1. Alfred Wegener Institute for Polar and Marine Research, Columbusstrasse, 27568 Bremerhaven, Germany
2. Laboratoire de Glaciologie et Geophysique de l'Environnement, CNRS-UJF, 54 rue Molière, 38400 Grenoble, France
3. Climate and Environmental Physics, Physics Institute, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
4. Present address: Centre for Ice and Climate, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen OE, Denmark.

Correspondence to: Hubertus Fischer¹ Correspondence and requests for materials should be addressed to H.F. (Email: hubertus.fischer@awi.de).

Abstract

Past atmospheric methane concentrations show strong fluctuations in parallel to rapid glacial climate changes in the Northern Hemisphere^{1, 2} superimposed on a glacial–interglacial doubling of methane concentrations^{3, 4, 5}. The processes driving the observed fluctuations remain uncertain but can be constrained using methane isotopic information from ice cores^{6, 7}. Here we present an ice core record of carbon isotopic ratios in methane over the entire last glacial–interglacial transition. Our data show that the carbon in atmospheric methane was isotopically much heavier in cold climate periods. With the help of a box model constrained by the present data and previously published results^{6, 8}, we are able to estimate the magnitude of past individual methane emission sources and the atmospheric lifetime of methane. We find that methane emissions due to biomass burning were about 45 Tg methane per year, and that these remained roughly constant throughout the glacial termination. The atmospheric lifetime of methane is reduced during cold climate periods. We also show that boreal wetlands are an important source of methane during warm events, but their methane emissions are essentially shut down during cold climate conditions.

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