Successful calculation of human and natural influence on cloud formation

Soot particles, dust lifted up by the wind or sea spray account for only some of the particles in the atmosphere. Others develop from certain trace vapours, for example when individual sulphuric acid and water molecules cluster as tiny droplets. This formation of new particles is known as nucleation. Clouds are formed by water condensing on the larger aerosol particles or what are known as cloud condensation nuclei. The more cloud droplets develop, the more sunlight is reflected back into space. Climate models show that the additional particles caused by human activity produce a cooling effect which partially offsets the greenhouse effect. It is, however, less than previously assumed.

Aerosol particles from sulphuric acid and ammonia emissions

The model calculations presented in "Science" prove that about half the cloud condensation nuclei in the atmosphere originate from nucleation. In the atmosphere today, particle formation is dominated almost everywhere by mechanisms where at least three chemical components must come together: apart from the two basic substances, i.e. sulphuric acid and water, these are either ammonia or specific organic compounds such as oxidation products from terpenes. Close to ground level, organic substances from natural sources are important, whilst ammonia plays a key role higher up in the troposphere. Ammonia and sulphur emissions have increased considerably over the past decades as a result of human activities.

11-year solar cycle has scarcely any influence

CLOUD has also investigated how the 11-year solar cycle influences the formation of aerosol particles in our present-day atmosphere. The model calculations show that the effects as a result of changes in ionisation through the sun are too small to make a significant contribution to cloud formation. Although the ions are originally involved in the development of almost one third of all newly formed particles, the concentration of the large cloud condensation nuclei in the course of the 11-year cycle changes by only 0.1 percent -- not enough to have any sizeable influence on the climate.

Cooling effects 27 percent less than expected

The CLOUD team has also presented first global model calculations for aerosol formation caused without the involvement of sulphuric acid and solely through extremely low volatile substances of biological origin (Gordon et al., PNAS). According to the findings, this process contributed significantly to particle formation above all in the pre-industrial atmosphere, since at that time far less sulphur components were released into the atmosphere. The number of particles in the pre-industrial atmosphere is now estimated to be far greater through the additional process than was shown in earlier calculations. The model calculations, which are based on data from the CLOUD experiment, reveal that the cooling effects of clouds are 27 percent less than in climate simulations without this effect as a result of additional particles caused by human activity: Instead of a radiative effect of -0.82 W/m2 the outcome is only -0.60 W/m².