They were most intense during a baby boom of stars, many of which blew up.

'The odds are 10,000 to 1 against this unexpected link between cosmic rays and the variable state of the biosphere being just a coincidence, and it offers a new perspective on the connection between the evolution of the Milky Way and the entire history of life over the last 4 billion years,' Dr Svensmark comments.

Dr Svensmark looked at the long record of life's bounty given by counts of heavy carbon atoms, carbon-13, in sedimentary rocks. When bacteria and algae in the ocean grow by taking in carbon dioxide, they prefer the ordinary carbon-12 atoms. As a result, the sea becomes enriched in carbon-13, which is acceptable to sea creatures building their carbonate shells. Variations in carbon-13 therefore record how much photosynthetic growth was in progress when the shell-makers were alive - in other words, how productive the biosphere was at that time.

To his surprise, Dr Svensmark noticed that the biggest fluctuations in productivity coincided with high star formation rates and cool periods in Earth's climate. Conversely, during a billion years when star formation was slow, cosmic rays were less intense and Earth's climate was warmer, the biosphere was almost unchanging in its productivity.

This reveals a link more subtle than any straightforward idea of, say, a warm climate being life-friendly or a cold climate deadly. The record shows that in all icy epochs the biosphere kept lurching between exceptionally low and exceptionally high productivity. The suggested reason is that, although ice is unfriendly to life, winds are stronger when the world is cold. By stirring the oceans, they improve the supply of nutrients in the surface waters so much that productivity can be higher than in a warm climate. And this, in effect, enlarges the fluctuations in biological productivity.

Most likely, the variations in cosmic radiation affected biological productivity through their influence on cloud formation. Hence, the stellar baby boom 2.4 billion years ago, which resulted in an extraordinarily large number of supernova explosions, had a chilling effect on Earth probably by increasing the cloud cover.

This is one of a number of new perspectives on climate change arising from the discovery that cosmic rays promote the formation of clouds, which have a cooling effect on the surface temperature of Earth. Recent experiments on how the cosmic rays influence cloud formation were reported in DNSC press release 3 October 2006.

Related Links
Danish National Space Center
Darwin Today At TerraDaily.com