In recent years, severe drought in southwestern North America has prompted intense interest in understanding the future trajectory of hydroclimate in this vulnerable region. Studying past greenhouse climates can help identify the processes that might drive future rainfall changes in the Southwest. One such interval, the mid-Pliocene Warm Period, which occurred 3.3 million years ago and featured CO2 concentrations similar to near-future values, presents an interesting conundrum. Simulations of the Pliocene suggest widespread dry conditions across the Southwest, just as models predict drying across much of the southwest by the end of the 21st century. However, multiple lines of geological evidence suggest wetter conditions in the Pliocene. This research project aims to resolve this apparent paradox using a combination of state-of-the-art, high-resolution climate model simulations and new reconstructions of southwestern rainfall spanning the Pliocene to the present. The reconstructions will provide some of the first constraints on the Pliocene history of regional climate features like the North American Monsoon. These data and model simulations will be used to evaluate sources of bias in model simulations of the Pliocene and to understand how this past interval of climate change may have differed from current warming. The project has direct applications to improving projections of southwestern rainfall in a warming world. The results of the work will be disseminated to the public and stakeholders via museum lectures, and will complement ongoing efforts to develop hands-on research experiences for high school students.
Paleoclimatic research can clarify the future behavior of regional climates by studying the past. This project exemplifies this goal by using new data from the mid-Pliocene Warm Period, specifically the mid-Piacenzian, to understand the response of North American Southwest rainfall to elevated greenhouse gases. This project tests the hypothesis that the spatial footprint of summer rainfall was much more extensive in the Pliocene. The research will reconstruct Plio-Pleistocene rainfall seasonality from marine sediment cores along the California margin and Baja California. Measurements of stable isotopes in leaf waxes will explicitly resolve changes in the balance of summer vs. winter rainfall. Complementary measurements of ocean surface temperature from organic proxies like alkenones will provide additional, independent constraints on regional environmental change. The project will also conduct a suite of Earth system model simulations to explore how changes in atmospheric parameterizations and model configuration influence simulated patterns of Pliocene rainfall. Proxy data and model simulations will be interpreted synergistically, with models helping inform proxy interpretations, and proxies helping to assess model skill. The research meets two major goals: first, it provides novel constraints on the Plio-Pleistocene evolution of key regional rainfall regimes like the North American Monsoon (NAM). Second, the project will help clarify the factors that influence model skill at simulating Pliocene hydroclimate, with broad implications for our understanding of the model configurations that most reliably predict regional responses to elevated CO2.
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.
