Strong sensitivity of late 21st century climate to projected changes in short-lived air pollutants

[1] This study examines the impact of projected changes (A1B ‘‘marker’’ scenario) in emissions of four short-lived air pollutants (ozone, black carbon, organic carbon, and sulfate) on future climate. Through year 2030, simulated climate is only weakly dependent on the projected levels of short-lived air pollutants, primarily the result of a near cancellation of their global net radiative forcing. However, by year 2100, the projected decrease in sulfate aerosol (driven by a 65% reduction in global sulfur dioxide emissions) and the projected increase in black carbon aerosol (driven by a 100% increase in its global emissions) contribute a significant portion of the simulated A1B surface air warming relative to the year 2000: 0.2 C (Southern Hemisphere), 0.4 C globally, 0.6 C (Northern Hemisphere), 1.5–3 C (wintertime Arctic), and 1.5–2 C ( 40% of the total) in the summertime United States. These projected changes are also responsible for a significant decrease in central United States late summer root zone soil water and precipitation. By year 2100, changes in short-lived air pollutants produce a global average increase in radiative forcing of 1 W/m; over east Asia it exceeds 5 W/m. However, the resulting regional patterns of surface temperature warming do not follow the regional patterns of changes in short-lived species emissions, tropospheric loadings, or radiative forcing (global pattern correlation coefficient of 0.172). Rather, the regional patterns of warming from short-lived species are similar to the patterns for well-mixed greenhouse gases (global pattern correlation coefficient of 0.8) with the strongest warming occurring over the summer continental United States, Mediterranean Sea, and southern Europe and over the winter Arctic.