Methane past concentrations

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) Qouzel 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 modem 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 modem values in excess of 1700-1800 (Fig. 18-15). Over tens of millennia, methane variations appear to correspond to northern hemisphere insolation changes, correlate with Vostok paleotemperatures (Chappel-... 

Brook, E. J., Sowers. T., and Orchardo, J. (1996). Rapid variations in atmospheric methane concentration during the past 110 000 years. Science 273, 1087-1091. 

Fig. 22.5. Concentrations of components (sulfate, sulfide, carbonate, methane, and acetate) and species (O2 and H2) that make up redox couples, plotted against temperature, during a model of the mixing of fluid from a hot subsea hydrothermal vent with cold seawater. Model assumes redox couples remain in chemical disequilibrium, except between 02(aq) and H2(aq). As the mixture cools past about 38 °C, the last of the dihydrogen from the vent fluid is consumed by reaction with dioxygen in the seawater. At this point the anoxic mixture becomes oxic as dioxygen begins to accumulate.

Atmospheric concentrations of (a) carbon methane, and (c) nitrous oxide over the past... 

Although atmospheric methane concentrations appear to have stabilized over the past few decades, melting of gas hydrates in permafrost and shallow marine sediments have the potential to rapidly release large quantities of this potent greenhouse gas. As noted in... 

FIGURE 14.17 Atmospheric methane concentrations over the past 1000 years. Different symbols represent data from ice cores in Antarctica and Greenland and the Antarctic firm layer. Line from 1978 includes air measurements at Cape Grim, Tasmania (adapted from Etheridge et al., 1998). 

There is growing evidence that the composition of the troposphere is changing. For example, analysis of historical ozone records has indicated that tropospheric ozone levels in both hemispheres have increased by a factor of 3-A over the last century. Methane concentrations have effectively doubled over the past 150 years and N2O levels have risen by 15% since pre-industrial times.Measurements of halocarbons have shown that this group of chemically and radiatively important gases to be increasing in concentration until relatively recently. 

Figure 5 Variability of (a) methane (using the same dataset as that of Figure 4(b)) and (b) carbon dioxide (using the same dataset as that of Figure 4(a)) concentrations over the past 1,000 yr. (c) The interpolar difference in methane, which is the difference between the Greenland ice-core record (d) and the Law Dome methane record (e, same as curve a) for corresponding years. Estimated 1 cr uncertainty for the interpolar difference is 10 ppb (Etheridge et al. (1998) reproduced by permission of the American Geophysical Union from J. Geophys. Res., 1998, 103, 15979-...

Figure 16. Temperature anomalies and methane and carbon dioxide concentrations over the past 220,000 wars as derived from the ice core records at Vostok, Antarctica (Schimel et al, 2000).

Fig. 16-3 Illustration of the linked behavior of radiatively important trace species concentrations over different time-scales. (a, b, c) Concentrations of methane, nss-SOj , and CO2 over the past 160 000 years found in ice cores from Vostok, Antarctica (temperature deduced from 02 also shown), (d, e, f) Secular trends in nitrous oxide, methane, and CO2 over the past 300 years, (g, h, i, j, k) Changes in October stratospheric ozone column burden over Antarctica, and chloroflourocarbon, methane, sulfate, and nitrate from south Greenland ice, and carbon dioxide concentrations over the past 30 years. Figures adapted from (a) Chappellaz et al. (1990) with the permission of Macmillan Magazines Ltd. (b) Legrand et al. (1988) with the permission of Macmillan Magazines Ltd. (c) Bamola et al. (1987) with the permission of Macmillan Magazines Ltd. ...

Because huge quantities of methane are buried in the seafloor, mainly as gas hydrates, there are concerns about potential climate effects, especially because methane has a greater greenhouse effect than C02. The concentration of atmospheric methane is also presently increasing at a more rapid rate than that of C02 (IPCC, 2001). Therefore, the abundance, distribution (in space and time) and the stability of methane hydrates have important implications for future and past global climate changes. 

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