Atmospheric aerosols versus greenhouse gases in the twenty-first century.

Looked at in a simplistic way, aerosols have counteracted the warming effects of greenhouse gases (GHG) over the past century. This has not only provided some 'climate protection', but also prevented the true magnitude of the problem from becoming evident. In particular, it may have resulted in an underestimation of the sensitivity of the climate system to the effect of GHG. Over the present century, the role of aerosols in opposing global warming will wane, as there are powerful policy reasons to reduce their emissions and their atmospheric lifetimes are short in contrast to those of the GHG. On the other hand, aerosols will continue to play a role in regional climate change, especially with regard to the water cycle. The end of significant climate protection by atmospheric aerosols, combined with the potentially very high sensitivity of the climate system, makes sharp and prompt reductions in greenhouse gas emissions, especially CO2, very urgent.