Published online 24 August 2011 | Nature | doi:10.1038/news.2011.504

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Experiment probes connection between climate change and radiation bombarding the atmosphere.

The CLOUD experiment is studying whether cosmic rays play a role in cloud formation. Maximilien Brice / CERN

It sounds like a conspiracy theory: 'cosmic rays' from deep space might be creating clouds in Earth's atmosphere and changing the climate. Yet an experiment at CERN, Europe's high-energy physics laboratory near Geneva, Switzerland, is finding tentative evidence for just that.

The findings, published today in Nature1, are preliminary, but they are stoking a long-running argument over the role of radiation from distant stars in altering the climate.

For a century, scientists have known that charged particles from space constantly bombard Earth. Known as cosmic rays, the particles are mostly protons blasted out of supernovae. As the protons crash through the planet's atmosphere, they can ionize volatile compounds, causing them to condense into airborne droplets, or aerosols. Clouds might then build up around the droplets.

The number of cosmic rays that reach Earth depends on the Sun. When the Sun is emitting lots of radiation, its magnetic field shields the planet from cosmic rays. During periods of low solar activity, more cosmic rays reach Earth.

Scientists agree on these basic facts, but there is far less agreement on whether cosmic rays can have a large role in cloud formation and climate change. Since the late 1990s, some have suggested that when high solar activity lowers levels of cosmic rays, that in turn reduces cloud cover and warms the planet. Others say that there is no statistical evidence for such an effect.

Polarizing lens

"People are far too polarized, and in my opinion there are huge, important areas where our understanding is poor at the moment," says Jasper Kirkby, a physicist at CERN. In particular, he says, little controlled research has been done on exactly what effect cosmic rays can have on atmospheric chemistry.

To find out, Kirkby and his team are bringing the atmosphere down to Earth in an experiment called Cosmics Leaving Outdoor Droplets (CLOUD). The team fills a custom-built chamber with ultrapure air and chemicals believed to seed clouds: water vapour, sulphur dioxide, ozone and ammonia. They then bombard the chamber with protons from the same accelerator that feeds the Large Hadron Collider, the world's most powerful particle smasher. As the synthetic cosmic rays stream in, the group carefully samples the artificial atmosphere to see what effect the rays are having.

Early results seem to indicate that cosmic rays do cause a change. The high-energy protons seemed to enhance the production of nanometre-sized particles from the gaseous atmosphere by more than a factor of ten. But, Kirkby adds, those particles are far too small to serve as seeds for clouds. "At the moment, it actually says nothing about a possible cosmic-ray effect on clouds and climate, but it's a very important first step," he says.

Scientists on both sides of the debate welcome the findings, although they draw differing conclusions. "Of course there are many things to explore, but I think the cosmic-ray/cloud-seeding hypothesis is converging with reality," says Henrik Svensmark, a physicist at the Technical University of Denmark in Copenhagen, who claims a link between climate change and cosmic rays.

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Others disagree. The CLOUD experiment is "not firming up the connection", counters Mike Lockwood, a space and environmental physicist at the University of Reading, UK, who is sceptical. Lockwood says that the small particles may not grow fast enough or large enough to be important in comparison with other cloud-forming processes in the atmosphere.

"I think it's an incredibly worthwhile and overdue experiment," says Piers Forster, a climatologist at the University of Leeds, UK, who studied the link between cosmic rays and climate for the latest scientific assessment by the International Panel on Climate Change. But for now at least, he says that the experiment "probably raises more questions than it answers".

Kirkby hopes that the experiment will eventually answer the cosmic-ray question. In the coming years, he says, his group is planning experiments with larger particles in the chamber, and they hope eventually to generate artificial clouds for study. "There is a series of measurements that we will have to do that will take at least five years," he says. "But at the end of it, we want to settle it one way or the other."