A team at the Danish National Space Center has discovered how cosmic rays from exploding stars can help to make clouds in the atmosphere. The results support the theory that cosmic rays influence Earth’s climate.

An essential role for remote stars in everyday weather on Earth has been revealed by an experiment at the Danish National Space Center in Copenhagen. It is already well-established that when cosmic rays, which are high-speed atomic particles originating in exploded stars far away in the Milky Way, penetrate Earth’s atmosphere they produce substantial amounts of ions and release free electrons. Now, results from the Danish experiment show that the released electrons significantly promote the formation of building blocks for cloud condensation nuclei on which water vapour condenses to make clouds. Hence, a causal mechanism by which cosmic rays can facilitate the production of clouds in Earth’s atmosphere has been experimentally identified for the first time. 

The Danish team officially announce their discovery on Wednesday in Proceedings of the Royal Society A, published by the Royal Society, the British national academy of science.

Finally

A missing link in climate theory

The experimental results lend strong empirical support to the theory proposed a decade ago by Henrik Svensmark and Eigil Friis-Christensen that cosmic rays influence Earth’s climate through their effect on cloud formation. The original theory rested on data showing a strong correlation between variation in the intensity of cosmic radiation penetrating the atmosphere and the amount of low-altitude clouds. Cloud cover increases when the intensity of cosmic rays grows and decreases when the intensity declines.

It is known that low-altitude clouds have an overall cooling effect on the Earth’s surface. Hence, variations in cloud cover caused by cosmic rays can change the surface temperature. The existence of such a cosmic connection to Earth’s climate might thus help to explain past and present variations in Earth’s climate.

Interestingly, during the 20th Century, the Sun’s magnetic field which shields Earth from cosmic rays more than doubled, thereby reducing the average influx of cosmic rays. The resulting reduction in cloudiness, especially of low-altitude clouds, may be a significant factor in the global warming Earth has undergone during the last century. However, until now, there has been no experimental evidence of how the causal mechanism linking cosmic rays and cloud formation may work.

‘Many climate scientists have considered the linkages from cosmic rays to clouds to climate as unproven,’ comments Eigil Friis-Christensen, who is now Director of the Danish National Space Center. ‘Some said there was no conceivable way in which cosmic rays could influence cloud cover. The SKY experiment now shows how they do so, and should help to put the cosmic-ray connection firmly onto the agenda of international climate research.’