Methane (CH4) is the most abundant organic species in the atmosphere. Like CO2, CH4 levels have risen dramatically over the last 200 years. Pre-industrial levels, which were ~750 ppb, now stand at ~1760 ppb in the northern hemisphere and ~1630 ppb in the southern hemisphere. CH4 has been increasing at 1.3% per year for most of the 20th century until the early 1990s (Blake and Rowland, 1988) as illustrated in Fig. 11.1. Since then the rate of growth has slowed to 0.6% per year (Steele et al., 1992). There are various reasons for the slowdown in the growth rate. Bekki et al. (1994) have argued that stratospheric ozone depletion was so severe in 1991-1992 that hydroxyl (OH) levels in the troposphere rose due to the increase in the short-wavelength radiation penetrating into the lower atmosphere. Their two-dimensional (2D) model calculations suggest that up to half of the decrease in CH4 growth rate in 1991 and 1992 could be attributed to this rise in OH levels. Wang et al. (2004) used a three-dimensional (3D) atmospheric model to carry out an inverse analysis of the CH4 budget. They concluded that the economic downturn of the former Soviet Union and east-European countries in the early 1990s, together with more efficient industrial practices, has been an important factor in the slowdown of CH4, in addition to potential changes in OH levels.

These model results suggest that the current slowdown may be temporary. Quay et al. (1999) and Fletcher et al. (2004) have used measurements of 13CH4, 12CH4 and 12CH3D to determine changes in source strengths for CH4 over the last 10-15 years. They concluded that there have been decreases in northern hemisphere sources such as bogs and landfills but increases in tropical and southern hemisphere sources such as swamps and biomass burning. Finally, Warwick et al. (2002) have shown that year-to-year fluctuations in meteorology can cause significant interannual fluctuations in the levels of atmospheric CH4 measured at the surface. Therefore, there are several potential reasons why CH4 trends have been fluctuating in recent years.

CH4 is a far more effective greenhouse gas than CO2, being 62 times more effective on a 20-year time horizon and 23 times more effective on a 100-year time horizon. The changes in effectiveness with time reflect the fact that CH4 is much shorter-lived in the atmosphere than CO2. The sources of CH4 are discussed in detail by Reay et al. (Chapter 9, this volume) and

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