Improving Estimates of Anthropogenic Aerosol Cooling and Climate Sensitivity

人们普遍认为，20世纪世纪温室气体增加的变暖效应在一定程度上被气溶胶污染的降温所抵消，而抵消的程度是检测和归因气候变化的一个核心问题。观测到的温度记录可能代表了被大量气溶胶冷却所掩盖的强烈的温室效应，或者是被较少的冷却所抵消的较弱的温室效应。 如果温室气体继续积累而气溶胶稳定或减少，这两种替代方案对未来变暖的后果截然不同。（本质上是阴影效应）利用表面温度对气溶胶的响应具有明显的空间模式的事实，复杂的“最佳指纹”技术已经被设计出来，以识别20世纪世纪气候模拟的模式。 该项目的工作扩展了方法，将表面温度与其他变量相结合，以帮助描述气溶胶效应。与温度相结合的两个变量是降水量和表面日照（直接和间接阳光）。 在气候模式模拟中，使用各种世纪气候强迫因子，包括所有强迫的组合，单独的气溶胶，以及除气溶胶外的所有强迫来识别模式。 其他工作试图将该方法应用于观测，同时考虑到观测网络的特征，包括台站的地理分布及其记录时期。这项工作具有更广泛的影响，因为更好地估计20世纪世纪气溶胶冷却将有助于得出对气候敏感性的观测约束，这是决定全球变暖造成的长期社会风险严重程度的最重要因素。 这种指导将直接有利于处理气候变化影响的决策者，对气候模型的开发也很有价值。这项工作还评估了特定类型的观测（例如降水和日照）对限制气溶胶冷却的价值，这些估计可能对优先投资新的观测基础设施很有价值。

It is generally accepted that the warming effect of greenhouse gas increases over the 20th century was to some extent offset by cooling from aerosol pollution, and the extent of the cancellation is a central question in the detection and attribution of climate change. The observed temperature record could represent strong greenhouse warming masked by substantial aerosol cooling or weaker warming offset by less cooling. The two alternatives have very different consequences for future warming if greenhouse gases continue to accumulate while aerosols level off or decline.Earlier work by the PI and others has attempted to separate out the aerosol cooling (essentially a shading effect) using the fact that the surface temperature response to aerosols has a distinct spatial pattern, and sophisticated "optimal fingerprinting" techniques have been devised to identify that pattern in simulations of 20th century climate. Work in this project extends the methodology by combining surface temperature with other variables to help characterize the aerosol effect. Two variables that show promise in combination with temperature are precipitation and surface insolation (direct and indirect sunlight). The patterns are identified in climate model simulations using various 20th century climate forcing factors including all forcings combined, aerosols alone, and all forcings except aerosols. Additional work seeks to apply the method to observations, taking into account characteristics of observing networks including the geographic distribution of the stations and their periods of record.The work has broader impacts because better estimates of 20th century aerosol cooling will be helpful to derive observational constraints on climate sensitivity, which is the most important factor determining the severity of the long-term societal risk posed by global warming. Such guidance would directly benefit policy makers addressing climate change impacts, and would also be valuable for climate model development. The work also provides an assessment of the value of particular types of observations (for instance precipitation and insolation) for constraining aerosol cooling, and these estimates could be valuable for prioritizing investments in new observational infrastructure.