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

University Of Texas At Austin, Austin TX

Investigators

Abstract

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.