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Modeling the Climate Evolution and Abrupt Changes over the Last 21,000 Years

Simulating snapshot time slices for the Last Glacial Maximum (LGM, ca. 21,000 years ago) and mid-Holocene (6000 years ago) with state-of-the-art climate models has allowed better understanding of both the models and the data. But the climate evolution over the last 21,000 years has not always been smooth to the changing forcings and boundary conditions between these two time slices. In fact, proxy records indicate that some regional changes may have been relatively abrupt as the climate transitioned from a glacial to its current interglacial state. A Transient simulation of the Climate Evolution from the Last Glacial Maximum to present (TraCE) employing the synchronously coupled Community Climate System Model (CCSM3) has been completed. This simulation considers the climatic responses to the forcing changes of greenhouse gas concentrations, orbital parameters, ice sheets and meltwater fluxes to the ocean.

I will present two illustrative examples where we used the TraCE and additional sensitivity simulations to dissect the forcings and mechanisms explaining the climatic responses recorded by various proxies during the deglacial period. The first example concerns the Greenland ice core records which indicate an abrupt warming of surface temperature, approximately 15 ° C in about 150 years, occurring around 15,000 years ago (the Bølling-Allerød warming). Synchronously, records of hydroclimate indicate wetter conditions starting over much of Africa (the African Humid Period). Our TraCE simulation is able to reproduce these different climatic responses, one at high latitudes and a temperature response, the other at low latitudes and a precipitation response. Our analyses of the TraCE and additional single-forcing sensitivity simulations allow us to attribute the observed responses to the superposition of the transient evolutions of the various forcings, with the changing atmospheric greenhouse concentrations and meltwater fluxes playing important roles in both regions.