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Title: Multidecadal variability of atmospheric methane and the Inter Polar Gradient: 0-1800 C.E
Author: Mitchell, L. and Brook, E.
Periodical: American Geophysical Union, Fall Meeting 2010, abstract #C23D-04
Abstract: Atmospheric methane is a potent greenhouse gas that is responsible for ~20% of the total increase in radiative forcing since the industrial revolution. Despite its importance there is a lack of scientific understanding regarding the controls on sources and sinks. Here we present high-precision, high-resolution records of atmospheric methane from the West Antarctic Ice Sheet (WAIS) Divide 05A ice core (WDC05A, 1000-1800 C.E., [Mitchell et al., submitted.]) and preliminary measurements from the WAIS Divide deep ice core (WDC06A, 0-1800 C.E.) and the Greenland ice core (GISP2D, 0-1800 C.E.). 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 which comes from Law Dome, Antarctica. The high degree of correlation between multiple ice cores demonstrates that the multidecadal variability is real and presents an opportunity to investigate the causes of this variability. We have compared our methane records to paleoclimate proxies for temperature and precipitation as well as anthropogenic activities which could have affected methane emissions over the past millennium. On multidecadal time scales there are generally low correlations with temperature and precipitation reconstructions with regional to global spatial scales. Correlations with temperature reconstructions are dominated by changes in northern hemisphere high latitudes between 1400-1600 C.E. suggesting that temperature changes during this time period affected methane emissions. Times of war and plague when large population losses could have reduced anthropogenic emissions appear coincident with periods of decreasing global methane concentrations however anthropogenic activity cannot explain all of the observed variability. We conclude that multidecadal variability of methane over the past millennium was not controlled by temperature, precipitation, or anthropogenic activity alone and instead by some combination of these parameters. Methane records from Antarctica and Greenland can be used to reconstruct the methane Inter-Polar Gradient (IPG) which is controlled by the latitudinal distribution of sources and can provide an additional constraint on possible source scenarios. Preliminary measurements reveal that the IPG over 0-1800 C.E. is ~43 ppb and has not changed significantly over this time interval despite an increase of ~40 ppb in global concentrations. This indicates that the latitudinal distribution of methane sources has also not experienced significant changes. Initial efforts to model the IPG will be presented.
Year: 2010