Ice Age Methane Revisited: Oceans, Lightning, and the Steady Wetland Source
Abstract
The concentrations of reactive greenhouse gases in the atmosphere are a result of the interplay between sources on land and in the oceans and the atmospheric sink. Methane (CH4) is the most important of the major, long-lived reactive trace gases, and over the past 400,000 years it has more closely paralleled the higher-frequency component of polar temperature records than any other measured gas. Analyses of ice core CH4 concentrations and carbon isotope composition (δ 13CH4) have suggested that changing CH4 emissions from wetlands drove prehistoric changes in ice-core CH4. As a reactive trace gas, the global CH4 budget is controlled not just by changes in source strength, but also by climate, changes in the flux of other reactive trace gases, and the nonlinear dynamics of atmospheric chemistry.
To investigate the effect of long-term climate change on the atmospheric concentration of CH4 we coupled climate, vegetation, and atmospheric chemistry models to simulate the natural emissions and atmospheric chemistry of the major reactive trace gases. Climate was simulated by a coupled AGCM/mixed-layer ocean model with simulations at 1000-year intervals from the Last Glacial Maximum (LGM, ca. 21 ka) to present. Terrestrial CH4 and Biogenic Volatile Organic Compound (BVOC) emissions were simulated using the BIOME4-TG global vegetation model, with simple algorithms for determining wetland area based on topography and soil moisture, CH4 emissions based on ecosystem carbon turnover in wet soils, and BVOC emissions based on vegetation type and density. We simulated atmospheric chemistry and transport with the LMDz-INCA 3D chemistry-transport model, and included a full prognostic simulation of nitrogen oxide (NOx) emissions from lightning based on simulated convective precipitation. Global wetland area decreased by 1x106 km2 from the LGM to the present (nearly 15%). However, CH4 emissions - 110 Tg yr-1 - were nearly unchanged over this same period. During the Pleistocene-Holocene transition CH4 emissions reached a maximum of ca. 130 Tg. LGM CH4 emissions were ca. 2\permil more depleted in δ 13CH4 compared to present because of the increase in tropical wetland activity relative to northern wetlands. Wetland CH4 emissions did not change drastically during the deglaciation because new wetland areas formed as ice sheets retreated, while other wetland areas were flooded by rising sea-level. Global emissions of BVOC increased significantly from the LGM to present, (350 Tg C yr-1 or 150%) because of increased vegetation density from warming climate and increased atmospheric CO2 concentrations. The simulated increase in sea surface temperatures (SST) from the LGM to present led to increased convective precipitation and a 10-30% increase in NOx emissions from lightning. Observed rapid changes in atmospheric CH4 concentrations over the last 21 ka cannot be completely attributed to climate change on millennial time-scales. However, the simulated changes in both the atmospheric BVOC and NOx burdens, which compete with CH4 as an OH sink, may have increased the lifetime of CH4 on the order of 30% at the present compared to LGM. This strong reduction in CH4 oxidation potential would have had long-term consequences for atmospheric CH4 concentrations and may explain much of the ice-core CH4 record without requiring major changes in the wetland CH4 source.- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2004
- Bibcode:
- 2004AGUSMGC21A..18K
- Keywords:
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- 0315 Biosphere/atmosphere interactions;
- 0322 Constituent sources and sinks;
- 0400 Biogeosciences;
- 1610 Atmosphere (0315;
- 0325);
- 1615 Biogeochemical processes (4805)