Aerosols are part of the climate system because they interact with both incoming solar and outgoing terrestrial radiation. Their role in radiative forcing has received increasing attention over the last years. While understanding on some of the important mechanisms has considerably improved, there remain many scientific uncertainties which do not yet allow drawing conclusive quantitative lessons about the impacts of the different types of aerosols on the radiative budget.

At the same time scientific knowledge on the impacts of aerosols (i.e., of fine particulate matter) on human health has substantially increased (e.g., WHO, 2003), so that the reduction of ambient levels of particulate matter (e.g., PM2.5) tops now the agenda of virtually all air quality managers around the world. Given these twofold impacts of aerosols on radiative forcing and human health, emission control strategies aimed at the protection of human health, as they are considered at the moment in many countries, will have (negative or positive) impacts on radiative forcing at the regional scale.

Earlier work at IIASA has highlighted reductions of emissions of particulate matter and of the associated health impacts as an important immediate and local co-benefit of global long-term decarbonisation strategies (Klaassen et al., 2004). Properly designed measures aimed at the reduction of the emissions of primary particles and the precursor emissions of secondary aerosols - including decarbonisation strategies - could yield important and tangible benefits, in the near-term on human health and in the long run on climate change.

Another environmental effect of aerosols that has not yet been fully explored is their impact on the hydrological cycle and resulting meteorological patterns related to their regional radiative forcing (e.g., Ramanathan et al., 2002). This aspect could be of high relevance for decision making, since it affects important endpoints (floods, droughts, storms) close to the sources where measures are taken and, due to the short lifetime of aerosols, responds quickly to changes in emissions.