AGS-PRF: Assessing Causes of the Divergence between Past and Projected Responses of Global Aridity to Greenhouse Warming

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This award provides funds for a 2-year Postdoctoral Research Fellowship for the PI, who will work under the mentorship of Professor Richard Seager at Columbia University. The goal of the project is to reconcile an apparent discrepancy between expected changes in hydroclimate due to global warming with paleoclimate proxy data from past climates. Global warming simulations typically suggest that water will become more scarce over the continents as temperatures rise, due to increases in evaporative demand that outpace increases in precipitation. But if drying accompanies warming then the same logic would suggest that colder climates, including ice age climates, should be wetter. This argument is apparently in conflict with the geological evidence, which suggests that the ice ages were in fact dryer than today's climate. Moreover, there is evidence that the warm climate of the early Cenozoic (about 55 million years ago) was wetter than today's world despite the higher temperatures.Three avenues of research are pursued to address this discrepancy, the first of which is a careful examination of modern-day climate simulations from climate models participating in the Coupled Model Intercomparison Project version 5 (CMIP5). The premise is that the drying produced in climate change simulations from these models could be associated with biases in the present-day simulations, thus models may overestimate future drying if they are consistently biased dry in their control simulations. The second activity is a diagnostic examination of continental hydroclimate in model simulations of the last glacial maximum (LGM, roughly 20 to 25 thousand years ago), taken from the Paleoclimate Model Intercomparison Project version 3 (PMIP3). While changes in precipitation and potential evapotranspiration have been examined in detail for future climate simulations, comparable diagnoses have not been performed for LGM simulations.The third is a reexamination of the paleo-proxy record to make sure that the evidence for LGM drying has not been misinterpreted. Much of this evidence is based on vegetation, and it is possible that the vegetation is responding to lower values of carbon dioxide (CO2) and a reduction in CO2 fertilization, rather than a reduction in available water. The PI proposes to address this possibility by looking at proxy data which is not related to vegetation, for example lake levels and evaporite deposits. If the non-vegetation proxies do not show the same aridity signal found in vegetation-based proxies (pollen, fossils, carbon isotope ratios), a role for CO2 fertilization would seem likely.The work has inherent broader impacts due to the profound consequences of water shortages for human well-being and natural ecosystems. The work specifically addresses the potential for climate change to produce water shortages and the question of how well we can anticipate such shortages based on climate model simulations. In addition, the postdoctoral research fellowship, under the mentorship of an established scientist, will help to build the career of the PI, thereby providing support for the future workforce in this scientific discipline.