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EVERY 100,000 years or so, the Earth plunges into an ice age. Though it鈥檚 the planet鈥檚 strongest climate variation, no one has ever explained what causes this inexorable cycle. But the latest idea focuses on fluctuations in the Sun鈥檚 magnetic activity.

The traditional explanation for ice ages is that variations in the Earth鈥檚 orbit called Milankovitch cycles change how much energy reaches the planet鈥檚 surface from the Sun. Yet nagging doubts remain. In particular, the 100,000-year ice-age cycle doesn鈥檛 match either of the two strongest orbital variations鈥攖he wobbling of the Earth鈥檚 axis once every 23,000 years, and the tilting of the Earth鈥檚 orbit every 41,000 years. Orbital ellipticity does follow a 100,000-year cycle, but its effect should be much weaker.

Fluctuations in solar activity could also affect climate, but they have been hard to document. Satellite measurements show that the intensity of solar radiation in the vicinity of Earth varies by about 0.1 per cent over the 11-year sunspot cycle, which is caused by fluctuations in the Sun鈥檚 magnetic activity. That variation is so small that it is swamped by other factors affecting climate, Nevertheless, Mukul Sharma, a geologist at Dartmouth College in New Hampshire, says that the Sun鈥檚 magnetic activity could still be influencing the world鈥檚 climate.

Sharma measured levels of beryllium-10, a radioactive isotope produced by cosmic rays hitting Earth鈥檚 atmosphere. The Sun鈥檚 magnetic activity blocks cosmic rays, so lower levels of beryllium-10 are laid down in rock when the Sun is more active. He found that beryllium-10 varied in phase with ice ages over the past 200,000 years, suggesting that a more active Sun causes ice ages.

But it鈥檚 still unclear how the Sun鈥檚 magnetic activity could affect temperatures on Earth so dramatically. 鈥淚t is quite a surprising result,鈥� says Sharma. He says he鈥檚 not sure if the Sun becomes hotter when it is magnetically active, or if something else links high solar activity to climate. He hopes that by matching beryllium-10 deposition rates to other markers he will be able to follow solar activity back about 800,000 years.

Last year, Gerard Bond of the Lamont-Doherty Earth Observatory at Columbia University found that over the past 12,000 years beryllium-10 levels varied in sync with the volume of rock grains dropped to the floor of the North Atlantic by passing icebergs. He suggested that this reveals a link between solar activity and a 1500-year climate cycle that affects the North Atlantic. But he found that other climate change during glacial intervals affected beryllium-10 deposition so strongly that the data became unreliable. That makes him wary of Sharma鈥檚 longer-term results. The idea of such a link is interesting, but the evidence is not yet convincing, he says.