Bjorn Stevens – Aerosol Forcing – Last Centuries Problem

Since its inception, modern climate science has accepted the fact that the aerosol is one of the climate systems most important and uncertain forcings. In the 1970s it was argued that the radiative effects of the aerosol might portend an accelerated arrival of a new Weltwinter (ice age), in the 1980s aerosol effects were linked to massive climate consequences of nuclear war, in the 1990s it was proposed that aerosols were masking a much larger sensitivity of global temperatures to rising concentrations of CO2, and in the first decade of this century aerosol effects on the hydrological cycle and weather extremes have received substantial attention. In this talk I will argue that the role of aerosol forcing has been greatly exaggerated, but to the extent the aerosol is responsible for a substantial forcing of the climate system this forcing was realized in the middle of the last century, and for both reasons the aerosol has ceased to be important for present or future changes in the global climate.