Fig. 18-15 Ice-core methane record from Vostok, Antarctica, for the past 220 000 years (Chappellaz et al, 1990; louzel et al, 1993).

1850 to 15ppbv/yr in 1975 (Etheridge et al, 1992), with the exception of a period from 1920 to 1945 when the growth rate was stable. The general increase in growth rate during the industrial revolution presumably reflects the expansion of anthropogenic methane sources.

Longer ice-core records show that methane concentrations have varied on a variety of time scales over the past 220 000 years (Fig. 18-15) (Jouzel et al, 1993; Brook et al., 1996). Wetlands in tropical (30° S to 30° N) and boreal (50° N to 70° N) regions are the dominant natural methane source. As a result, ice-core records for preanthropogenic times have been interpreted as records of changes in methane emissions from wetlands. Studies of modern wetlands indicate that methane emissions are positively correlated with temperature, precipitation, and net ecosystem productivity (Schlesinger, 1996).

Over the past 220 000 years methane concentrations ranged between 350 and 750ppbv, compared to modern values in excess of 17001800 (Fig. 18-15). Over tens of millennia, methane variations appear to correspond to northern hemisphere insolation changes, correlate with Vostok paleotemperatures (Chappel laz et al, 1990) and have a strong 20 000 year periodicity, corresponding to the Earth's precession cycle, which dominantly influences low-latitude climate. Changes in tropical monsoon strength have been proposed as a possible link; a more active monsoon and warmer temperatures could cause both greater wetland area and greater methane production in the tropics (Chappellaz et al, 1990). Recently, analysis of new 400000+ year record from the Vostok site (Delmotte et al, 1998) suggests that a 40 000 year periodicity is also evident.

High-resolution methane records from Greenland (which in some cases resolve sub-century variation) considerably expand this picture of methane variability (Chappellaz et al, 1993; Brook et al., 1996). These records confirm the long-term patterns but also show higher-frequency millennial-scale variability closely associated with rapid temperature changes inferred from isotopic records; all of the interstadial events in the central Greenland d180 record have correlated increases in atmospheric methane mixing ratio (Fig. 18-16). The methane response to rapid climate change during the last glacial period demonstrates that these rapid

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