Ice cores provide a record of the concentration of beryllium-10 in the atmosphere. This is produced when high-energy particles from space bombard the atmosphere, but when the Sun is active its magnetic field protects the Earth from these particles and levels of beryllium-10 are lower.
But he told New Scientist that when he saw the data converted to sunspot numbers he thought, 'why the hell didn't I do this?' It makes the conclusion very stark, he says. 'We are living with a very unusual sun at the moment.'
The findings may stoke the controversy over the contribution of the Sun to global warming. Usoskin and his team are reluctant to be dragged into the debate, but their work will probably be seized upon by those who claim that temperature rises over the past century are the result of changes in the Sun's output (New Scientist, print edition, 12 April 2003). The link between the Sun's magnetic activity and the Earth's climate is, however, unclear.