THE idea that changes in the sun's activity can influence the climate is making a comeback, after years of scientific vilification, thanks to major advances in our understanding of the atmosphere. The findings do not suggest - as climate sceptics frequently do - that we can blame the rise of global temperatures since the early 20th century on the sun.
"There are extravagant claims for the effects of the sun on global climate," says Giles Harrison, an atmospheric physicist at the University of Reading, UK. "They are not supported."
Where solar effects may play a role is in influencing regional weather patterns over the coming decades. Predictions on these scales of time and space are crucial for nations seeking to prepare for the future.
Over the famous 11-year solar cycle, the sun's brightness varies by just 0.1 per cent. This was seen as too small a change to impinge on the global climate system, so solar effects have generally been left out of climate models.
However, the latest research has changed this view, and the next report by the Intergovernmental Panel on Climate Change (IPCC), due in 2013, will include solar effects in its models.
So far, three mechanisms have come to light (see diagram). The best understood is what is known as the top-down effect, described by Mike Lockwood, also at the University of Reading, and Joanna Haigh of Imperial College London. Although the sun's brightness does not change much during solar maxima and minima, the type of radiation it emits does.
During maxima the sun emits more ultraviolet radiation, which is absorbed by the stratosphere.This warms up, generating high-altitude winds. Although the exact mechanism is unclear, this appears to have knock-on effects on regional weather: strong stratospheric winds lead to a strong jet stream.
The reverse is true in solar minima, and the effect is particularly evident in Europe, where minima increase the chances of extreme weather. Indeed, this year's cold winter and the Russian heatwave in July have been linked to the sun's current lull, which froze weather systems in place for longer than normal.
The second effect is bottom-up, in which additional visible radiation during a solar maximum warms the tropical oceans, causing more evaporation and therefore more rain, especially close to the equator.
On its own, the effect may not be sufficient to cause noticeable differences. "It's too weak a forcing," says Tim Woollings of the University of Reading. But a study by Katja Matthes at the GFZ German Research Centre for Geosciences in Potsdam and colleagues suggests the two effects could work together to greater effect.
For example, observations show that monsoon rains in south-east Asia tend to be stronger during peak solar years. The researchers found that they were only able to reproduce this in models if they included both effects (Science, vol 325, p 1114).
The third solar influence on climate is extraterrestrial. Earth is bombarded by cosmic rays from exploding stars, which are largely deflected by the solar wind during solar maxima and to a slightly lesser degree in minima.
One theory held that cosmic rays cool the planet by helping to form airborne particles that water vapour condenses onto, increasing cloud cover. However, models suggest the effect is tiny (Nature, vol 460, p 332).
Just to be sure, though, the idea is being tested by the CLOUD experiment at CERN in Geneva, Switzerland. Initial results are expected in the next six months.
A theory that has more traction with climate scientists says the rays may change cloud behaviour rather than formation. Using weather balloon measurements, Harrison has shown that clouds have charged layers at their top and bottom, and he suggests that ions produced by cosmic rays might be responsible (Geophysical Research Letters, DOI: 10.1029/2010GL043605).
"The charge might make it easier for larger water droplets to form," he says, causing rain to fall sooner during solar minima. "But that's just one of many possibilities."
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