CLOUD Experiment

Project leader: Markku Kulmala    Personnel

Atmospheric aerosol particles play an important role in atmospheric physics and chemistry. They influence the climate by two distinct mechanisms: the direct reflection of solar radiation by aerosols, and the indirect increase in cloud reflectivity caused by greater numbers of cloud condensation nuclei (CCN) and ice nuclei (IN). New particle formation (NPF) and growth is important for climate since it gives rise to more than half of global CCN. Since pre-industrial times, CCN and IN have increased and have offset a significant but poorly understood fraction of the warming from increased greenhouse gases. A large part of this uncertainty results from the poorly known base-line aerosol state of the pristine pre-industrial atmosphere. Anthropogenic increases of aerosols and clouds are considered by the Intergovernmental Panel on Climate Change (IPCC) to be the major uncertainty limiting our understanding of Earth’s climate sensitivity, which, in turn, limits the ability of climate models to make precise climate projections for the 21st century.