Collaborative Research: A paleoclimate perspective on the response of Southwest North American rainfall to elevated greenhouse gases

近年来，北美西南部的严重干旱引起了人们对了解这一脆弱地区水文气候未来轨迹的强烈兴趣。研究过去的温室气候可以帮助确定可能导致西南地区未来降雨变化的过程。上新世中期暖期就是这样一个间歇期，它发生在330万年前，其特征是二氧化碳浓度与不久的将来的值相似，这提出了一个有趣的难题。对上新世的模拟表明，西南地区普遍存在干旱状况，正如模型预测到21世纪末西南大部分地区将出现干旱一样。然而，多条地质证据表明，上新世的环境更为湿润。该研究项目旨在通过结合最先进的高分辨率气候模型模拟和新重建的西南地区从上新世到现在的降雨量来解决这个明显的悖论。这些重建将为上新世地区气候特征的历史提供一些初步的约束，比如北美季风。这些数据和模式模拟将用于评估上新世模式模拟中的偏差来源，并了解过去的气候变化间隔可能与当前的变暖有何不同。该项目可直接应用于在全球变暖的情况下改善对西南地区降雨的预测。这项工作的结果将通过博物馆讲座向公众和利益相关者传播，并将补充正在进行的努力，为高中生开发实践研究经验。古气候研究可以通过研究过去来阐明区域气候的未来行为。该项目通过使用上新世中期暖期（特别是皮亚琴世中期）的新数据来了解北美西南部降雨对温室气体升高的响应，从而实现了这一目标。该项目验证了上新世夏季降雨的空间足迹更为广泛的假设。该研究将从加利福尼亚边缘和下加利福尼亚州的海洋沉积物岩心中重建上新世至更新世的降雨季节性。测量叶蜡中的稳定同位素将明确地解决夏季与冬季降雨平衡的变化。利用烯烃等有机代用物对海洋表面温度进行补充性测量，将为区域环境变化提供额外的、独立的约束条件。该项目还将进行一套地球系统模型模拟，以探索大气参数化和模型配置的变化如何影响上新世降雨的模拟模式。代理数据和模型模拟将协同解释，模型帮助告知代理解释，代理帮助评估模型技能。该研究实现了两个主要目标：第一，它为像北美季风（NAM）这样的关键区域降雨制度的上新世-更新世演化提供了新的约束。其次，该项目将有助于澄清影响模型模拟上新世水文气候技能的因素，对我们理解最可靠地预测区域对二氧化碳升高的响应的模型配置具有广泛的意义。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.