Limited response of peatland CHsub4/sub emissions to abrupt Atlantic Ocean circulation changes in glacial climates

摘要

Ice-core records show that abrupt Dansgaard–Oeschger (D–O) climatic warming eventsof the last glacial period were accompanied by large increases in theatmospheric CH₄ concentration (up to 200 ppbv). These abrupt changesare generally regarded as arising from the effects of changes in the AtlanticOcean meridional overturning circulation and the resultant climatic impact onnatural CH₄ sources, in particular wetlands. We use two differentecosystem models of wetland CH₄ emissions to simulate northern CH₄sources forced with coupled general circulation model simulations of fivedifferent time periods during the last glacial to investigate the potentialinfluence of abrupt ocean circulation changes on atmospheric CH₄ levelsduring D–O events. The simulated warming over Greenland of 7–9 °Cin the different time periods is at the lower end of the range of11–15 °C derived from ice cores, but is associated with strongimpacts on the hydrological cycle, especially over the North Atlantic andEurope during winter. We find that although the sensitivity of CH₄emissions to the imposed climate varies significantly between the twoecosystem emissions models, the model simulations do not reproduce sufficientemission changes to satisfy ice-core observations of CH₄ increasesduring abrupt events. The inclusion of permafrost physics and peatland carboncycling in one model (LPJ-WHyMe) increases the climatic sensitivity of CH4₄emissions relative to the Sheffield Dynamic Global Vegetation Model (SDGVM) model, which does not incorporate theseprocesses. For equilibrium conditions this additional sensitivity is mostlydue to differences in carbon cycle processes, whilst the increasedsensitivity to the imposed abrupt warmings is also partly due to the effectsof freezing on soil thermodynamics. These results suggest that alternativescenarios of climatic change could be required to explain the abrupt glacialCH₄ variations, perhaps with a more dominant role for tropical wetlandCH₄ sources.