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Title: The isotope records from WAIS Divide and US ITASE: climate in West Antarctica over the past two millennia (Invited)
Author: Steig, E.J., White, J.W., Kuettel, M., Ding, Q., Hoffmann, G., Schneider, D.P., Mayewski, P.A., Dixon, D.A. and Taylor, K.
Periodical: American Geophysical Union, Fall Meeting 2010, p.01
Abstract: Central West Antarctica has warmed at the surface and in the troposphere over the last 50 years. We use stable isotope records from US ITASE and the West Antarctic Ice Sheet (WAIS) Divide ice cores to place this warming in a long term context. The mean of the 14 records provides an estimate of mean δ18O in precipitation over West Antarctica for the last 200 years, while the standard error provides an estimate of the uncertainty in individual annual or decadal values. Given a linear δ18O/temperature scaling of 0.8‰/°C, the ice core isotope ratios show same the magnitude of warming since 1957 as instrumental temperatures. It is very likely (95% confidence) the two decades 1935-1944 and 1991-2000 are the most isotopically elevated in the last 200 years, and likely (90% confidence) that the 1990s were more elevated than the 1935-1945 decade. This is significant because these two decades are also the warmest globally. Winter warming in West Antarctica is strongly related to atmospheric circulation anomalies forced by warming in the central tropical Pacific (Ding et al., this meeting, and in review), which was also anomalously warm in 1935-1944 and the 1990s. Tracer-enabled general circulation model experiments show that the same circulation anomalies lead to enrichment of stable isotope ratios in West Antarctic precipitation, with δ18O/temperature scaling similar to observations. The 2000 year long record from WAIS Divide shows that the 1990s in West Antarctica is not unprecedented, but that events of similar magnitude occur only a few times in the last 2000 years. High frequency variability in West Antarctic δ18O is superimposed on an overall decline of about 1.6‰ in the last 2000 years, similar to that observed in East Antarctic cores such as Vostok and Taylor Dome. The long term decline in δ18O at WAIS Divide contrasts with the records from Byrd and Siple Dome, which show rising isotope profiles over the past 5000 years. The parsimonious explanation is that Antarctic-wide cooling (as expected from a direct response to Milankovich forcing) is compensated by elevation changes at these other West Antarctic sites, producing local warming trends simply due to the lapse rate. In particular, it suggests that Siple Dome has thinned in the last 2000 years (though we emphasize that the recent warming is too fast to be explained by elevation change). The magnitude of implied thinning at Siple Dome (~200 m) is small enough that it cannot be ruled out by existing geophysical data. These results obviate the need to call on more complex meteorological relationships in explaining the apparent discrepancy between previously-existing West and East Antarctic ice core records during the Holocene. We conclude that recent warming in West Antarctica reflects the large-scale global warming trend, superimposed on the millennial scale cooling forced by insolation.
Year: 2010