Methane

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Methane is a greenhouse gas, 20 times more powerful per molecule than CO2 at current concentrations (see Chapter 4). Methane has natural sources as well as additional anthropogenic sources to the atmosphere (Table 10.1). Once released into the atmosphere, methane reacts slowly with activated oxygen compounds to oxidize back to CO2. The reactive oxygen compounds are produced by sunlight. In the absence of sunlight methane and O2 gas coexist in ice core bubbles for hundreds of thousands of years with no reaction. Put it in the sunlight, and it slowly burns up.

The rate of methane emission and the atmospheric lifetime of methane together determine the concentration of methane in the atmosphere. Let's assume that the methane release rate is steady from one year to the next for a long time. Let's also assume that the emission of methane must be balanced by the rate of methane decomposition. The fluxes balance exactly if the methane concentration were constant with time. As it is, methane is rising with time, so our steady-state assumption is not strictly correct, but it is close enough to be useful. The steady-state assumption is

Emission [Gton C/year] = Decomposition [Gton C/year]

The lifetime of a methane molecule in the present-day atmosphere is about a decade. Methane is consumed by reactive oxygen compounds in the atmosphere, in particular, a molecule called OH radical. OH radical is related to ozone, so one could imagine a change in OH radical driven by the change in ozone chemistry of the atmosphere. It could be that if the methane concentration were higher, the lifetime might be longer because degradation might be limited by the availability of OH radical molecules. If however we toss out these potential complications, and just assume that the atmospheric lifetime of methane is and will always be a decade, we could just write that the degradation rate of methane is

Decomposition [Gton C/year] = Inventory [Gton C]/Lifetime [years]

Combining these two equations, we get

Emission [Gton C/year] = Inventory [Gton C]/Lifetime [years] Rearranging,

Inventory [Gton C] = CH4 emission [Gton C/year] * Lifetime [years]

This relation tells us that the methane concentration in the atmosphere is linearly related to the methane source to the atmosphere, as long as the lifetime of methane stays the same. If we doubled the emission, after a few decades, the steady-state concentration would double. The real world may be a bit more complicated because the lifetime might change as the methane concentration goes up.

One of the natural sources of methane to the atmosphere is the degradation of organic carbon in freshwater swamps. Organic carbon degrades first by reaction with O2, as we have discussed in Chapter 8. In seawater, after the O2 is gone, organic carbon reacts with sulfate ion, SO2-, to produce hydrogen sulfide, H2S. After oxygen and sulfate are depleted, methane is produced from organic carbon by fermentation. This is how the methane is produced that freezes into clathrate deposits below the sea floor.

In freshwater, there is not much SO4-, so as soon as oxygen is gone, methane production begins. Methane is found much shallower in freshwater mud than in salt water mud. If you step in mucky swampy freshwater mud, you may see bubbles of methane rising up around your legs. Methane is sometimes referred to as swamp gas for this reason, and it is one of the usual suspects blamed for sightings of UFOs, flying saucers. Bubbles of ancient atmosphere preserved in ice cores tell us that the methane concentration has fluctuated with climate state over the past 400,000 years (the longest ice core yet available), with lower methane concentrations during colder, drier climate stages (Fig. 8.3). This is interpreted to be the result of the changing abundance of swamps.

Anthropogenic sources of methane include production in the guts of ruminant animals, and release as leakage by the fossil fuel industry. Rice paddies often provide ideal anoxic freshwater environments for methane production. The methane concentration has doubled over its preanthropogenic concentration (Fig. 10.1), and is responsible for a quarter of anthropogenic greenhouse heat trapping (Fig. 10.2).

There are two main anthropogenic sources of CO2 to the atmosphere. One is deforestation. A heavily wooded forest holds more carbon per area than does a plowed agricultural field. The world's forests started to feel the axe thousands of years ago with the development of agriculture and the growing of the human population. Most of the temperate latitudes have been cut long since, and the tropics are currently being cut. The year 1750 has been taken as the beginning of the anthropogenic CO2 rise, although it has been argued that both CO2 and methane may have started rising from their "natural" trajectories thousands of years ago. The rise in atmospheric CO2 after sac sec

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Fig. 10.1 History of CO2 and CH4 concentrations in the atmosphere, from ice cores (symbols) and atmospheric measurements (solid lines). Replotted from IPCC (2001).

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Fig. 10.1 History of CO2 and CH4 concentrations in the atmosphere, from ice cores (symbols) and atmospheric measurements (solid lines). Replotted from IPCC (2001).

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