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.
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