|Title:||Constraints on the Late Holocene Anthropogenic Contribution to the Atmospheric Methane Budget|
|Author:||Mitchell, L., Brook, E., Lee, J., Buizert, C. and Sowers, T.A.|
|Periodical:||American Geophysical Union, Fall Meeting 2013, abstract #B51E-0343|
The origin of the late pre-industrial Holocene (LPIH) increase in atmospheric methane concentrations has been much debated. Hypotheses invoking changes in solely anthropogenic sources or solely natural sources have been proposed to explain the full increase in concentrations. Measurements of methane from Antarctica (the West Antarctic Ice Sheet (WAIS) Divide 05A ice core (WDC05A, ~1.0-0.2 ka [Mitchell et al., 2011]), and WAIS Divide deep ice core (WDC06A, ~4.7-0.2 ka)) as well as Greenland (GISP2D, ~2.8-0.2 ka) have been completed. These records have decadal scale resolution, analytical precision of <3 ppb, and are highly correlated with the only previous high resolution ice core methane record from Law Dome, Antarctica. The high degree of correlation between multiple ice cores demonstrates that the observed multidecadal variability is real and allows us to create a common chronology between these ice cores with unparalleled precision because methane is a globally distributed greenhouse gas. We use these records to construct a high-resolution record of the methane inter-polar difference (IPD). We find that the IPD has remained essentially constant (781 B.C.E.-1803 C.E. mean 41.6 ppb; trend 0.9 ?? 0.3 ppb/ka) throughout the LPIH despite a 115 ppb (17%) increase in the global concentration of methane. The IPD constrains the latitudinal distribution of emissions and we use a box model to demonstrate that LPIH emissions increased primarily in the tropics with secondary increases in the subtropical northern hemisphere. Anthropogenic and natural sources have different latitudinal characteristics, which we exploit to demonstrate that both anthropogenic and natural sources are needed to explain LPIH methane concentration changes. This work demonstrates a need for additional high resolution modeling of wetland emissions during the late Holocene.