Unexpected Changes to the Global Methane Budget over

We report a 2000-year Antarctic ice-core record of stable carbon isotope mea-surements in atmospheric methane (d13CH4). Large d 13CH4 variations indicate that the methane budget varied unexpectedly during the late preindustrial Holocene (circa 0 to 1700 A.D.). During the first thousand years (0 to 1000 A.D.), d13CH4 was at least 2 per mil enriched compared to expected values, and during the following 700 years, an about 2 per mil depletion occurred. Our modeled methane source partitioning implies that biomass burning emissions were high from 0 to 1000 A.D. but reduced by almost È40 % over the next 700 years. We suggest that both human activities and natural climate change influenced preindustrial biomass burning emissions and that these emissions have been previously understated in late preindustrial Holocene methane budget research. Methane is an important greenhouse gas, and measurements of its atmospheric concentra-tion, ECH 4 ^, from ice cores demonstrate a slow increase over the late preindustrial Holocene (the LPIH, circa 0 to 1700 A.D.) (1) and a rapid increase to unprecedented levels over recent centuries (1–3). However, the causes of these variations are not well understood (1–3), because emission rates from the diverse methane sources are spatially and temporally variable, sometimes small, and distributed globally. Additionally, methane emissions vary with climate (1) and possibly with preindustrial human activities such as rice cultivation, cattle farming, and biomass burning (4, 5). The sources of atmospheric methane can be separated into three general categories based on their stable carbon isotope ratios d13C