Abstract
Abstract. We investigate the climate response to changing US anthropogenic aerosol sources over the 1950–2050 period by using the NASA GISS general circulation model (GCM) and comparing to observed US temperature trends. Time-dependent aerosol distributions are generated from the GEOS-Chem chemical transport model applied to historical emission inventories and future projections. Radiative forcing from US anthropogenic aerosols peaked in 1970–1990 and has strongly declined since due to air quality regulations. We find that the regional radiative forcing from US anthropogenic aerosols elicits a strong regional climate response, cooling the central and eastern US by 0.5–1.0 °C on average during 1970–1990, with the strongest effects on maximum daytime temperatures in summer and autumn. Aerosol cooling reflects comparable contributions from direct and indirect (cloud-mediated) radiative effects. Absorbing aerosol (mainly black carbon) has negligible warming effect. Aerosol cooling reduces surface evaporation and thus decreases precipitation along the US east coast, but also increases the southerly flow of moisture from the Gulf of Mexico resulting in increased cloud cover and precipitation in the central US. Observations over the eastern US show a lack of warming in 1960–1980 followed by very rapid warming since, which we reproduce in the GCM and attribute to trends in US anthropogenic aerosol sources. Present US aerosol concentrations are sufficiently low that future air quality improvements are projected to cause little further warming in the US (0.1 °C over 2010–2050). We find that most of the warming from aerosol source controls in the US has already been realized over the 1980–2010 period.
Highlights
Global mean surface temperatures increased by 0.74 ± 0.18 ◦C between 1906 and 2005 due to increasing greenhouse gases (Trenberth et al, 2007)
Mean fine aerosol concentrations are archived from the second year for use in the general circulation model (GCM) including sulfatenitrate-ammonium (SNA), primary organic aerosol (POA), secondary organic aerosol (SOA), and black carbon (BC) (Park et al, 2006; Liao et al, 2007)
We previously showed that our simulated trend of sulfate over the US is in good agreement with observations, at least for 1990–present, while the simulated decreasing trend of black carbon is too low by a factor of 2 (Leibensperger et al, 2012)
Summary
Global mean surface temperatures increased by 0.74 ± 0.18 ◦C between 1906 and 2005 due to increasing greenhouse gases (Trenberth et al, 2007). The eastern US experienced a cooling between 1930 and 1990 (Fig. 1). The net cooling effect of anthropogenic aerosols is known to have mitigated some of the global warming from greenhouse gases (Hegerl et al, 2007), but the importance of aerosol cooling on temperature trends in the US has received little attention. As US aerosol sources are increasingly controlled to improve air quality, the associated cooling is undone resulting in accelerated warming (Andreae et al, 2005; Brasseur and Roeckner, 2005; Kloster et al, 2009; Mickley et al, 2012).
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