The Role of Evolving Spatial Patterns of Anthropogenic Aerosol Emissions in the Trajectory of Climate Change

过去一个世纪的观测记录是理解和应对气候变化的关键资源，但很难解释这种记录，因为温室气体(GHG)的增加并不是推动全球变暖和区域气候演变的单独行动。在混杂因素中，可能最大的是人为气溶胶(AAs)的降温效应，特别是尾气和烟囱排放的雾霾，它们可能抵消了高达一半的温室气体增加的变暖效应。AAs的抵消作用促使了许多检测和归因研究，试图通过确定每个因素的不同地理气候响应模式来区分温室气体、AAs和其他因素。这些研究通常确定一种单一的固定模式作为对AA冷却的反应，如果目标是比较记录的两个不同时期，这是有意义的。但单一的固定模式是一个粗略的近似值，因为排放在过去一个世纪里发生了相当大的变化，本世纪中叶，美国东部和西欧的AA源占主导地位，随后中国和印度过渡到主要源。震源地区的变化自然会产生一种不断演变的反应模式，也可能导致反应强度的变化，因为一些地区的气候比其他地区对强迫更敏感。该奖项的研究考虑了AA反应的演变和随时间演变的反应模式的影响。一个目标是区分由于AA强迫的变化引起的响应的演变和由于气候平衡到新强迫的动力调整的速度而发生的演变。另一个是了解不断演变的AA响应模式如何影响从观测记录得出的气候敏感度估计值(二氧化碳增加一倍所导致的变暖量)。除了观测记录外，这项工作还利用了共同体地球系统模型(CESM)和其他气候模型的模拟集合，使强制气候响应与自然气候变异性的噪音分开。该项目中使用的集合包括20世纪所有已知外部强迫的模拟和对21世纪气候强迫的预测，以及单独应用温室气体和AAs的单强迫模拟。更多的模拟将使用复杂程度较低的CESM版本进行，包括仅限大气的版本和用静止的“平板”取代海洋的版本。鉴于需要了解20世纪的人为气候变化及其对未来的启示，这项工作具有社会意义。AAs在观测记录中的作用尤其重要，因为它使确定气候对温室气体增加的敏感性的努力复杂化：20世纪观测到的变暖可能是由于低气候敏感性与弱AA冷却相结合，也可能是由于高气候敏感性和强AA冷却。这两种可能性基本上区分了可能的气候未来，因此偏向其中一种的结果将具有独特的价值。此外，该项目还支持一名博士后研究员、一名研究生和一名本科生，从而建设未来的气候科学工作队伍。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

The observational record of the past century is a critical resource for understanding and addressing climate change, but the record is hard to interpret as greenhouse gas (GHG) increases do not act alone in driving global warming and regional climate evolution. Among the confounding factors perhaps the largest is the cooling effect of anthropogenic aerosols (AAs), in particular haze from tailpipe and smokestack emissions, which may have offset up to half of the warming effect of GHG increases. The offsetting effect of AAs has prompted numerous detection and attribution studies seeking to discriminate between GHGs, AAs, and other factors by identifying distinct geographical climate response patterns for each. These studies have commonly identified a single fixed pattern as the response to AA cooling, which makes sense if the goal is to compare two distinct periods of the record. But a single fixed pattern is a crude approximation since emissions evolved considerably over the century, with the mid-century dominated by AA sources in the eastern US and Western Europe followed by a transition to dominant sources in China and India. The change in source regions would naturally produce an evolving response pattern and could also cause changes in the strength of the response, as climate is more sensitive to forcing in some regions than in others.Research under this award considers the evolution of the AA response and the implications of a time-evolving response pattern. One goal is to distinguish between the evolution of the response due to changes in AA forcing and the evolution that occurs due to the pace of the dynamical adjustments through which the climate equilibrates to new forcing. Another is to understand how the evolving AA response pattern affects estimates of climate sensitivity (the amount of warming caused by a doubling of carbon dioxide) derived from the observed record. In addition to the observational record the work takes advantage of ensembles of simulations from the Community Earth System Model (CESM) and other climate models, which allow the forced climate response to be separated from the noise of natural climate variability. The ensembles used in the project include simulations forced with all known external forcings over the 20th century and projections of climate forcings over the 21st century, as well as single-forcing simulations in which GHGs and AAs are applied in isolation. Additional simulations will be performed with reduced complexity versions of CESM including an atmosphere-only version and a version in which the ocean is replaced by a motionless "slab".The work has societal relevance given the need to understand the anthropogenic climate change of the 20th century and its lessons for the future. The role of AAs in the observational record is particularly important as it complicates efforts to determine the sensitivity of climate to GHG increases: the observed warming over the 20th century could either be due to low climate sensitivity combined with weak AA cooling or high climate sensitivity and strong AA cooling. The two possibilities essentially bracket the spectrum of possible climate futures, thus results that favor one over the other would be uniquely valuable. In addition, the project supports a postdoctoral researcher, a graduate student, and an undergraduate, thereby building the future climate science workforce.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.