The Intergovernmental Panel on Climate Change (IPCC) released a major report in 2007 saying that most of the Earth’s warming over the last 50 years has been manmade. However some researchers believe that the observed temperature changes could instead be caused primarily by variations in natural phenomena — including changes to the flux of galactic cosmic rays striking the Earth’s atmosphere.

Now, two particle physicists in the UK claim to have shown that there is little evidence that variations in the cosmic ray flux affect Earth’s climate — although a group in the Ukraine believes that such a link can explain long-term temperature trends.

Cosmic rays and clouds

The idea that cosmic rays — high-energy particles that bombard Earth from space — could be affecting the Earth’s climate was put forward by physicist Henrik Svensmark of the Danish Space Research Institute in Copenhagen and colleagues in the late 1990s. Svensmark found that variations in global cloud cover at altitudes of up to 3 km, as revealed by satellite data from 1983 onwards, correlated neatly with the incident cosmic ray flux measured by neutron counters located around the world. Furthermore these variations matched changes in sunspot activity, which varies on an 11-year cycle, with the peak in sunspot activity, which occurred around 1990, corresponding to a minimum in incident cosmic rays and coverage of low clouds.

Svensmark and colleagues proposed that greater sunspot activity, which causes the Sun to emit larger numbers of charged particles (the solar wind), decreases the flux of cosmic rays reaching the Earth from elsewhere in our galaxy because the solar wind’s increased magnetic field deflects more of them away from our planet. Because, he claims, these cosmic rays ionize the atmosphere and water droplets then condense on the ions, a decrease in the cosmic ray flux will lead to a reduction in cloud cover. Lower cloud cover, he says, will then, on balance, cause the Earth to heat up.

Significant uncertainties

However, significant uncertainties in Svensmark’s theory have meant that the IPCC did not include cosmic rays as a possible cause of climate change in their report issued last year.

Now, new research by Terry Sloan of the University of Lancaster and Arnold Wolfendale of the University of Durham has cast further doubt on the link between cosmic ray flux and cloud cover (Environ. Res. Lett. 3 024001 ). The pair says that the observed correlation between cosmic rays and cloud cover does not imply that variations in the former cause changes in the latter. They came to this conclusion after looking for correlations between the two observables beyond the simple global averaged data from the sunspot cycle.

No such correlation

The first of two such correlations they looked for was that between cosmic ray flux and cloud cover at different magnetic latitudes. Galactic cosmic rays are also deflected by the Earth’s own magnetic field, but this deflection is lower at the poles than it is at the equator, so it is at the former that the full effect of changes to the solar wind is felt and hence the “dip” in cosmic ray flux at the time of the solar maximum is more pronounced here. Sloan and Wolfendale looked to see if there was a corresponding variation in the cloud cover dip across magnetic latitudes but found none. They also looked to see if sudden bursts or reductions in the cosmic ray flux, which do occur throughout a solar cycle, were accompanied by increases or decreases in low cloud cover. Again they found no such correlation.

We have shown that Svensmark has no ground on which to challenge the IPCCTerry Sloan, University of Lancaster

By performing a statistical analysis on their first correlation, the UK researchers concluded that no more than 23% of the reduction in global low cloud cover at the time of the 1990 solar maximum was caused by the lower cosmic ray flux, pointing out that there are all sorts of other effects that could have been to blame instead. They believe their analysis can be used by climatologists to put an upper limit on the impact of cosmic rays on global warming. “We have shown that Svensmark has no ground on which to challenge the IPCC,” says Sloan.

Direct connection?

However, Sloan and Wolfendale are not the only physicists to have recently turned their attention to the cosmic ray hypothesis. Vitaliy Rusov of the National Polytechnic University in Odessa, Ukraine and colleagues do not agree with the IPCC’s view that man is to blame for the recent warming. To prove their point, they looked for a direct connection between cosmic ray flux and temperature.

The team constructed a model of the Earth’s climate in which the only significant inputs were variations in the Sun’s power output and changes to the galactic cosmic ray flux (arXiv: 803.2765 ). They found that the model’s predicted evolution of Earth’s surface temperature over the last 700,000 years agrees well with proxy temperature data taken from Antarctic ice cores (arXiv: 0803.2766 ).

Rusov agrees that Svensmark’s cosmic ray ionization mechanism cannot fully account for the observed correlation between cosmic ray flux and cloud cover, as Sloan and Wolfendale have demonstrated. But he believes that a small but direct link between cosmic rays and clouds could itself trigger a mechanism which causes further, and greater, changes in cloud cover.