Posted 28 August 2011, by Jorn van Dooren, Bits of Science, bitsofscience.org
Clouds can have a large impact on global climate. Depending on conditions they can either trap or reflectthe sun’s heat. Scientists at CERN have now determined that organic vapours released by Earth’s organisms play a far more significant role in cloud formation than previously suspected.
Cloud droplets can form only on tiny airborne particles called aerosols. The basis for these aerosols is created when sulphuric acid and ammonia molecules cluster together, sometimes jumpstarted by cosmic rays. Previous assumptions led us to believe that sulphuric acid and ammonia accounted for the formation of a majority of aerosols. But as the results of the research presented in Nature show, their clustering can only account for a tenth to a thousandth of observed rates.
To let nucleation occur at a substantial rate in the low atmosphere the presence of an organic trace vapour is needed besides sulphuric acid and ammonia. If this is significant on a global scale, the role of the organic component and thus that of the biosphere on cloud formation is more significant than previously thought.
In the large stainless steel chamber filled with highly purified air where the Cloud experiment took place, the scientists also tested the effect of cosmic rays on cloud formation, as some physicists believe these are responsible for global warming. The team did find that the rays increased nucleation by 2 to 10 times, which is small compared to the effect of ammonia increase (100-1000 fold). Furthermore this increase in nucleation yields particles around the size of 1 nanometre, which does not necessarily translate in an increase of 50 nanometre particles needed for cloud formation.
The apparent effect of the biosphere on cloud formation has significant effect on the role attributed to aerosols in current climate models. Maybe more importantly, since our activities greatly influence the biosphere, it may add an extra unexpected factor to human contribution to climate forcing.
Aerosols and clouds play an important role in climate modelling, but were never properly measured. The Cloud experiment aims to create a better understanding of cloud formation and will result in better computer models of how the Earth’s climate is influenced by clouds.
The focus of the study is on how cosmic rays influence cloud formation, of which they so far found only a marginal effect. So the effect of the biosphere was somewhat of a surprising, but nevertheless important, bonus. It is another step towards our understanding of how we can affect global climate, whether it is intentional through cloud geoengineering or in some other less intentional fashion.
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