Aerosol forcing of climate over the industrial period is estimated by the IPCC fourth assessment to be -0.4 to -1.8 Wm-2 due to the impact of aerosol changes on cloud albedo – the so called aerosol indirect effect on climate. This large aerosol uncertainty compares to 1.7±0.2 Wm-2 due to CO2. The aerosol indirect effect therefore has a major influence on climate change and the potential to mask a significant portion of greenhouse gas warming. But the aerosol forcing has by far the largest uncertainty, a situation that has persisted through all IPCC assessments.
The aerosol indirect effect has proved enormously challenging to quantify and has become one of the grand challenges in aerosol science. The fundamental driver of the indirect effect is the change in aerosol from pre-industrial to present-day periods, which controls the change in cloud drop concentrations. GASSP aims to understand the processes that drive cloud-active aerosol so that we have a firm basis upon which to calculate the impact of aerosols on climate.