Abrupt changes in biomass burning during the last glacial period.

Understanding the causes of past atmospheric methane (CH) variability is important for characterizing the relationship between CH, global climate and terrestrial biogeochemical cycling. Ice core records of atmospheric CH contain rapid variations linked to abrupt climate changes of the last glacial period known as Dansgaard-Oeschger (DO) events and Heinrich events (HE). The drivers of these CH variations remain unknown but can be constrained with ice core measurements of the stable isotopic composition of atmospheric CH, which is sensitive to the strength of different isotopically distinguishable emission categories (microbial, pyrogenic and geologic). Here we present multi-decadal-scale measurements of δC-CH and δD-CH from the WAIS Divide and Talos Dome ice cores and identify abrupt 1‰ enrichments in δC-CH synchronous with HE CH pulses and 0.5‰ δC-CH enrichments synchronous with DO CH increases. δD-CH varied little across the abrupt CH changes. Using box models to interpret these isotopic shifts and assuming a constant δC-CH of microbial emissions, we propose that abrupt shifts in tropical rainfall associated with HEs and DO events enhanced C-enriched pyrogenic CH emissions, and by extension global wildfire extent, by 90-150%. Carbon cycle box modelling experiments suggest that the resulting released terrestrial carbon could have caused from one-third to all of the abrupt CO increases associated with HEs. These findings suggest that fire regimes and the terrestrial carbon cycle varied contemporaneously and substantially with past abrupt climate changes of the last glacial period.