Improving Estimates of Anthropogenic Aerosol Cooling and Climate Sensitivity

George Mason University, Fairfax VA

Investigators

Abstract

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.