Simulating idealized Dansgaard-Oeschger events and their potential impacts on the global methane cycle

Abstract

Ice-core records indicate that during the last glacial period, atmospheric CH 4 and Greenland temperature rose abruptly in a series of Dansgaard-Oeschger (D-O) events. These increases attained up to two-thirds of the glacial-interglacial amplitude for CH 4. We use these major changes as possible constraints on the mechanisms of D-O variability. A series of simulations are performed with a coupled atmosphere-ocean general circulation model with a time-dependent freshwater forcing which induces rapid variations in the Atlantic meridional overturning circulation (AMOC). The transient climate output is then used to drive a dynamic vegetation model which simulates wetland CH 4 emissions. During freshwater input, emissions are reduced and, in sensitivity simulations, demonstrate strong dependence on the background climate (orbital insolation or ice sheet extent). In the reverse situation, the strengthening of the AMOC leads to warming over Northern Eurasia, but only minor change to the hydrological cycle in the tropics where most CH 4 emissions occur. This is a robust result from the climate models examined and is of central importance to our results, which suggest that AMOC driven CH 4 variations are considerably smaller than observed D-O events. Modelled emissions of volatile organic compounds (VOC) lead to a significant effect on the CH 4 lifetime during these events, but both the VOC emissions model and VOC effects on CH 4 lifetime are subject to uncertainty. The model results can only be reconciled with some of the larger changes in the ice-core record by including the effect of VOCs and by taking the total amplitude of AMOC perturbation from weak to very strong. Our results suggest that either the model is too insensitive to change, or that further mechanisms may be important for the large CH 4 changes during D-O events. In particular, a strengthening of the AMOC alone cannot reproduce the observed abrupt CH 4 increases. Comparisons with independent models would help to identify possible avenues for further work.