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Title: New high-precision, high-resolution records of atmospheric methane from Greenland and Antarctic ice cores: 0-1800 A.D
Author: Mitchell, L.E. and Brook, E.
Periodical: American Geophysical Union, Fall Meeting 2009, abstract #PP41B-1524
Abstract: Atmospheric methane has caused the second largest increase in radiative forcing from greenhouse gases since the start of the industrial revolution, and methane sources and sinks are sensitive to climate variations. 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 A.D., [Mitchell et al., in prep.]) and preliminary results from the WAIS Divide deep ice core (WDC06A, 0-1270 A.D.) and the Greenland ice core (GISP2D, 0-1800 A.D.). These records have decadal scale temporal resolution and an analytical precision of <3 ppb, and when complete will provide a high resolution interpolar gradient (IPG) record allowing investigation of geographical changes in atmospheric methane sources on multi-decadal timescales. The WDC05A methane record is highly correlated with the only previous high resolution record which comes from Law Dome (r2 = 0.73). The high degree of correlation increases confidence in both records and shows that the multidecadal variability present in both records is real. An offset between the gas age timescales is apparent and the maximum correlation (r2 = 0.81) is attained when one of the timescales is shifted by 9 years which is within the published uncertainty of ± 10 years for WAIS Divide [Mischler et al., accepted]. Large changes in methane concentration during large, rapid climate changes (i.e. Dansgaard-Oeschger events) have been used to synchronize gas age timescales from a variety of ice cores. Our results demonstrate that high-precision records of methane with decadal scale sample resolution from ice cores that experience moderate smoothing from the firn can be used to synchronize gas age timescales between different ice cores on fine time scales. Finally we compare these atmospheric methane records with numerous paleoclimate archives that could have played a dominant role in regulating past emissions, namely records of temperature, hydrologic, and anthropogenic changes. We find no correlation with hemispheric scale temperature reconstructions, but it is possible that specific events occurring in methane source regions such as the large temperature drop from 1550-1600 A.D. in the high latitude northern hemisphere were significant enough to cause a reduction in global methane concentration. We have also identified a possible correlation with a proxy for East Asian monsoon strength. Comparison with records of anthropogenic activities does not provide compelling evidence for humans having a significant impact on methane emissions prior to the start of the Industrial Revolution in the 18th century.
Year: 2009