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

随着大气中二氧化碳的增加和全球变暖，美国西南部的水资源供应仍然是一个紧迫的问题。持续的人口增长加上一千多年来最严重的干旱之一，给该地区的水资源带来了巨大的压力。模拟全球气候的模型普遍发现，随着大气中二氧化碳的增加，美国西南部将变得更加干旱，因为更高的温度将增加地表水分的蒸发。与模式预测相反，在过去的地质时期，当大气中二氧化碳含量较高时，美国西南部似乎更湿润。模型预测和地质数据之间的分歧挑战了我们理解美国西南部水资源供应将如何随着大气中二氧化碳的增加而变化的能力。这项研究的重点是通过研究不同的气候模式如何模拟水循环的变化，并将这些模拟结果与1500万年前中新世中期全球变暖时期的新数据进行比较，来理解这种分歧。这项工作将提高社会理解未来美国西南部水循环将如何变化的能力。该项目将收集新墨西哥州北部Santa Fe群的沉积碳酸盐稳定同位素数据，这些数据跨越中新世气候优化（MCO）。沉积稳定同位素值对水汽来源、降水季节、温度和初级生产力敏感。反过来，目标盆地中大量的火山灰沉积物为高分辨率的绝对年代学提供了潜力，可以将其与MCO的海洋记录联系起来。研究人员将分析现有的千年耦合（海洋-大气-陆海冰）气候模式模拟和大型初始条件集合，以区分内部变率、瞬态和平衡水文气候对CO2强迫和各种海温模式的响应。进一步的模拟将采用一种新开发的模拟水文气候格林函数的方法，以快速计算局部P对大范围海温模式变化的响应，这些变化被假设为迫使美国西南部降雨。这些任务的结合将使研究人员能够确定美国西南部水文气候变化的驱动因素，并将模型模拟和古档案与变暖或查明差异进行协调。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。

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.