The cooling effect of aerosol emissions in the atmosphere has masked about one third of warming associated with greenhouse gas emissions over land between 1964 and 2010, reports a paper published online this week in Nature Geoscience. A second paper shows that, on a more regional scale, reductions in European pollution levels - specifically sulfur emissions - have substantially contributed to amplifying Arctic warming over the past three decades.
Sulfate aerosol emissions have historically co-occurred with greenhouse gas emissions. High levels of sulfate aerosols in the atmosphere lead to climate cooling and the dimming of incoming solar radiation, which can mask part of the warming effect of greenhouse gases.
Trude Storelvmo and colleagues analyse observations of greenhouse gas concentrations, temperatures and surface radiation at 1,300 sites globally - a method entirely independent of estimates based on climate model simulations. They attribute temperature changes from 1964 to 2010, to distinguish between those from increasing levels of greenhouse gases and those from changes in aerosol load. Using this attribution, they estimate that aerosol cooling has offset one third of global warming over land, and that global temperatures will reach 2°C above pre-industrial levels when CO2 concentrations reach double pre-industrial concentrations.
Looking specifically at the impacts of European air pollution control on Arctic warming, Annica Ekman and colleagues present Earth system model simulations that suggest that 0.5°C of Arctic warming since 1980 can be explained by reductions in European sulfur emissions. They find that the warming begins in summer, with increased incoming solar radiation and enhanced poleward heat transport in the ocean and atmosphere leading to reduced sea ice cover, which thereby facilitates release of heat to the atmosphere in autumn and winter.
In a News & Views article accompanying the paper by Ekman and colleagues, Thorsten Mauritsen writes, “ultimately, it is not the uncertain temporary aerosol cooling, but the warming induced by long-lived greenhouse gases such as carbon dioxide that has warmed the Arctic in the past - and will continue to do so.”