An analysis of more than 50 years' worth of climate data has found scant evidence for a controversial theory that attempts to link cosmic rays and global warming. The theory suggests that solar variations can affect the number of cosmic rays reaching the Earth, which in turn influences climate by impacting on cloud formation. The latest study was done by Rasmus Benestad of the Norwegian Meteorological Institute and he concludes that changes to the Sun cannot explain global warming.

Benestad compared variations in the 1951–2006 annual mean galactic cosmic-ray-flux data with annual variations in temperature, mean sea-level barometric pressure and precipitation. The cosmic-ray data were obtained using a high-altitude neutron monitor located in Climax, Colorado.

He looked for meteorological responses to cosmic rays over timescales of more than a year, and for "fingerprint" patterns in both time and space. He also checked for responses to greenhouse-gas concentrations and the El Niño Southern Oscillation.

Little evidence

"The significance of the findings was that the results were negative – I found little evidence of the cosmic rays having a discernible affect on a range of common meteorological elements: temperature, the barometric pressure or precipitation," says Benestad. "Not for the global mean at least. One possible exception may have been for parts of Europe, however."

The galactic cosmic-ray flux was associated with lower temperatures in parts of Eastern Europe. Benestad is intrigued whether these results were a coincidence or do indeed show a connection between cosmic rays and both temperature and sea-level pressure. He plans to investigate further. "Why would a solar effect be seen only in a limited region?" he wonders. "This region is affected by the North Atlantic Oscillation, and this phenomenon is a bit special – a variation in the sea-level pressure over timescales of up to several years. The persistence in these variations may match the variations in the Sun by accident, but it could also be sensitive to variations in the Sun." If there is a real connection between changes to the Sun and the North Atlantic Oscillation, Benestad believes that this knowledge could benefit decadal predictions.

On a larger scale, the analysis indicated that the weak global mean-temperature response associated with cosmic-ray flux could easily be down to chance. What is more, there has been no long-term trend in cosmic-ray flux. "Hence, there is little empirical evidence that links galactic cosmic-ray flux to recent global warming," wrote Benestad in Environmental Research Letters.

Do we see a clear effect?

"To me, the question was whether galactic cosmic rays really matter when it comes to climate change – do we see a clear effect?" Benestad asks. "There have been some discussions about climate change over the last decade and whether these cosmic rays have played a role in the recent global warming – even to the extent to suggest the exclusion of other explanations such as greenhouse gases."

Cosmic rays ionize the atmosphere and some scientists believe that water droplets can condense on the resulting ions and aerosol particles, assisting cloud formation. Experimental work in this area has not been conclusive. Sunspot activity, which ebbs and flows on an 11-year cycle, decreases the cosmic-ray flux by increasing the solar wind – charged particles emitted by the Sun. The solar wind's greater magnetic field then deflects away some of the cosmic rays that would otherwise hit the Earth from elsewhere in the galaxy. So, if the theory linking cosmic rays and cloud formation is correct, then increased sunspot activity could potentially reduce cloud cover.

However, other researchers have found that the relationship between cosmic rays and changes in cloud cover, and hence the Earth's surface temperature, is limited. And Benestad has found that there is no evidence of a link between cosmic rays and climate variables such as temperature, pressure and precipitation.

False dichotomy

"Some scholars have implied a false dichotomy between galactic cosmic rays and greenhouse gases, arguing that global warming caused by galactic cosmic-ray flux would be at the expense of an effect from rising concentrations of greenhouse gases," Benestad explains. "Such propositions have resulted in public controversy."

Benestad's study was part of the European TOSCA project, which aims to understand the impact of solar variability on the Earth's climate. The purpose of his research is to look for a clear, direct and lasting response to cosmic rays in general meteorological parameters affecting people, rather than to assess whether cosmic rays are important for physical processes in the Earth's atmosphere.

"There has been a tremendous amount of effort [put] into collecting all the data – from the neutron counter, thermometers around the world, rain gauges and barometric readings – organizing the data, and making them available," says Benestad. "The novel aspect was the strategy used for analysing the results – combining linear algebra with regression analysis. This approach made it possible to investigate a large number of measurements simultaneously."

Benestad reported his results in Environmental Research Letters.