Reconciling US Southwest Hydroclimate Model Projections and Geologic Data: Constraints from the Miocene Climate Optimum

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As atmospheric CO2 rises and the world warms, water availability in the US Southwest remains a pressing concern. Ongoing population growth combined with one of the worst droughts in more than a millennia has placed tremendous pressure on water resources in the region. Models that simulate global climate generally find that the US Southwest will become more arid as atmospheric CO2 rises, because higher temperatures will increase evaporation of water from the surface. In contrast to model projections, the Southwest US appears to be wetter during periods in the geologic past when atmospheric CO2 was higher. This disagreement between model projections and geologic data challenges our ability to understand how water availability in the US Southwest will change as atmospheric CO2 rises. This research focuses on understanding this disagreement by investigating how different climate model simulations project changes in the water cycle and comparing these simulations with new data from a globally warmer period 15 million years ago during the middle Miocene. This work will improve society’s ability to understand how the water cycle in the US Southwest will change in the future.This project will collect new sedimentary carbonate stable isotope data from Santa Fe Group of northern New Mexico that span the Miocene Climate Optimum (MCO). The sedimentary stable isotope values are sensitive to moisture source, precipitation seasonality, temperature, and primary productivity. In turns, numerous ash deposits in the targeted basin provide the potential for a high-resolution, absolute chronology to link to the oceanic record of the MCO. The researchers will analyze existing millennial-long coupled (ocean-atmosphere-land-sea ice) climate model simulations and large initial condition ensembles to differentiate between internal variability, the transient, and the equilibrium hydroclimate response to CO2 forcing and various SST patterns. Further simulations will apply a newly developed method of simulating a hydroclimate Green’s function to rapidly compute local P responses to a large range of SST pattern changes, which are hypothesized to force US Southwest rainfall. The combination of these tasks will permit the researchers to identify drivers of US Southwest hydroclimate change and reconcile model simulations and paleo archives to warming or pinpoint the discrepancies.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.