Attributing Accelerated Summertime Warming in the Southeast United States to Recent Reductions in Aerosol Burden: Indications from Vertically-Resolved Observations

Mika Tosca,Michael Garay,James Campbell,Felix Seidel,Simone Lolli,Jared Marquis,Olga Kalashnikova

doi:10.3390/rs9070674

Abstract

During the twentieth century, the southeast United States cooled, in direct contrast with widespread global and hemispheric warming. While the existing literature is divided on the cause of this so-called “warming hole,” anthropogenic aerosols have been hypothesized as playing a primary role in its occurrence. In this study, unique satellite-based observations of aerosol vertical profiles are combined with a one-dimensional radiative transfer model and surface temperature observations to diagnose how major reductions in summertime aerosol burden since 2001 have impacted surface temperatures in the southeast US. We show that a significant improvement in air quality likely contributed to the elimination of the warming hole and acceleration of the positive temperature trend observed in recent years. These reductions coincide with a new EPA rule that was implemented between 2006 and 2010 that revised the fine particulate matter standard downward. Similar to the southeast US in the twentieth century, other regions of the globe may experience masking of long-term warming due to greenhouse gases, especially those with particularly poor air quality.

Highlights

During the latter half of the twentieth century, while the globally-averaged surface temperature increased [1], the southeastern United States (SEUS) experienced cooling [2,3,4]
The twentieth century warming hole was especially notable during the summer (Figure 2a) when anthropogenic aerosol concentrations in the SEUS are highest [49]
The FLG radiative transfer model was used to estimate the direct effect of these reductions in aerosol burden on the surface energy budget and column radiative heating profile, and to examine how that effect compares with the observed temperature trends previously discussed

Summary

Introduction

During the latter half of the twentieth century, while the globally-averaged surface temperature increased [1], the southeastern United States (SEUS) experienced cooling [2,3,4]. Annually-averaged surface temperature observations calculated from three widely recognized datasets (Figure 1a, described in Section 2.1) show a warming trend of +0.54 ± 0.30 ◦ C century−1 between 1900 and 2008 for the continental United States, for instance, but a minimal cooling trend of −0.02 ± 0.39 ◦ C century−1 in the SEUS (30–35◦ N; 95–80◦ W) (Figure 1b). While this so-called “warming hole” is noted in the literature, its origin is still unknown, though several recent studies link it to changes in large-scale convective precipitation [5], low-level circulation [6], decadal swings of the Pacific Decadal. Observations of net surface solar radiation over the United States show a corresponding multi-decadal decrease from 1961 through 1990 [15]

Methods

Results

Conclusion