489-2 © 2003 John Wiley & Sons, Inc.

Concentration CH, (ppbv)

Concentration CH, (ppbv)

Concentration CH4 (ppbv)

1000 1200 1400 1600 1800 2000 1795 1835 1875 1915 1955 1995

Year Yea-

Figure 1 Concentrations of methane (a) over the last 1000 years and (b) over the last 200 years. Data of Etheridge et al. (1992) (•) and Rasmussen and Khalil (1984) (+) are from ice core samples. Data of Khalil and Rasmussen (o) are global averages from weekly flask samples collected at various latitudes. Trends of methane concentrations (c) during the last 1000 years and (d) over the last 200 years are calculated from the data shown in (a) and (b). Rapid increases in methane started only about 200 years ago. These are linear regression estimates of trends over various (nonoverlapping) periods of time between about a.d. 0 and the present. For data between 1840 and 1940, trends were calculated over 20-year periods; for 1940-1980, over 10-year periods; and for 1980-1992 over 2-year periods. The calculated trends were placed at the middle of the time span in each calculation. The earlier data are more sparse. Trends for the period between a.d. 0 and 1800 were calculated for every 10 data points, and the trends are placed at the average time spanned by the 10 data points.

1000 1200 1400 1600 1800 2000 1795 1835 1875 1915 1955 1995

Year Yea-

Figure 1 Concentrations of methane (a) over the last 1000 years and (b) over the last 200 years. Data of Etheridge et al. (1992) (•) and Rasmussen and Khalil (1984) (+) are from ice core samples. Data of Khalil and Rasmussen (o) are global averages from weekly flask samples collected at various latitudes. Trends of methane concentrations (c) during the last 1000 years and (d) over the last 200 years are calculated from the data shown in (a) and (b). Rapid increases in methane started only about 200 years ago. These are linear regression estimates of trends over various (nonoverlapping) periods of time between about a.d. 0 and the present. For data between 1840 and 1940, trends were calculated over 20-year periods; for 1940-1980, over 10-year periods; and for 1980-1992 over 2-year periods. The calculated trends were placed at the middle of the time span in each calculation. The earlier data are more sparse. Trends for the period between a.d. 0 and 1800 were calculated for every 10 data points, and the trends are placed at the average time spanned by the 10 data points.

ing gained by such a discussion, we can evaluate plausible expectations for future concentrations and the resulting environmental impact of methane.

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