Landfills

Termites

Wetlands

Other Natural Oceans

Fossil Fuel

Ruminants

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The diversity of sources of methane, and their biological nature, results in a complex source pattern, something that makes it quite difficult to assemble an accurate source inventory There are a considerable number of different rice farming practices, for example, all of which result in substantially different amounts of methane emitted per amount of rice produced, or per hectare farmed. On the other hand, this diversity provides a chance for mitigation by the selection of practices that result in low methane emission factors. Asa result, considerable effort has gone into refining our understanding of the processes that control methane emission from the various source types, and into producing more-accurate source estimates.

A very different picture prevails on the sink side of the methane budget. The term labeled "Storage" in Figure 2.2 represents the amount of methane accumulating annually in the atmosphere, a value that can be relatively easily and accurately obtained from the existing measurement network. It appears in the budget as a "sink," in the sense of being a reservoir where some of the methane introduced into the atmosphere ends up, at least for some years. The true sinks - the processes that remove methane from the atmosphere - are dominated by one process, the tropospheric oxidation of methane through its reaction with the hydroxyl radical OIL This sink is so large that even a relatively small uncertainty in its magnitude, some 10%-20%, is as large as many of the individual methane sources.

It may seem, at first glance, that this sink is rather well knovsn and predictable. The global mean O! I concentration can be derived from analysis of the distribution in time and space of compounds such as methyl chloroform, as we have discussed above. The reaction rate constant of methane with OH and the global concentration distribution of methane are also well known. Using present-day measured CH4 and model-predicted OH concentrations, we obtain a photochemical sink that is of the right size to balance the methane budget. Yet how well can we extrapolate this knowledge back into the past, or, more importantly, forward into the future?

Model calculations suggest that tropospheric OH concentrations have decreased by some 25% since the industrial revolution (Crutzen, 1995a). This value is, however, highly uncertain, and other models have predicted much smaller or much larger changes ( Thompson, 1992). Furthermore, there is an ongoing controversy about changes in tropospheric Of I over the past few decades (Krol et al., 1998; Prinn et aL, 1995). The outcome of this discussion has important implications for our understanding of the current methane budget. If the methane lifetime had not been significantly affected by likely changes in OH, the decreasing rate at which methane accumulates in the atmosphere would be consistent with methane sources having reached a plateau, and the atmospheric methane concentration approaching a new steady state {Dlugokencky et aL, 1998). If, on the other hand, OH had been growing significantly over the past decade, we would have to assume that methane sources also are currently increasing.

To look into the future, we must remember that most methane oxidation takes place in the Tropics because of the high concentrations of OH resulting from high amounts of water vapor and high UV flux in that region (Andreae and Crutzen, 1997). The Tropics are also the world's most rapidly changing region. Deforestation of the Amazon Basin and subsequent agricultural and industrial development are likely to

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substantially change the amounts of hydrocarbons and nitrogen oxides released into the tropical atmosphere, resulting in elevated ozone over the region. At the same time, deforestation would change the regional water balance, incl uding the atmospheric water vapor content. Because ozone and water vapor are the precursors of the Ol I radical, these changes must be expected to have a pronounced influence on OII and consequently on the lifetimes of GH4, CO, and all the other atmospheric trace gases that are being removed by reaction with OH. Changes in cloudiness resulting from a perturbation of the hydrological cycle in the humid tropics would also impact the Oil distribution because of the radiative and chemical effects of clouds on this radical (Mauldin et al., 1997),

The challenge of predicting future CH4 Levels is further complicated by the fact that atmospheric CH4, CO, and OH are part of a coupled chemical reaction scheme, with complex, nonlinear behavior resulting from simple perturbations (Prather, 19%). Adding CO to this system actually leads to increased CH4 concentrations, and the couplings and feedbacks in the system result in effects that take longer to decay than the lifetimes of the individual molecules involved. The noniinearities in this system increase with the concentrations of methane (and CO present in the atmosphere); and at methane source fluxes around three times the present size, runaway growth of methane could occur.

When the possibilities of additional feedbacks with climate and biota are considered, even more complex feedbacks can be expected. The warming resulting from the greenhouse effect may release additional methane from wetlands due to enhanced microbial activity at elevated temperatures, at least initially (Caoet al., 1998; Chapman and Thurlow, 1996; Christensen and Cox, 1995; Lashof et al., 1997; Oechel and Yourlitis, 1994). At longer time scales, effects of changing water table levels and soil moisture content resulting from climate change may reverse the direction of this feedback. Global warming may also liberate methane locked into clathrates in continental slope sediments and permafrost (Harvey and Huang, 1995). This effect may be counteracted in part by stabilization of clathrates due to increased pressure resulting from rising sea levels (Gornitz and Fung, 1994). The overall impact of this feedback process is difficult to assess because of great uncertainties about the amounts of CH+ present in clathrates, but it is thought to be important mostly in the more distant future (beyond the 21st century) and for high climate sensitivities.

2.5 Indirect Sources and Sinks of Climatically Active Gases: CO, O3

In the preceding section, we pointed out that gases that are not themselves greenhouse gases may have a climatic effect because they change the rates of production or destruction of greenhouse gases. In this sense, we can attribute a climate forcing and greenhouse warming potential to gases such as CO, which has no significant radiative effect of its own. This is because adding CO to the atmosphere increases the lifetime and abundance of methane, results in the production of ozone, and, following oxidation, adds some CO2 to the atmosphere. W hen these effects were simulated in a photochemical model, the cumulative radiative forcing due to CO emissions exceeded at shorter time

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scales (<15 years) that due to anthropogenic NSO, one of the important greenhouse gases (Daniel and Solomon, 1998).

Tropospheric ozone, a gas that has no direct emission sources, is the third most important greenhouse gas after CO2 and CI 14 (1 ioughton et al., 1996; Portmann et al., 1997; Roelofs et al, 1997; Shine and Förster, 1999; van Dorland et al, 1997), Because the chemical lifetime of ozone in the troposphere is of the same order as the time scales of many atmospheric transport processes (days to weeks), its temporal and spatial distribution is highly in homogeneous. In ¡he absence of vertically resolved and globally representative data sets on O3 concentrations, the climatic effect due to this gas must therefore be estimated based on model calculations. The chemical precursors of ozone are hydrocarbons (including methane and NMHC), CO, and the oxides of nitrogen, NO*. The latter play an especially important role in the ozone budget, because their abundance determines whether the photochemical oxidation of hydrocarbons and (X) results in net O3 production or destruction (Crutzen, 1995b; National Research Council [U.S.J Committee on Tropospheric Ozone Formation and Measurement, 1991).

In many regions of the Earth, especially on the continents, biogenic N\ 11 iC emissions are relatively abundant (Fehsenfeid et al., 1992; Guenther el al., 1995), and, in the absence of strong NOx emissions, their photooxidation results in net O3 destruction, When NOx emissions in these regions increase due to development or because deforestation lets NOx from soil microbial production escape more readily into the troposphere, the system can switch to net O3 production, strongh enhancing ozone levels (Keller et al., 1991). This is especially critical in the Tropics, where O; can be entrained into the intertropical convergence zone (ITC2) transported by deep convection into the upper troposphere, where it has the strongest climatic effect. Modeling studies suggest that input of pollutants into convective regions may have strong effects on O3 levels in the free troposphere (Ellis et al., 1996).

Figure 2A shows the sources of nitrogen oxides in the troposphere. Of particular importance to the iropical atmosphere are the emissions from biomass burning, most

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