A Reassessment of Greenland Climate History Using a Proxy System Model for Accumulation of the Isotope Record in Snow
Wednesday, 17 December 2014: 5:30 PM
It is well known that the climate history of many millennia is recorded by the stable isotope ratios of ice from polar ice sheets. Indeed the use of stable isotope ratios as a proxy for past temperature has been considered a gold standard in establishing regional climate change near proxy locations. It is also known that environmental factors other than temperature can significantly influence the isotopic ratios of ice. Recognizing that multiple factors are at play introduces some ambiguity in simple interpretations, but also suggests the records are underutilized if interpretations focus solely on temperature. As a case in point, we consider isotope records from Greenland snow. To disentangle the various processes that may influence these isotope ratios (condensation temperature, atmospheric circulation, post deposition vapor-snow exchange, cloud microphysics), both an adequate model and the relevant data are required. Advances in measurement technologies now allow many of the appropriate data to be collected, and models can be developed to capture some of the additional processes that are now observed. The mechanistic approach offered by the model allows a quantitative explanation of post deposition change in snow isotopic composition and shows that variability in the isotope ratio of vapor over ice sheets is an important control. Boundary layer conditions and the frequency of synoptic-scale storms emerge as critical factors influencing vapor isotope ratios and therefore are potentially reconstructable from the ice record. We show that many of the variations over the last glacial cycle accredited to local temperature are accompanied with changes in the frequency of storms and changes in the structure of the boundary layer over the ice sheet. Since these mechanisms correlate with temperature, our findings provide a fundamental basis for why the isotope record is a robustly traces the climate history, whose quantitative interpretation need not be limited to temperature alone.