Strong evidence that changes in the Earth‘s climate follow changes in the Sun‘s magnetic behaviour came from the Copenhagen scientists Eigil Friis-Christensen and Knud Lassen in 1991. In principle it might explain most of the warming in the 20th Century, but no one knew how the Sun could affect the climate so much.

One effect of solar changes is to vary the number of cosmic rays reaching the Earth from the Galaxy. In 1995 Henrik Svensmark, also in Copenhagen, began to wonder if the cosmic rays could affect cloud cover. When he compared satellite observations of clouds with the varying counts of cosmic rays from year to year, he found an amazing link. A stronger Sun and fewer cosmic rays meant fewer clouds and a warmer world. Friis-Christensen agreed with this explanation for the Sun‘s role.