Cloud-making: Another human effect on the climate


Cloud-making: Another human effect on the climate


Posted 24 August 2011, by Michael Le Page, New Scientist (Reed Business Information, Ltd.),


IN HIS Gaia hypothesis, James Lovelock famously suggested that living organisms could affect clouds – and he was eventually proved right. Now it seems the effect may be even stronger than we thought. Organic vapours released by organisms such as trees, marine bacteria and livestock appear to play a far more important role in cloud formation than suspected.

“This was a big surprise,” says Jasper Kirkby at CERN near Geneva, Switzerland, whose team made the finding. Since our activities have such a huge impact on the biosphere, this hints at a previously unknown way in which humans can affect the climate, he says.

Anything that affects cloud formation can in theory affect climate, because clouds can either reflect or trap the sun’s heat depending on conditions. Cloud droplets can form only on particles above 50 nanometres. In much of the atmosphere, dust, smoke and sea-spray provide more than enough of these cloud condensation nuclei, or CCNs.

High in the atmosphere, however, such particles are scarce. Here, cloud formation depends partly on trace gases condensing to form particles just 1 nanometre across, which can then grow large enough to act as CCNs.

Kirkby is part of the CLOUD experiment at CERN to investigate whether cosmic rays influence cloud formation. The team started by looking at the formation of the very small particles – a process called aerosol nucleation – by mimicking atmospheric conditions inside an ultraclean steel “cloud chamber”, which Kirkby says is the cleanest ever created. Cleanliness is vital since contaminants can themselves become sites of aerosol nucleation.

Aerosol nucleation is known to require sulphuric acid, but Kirkby’s team found that it is not enough by itself at low altitudes – the presence of an additional organic trace vapour is needed (Nature, DOI: 10.1038/nature10343). “If there is too little of either component then nucleation will not occur at an appreciable rate in the low atmosphere,” says Kirkby. That means the organic component – and thus the role of living organisms – is more important than had been thought, although the full implications are not yet understood.

“If it is significant on a global scale, it might mean that the natural emissions of organics is also important in cloud formation,” says Bart Verheggen of the Energy Research Centre of the Netherlands in Petten.

Some physicists think galactic cosmic rays – high-energy particles originating from faraway stars – might affect cloud formation. To test their effect on aerosol nucleation, Kirkby’s team fired beams similar to cosmic rays through the chamber and found it increased nucleation between 2 and 10 times. But he points out that an increase in 1 nanometre particles does not necessarily translate into the 50 nanometre CCNs needed for cloud formation.

Other evidence shows that even if cosmic rays do affect the climate, the effect must be small. Changes in the number of cosmic rays hitting the atmosphere due to changes in solar activity cannot explain global warming, as average cosmic ray intensities have been increasing since 1985 even as the world has warmed – the opposite of what should happen if cosmic rays produce climate-cooling clouds.


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