The sun's cooling down - so what does that mean for us?

The sun's activity is winding down, triggering fevered debate among scientists about how low it will go, and what it means for Earth's climate. Nasa recorded no sunspots on 266 days in 2008 - a level of inactivity not seen since 1913 - and 2009 looks set to be even quieter. Solar wind pressure is at a 50-year low and our local star is ever so slightly dimmer than it was 10 years ago.

Sunspots are the most visible sign of an active sun - islands of magnetism on the sun's surface where convection is inhibited, making the gas cooler and darker when seen from Earth - and the fact that they're vanishing means we're heading into a period of solar lethargy.

Where will it all end? Solar activity varies over an 11-year cycle, but it experiences longer-term variations, highs and lows that can last around a century.

"A new 11-year cycle started a year or two ago, and so far it's been extremely feeble," says Nigel Weiss of the University of Cambridge. With Jose Abreu of the Swiss Federal Institute of Aquatic Science and Technology in Dübendorf and others, Weiss recently predicted that the long-term solar high we've been enjoying since before the second world war is over, and the decline now under way will reach its lowest point around 2020. Their prediction is based on levels of rare isotopes that accumulate in the Earth's crust when weak solar winds allow cosmic rays to penetrate the Earth's atmosphere.

There's even a chance, says Weiss, that we might be heading for a low as deep as the Maunder minimum of the 17th century. Either side of that trough, Europe shivered through the Little Ice Age, when frost fairs were held on the Thames and whole Swiss villages disappeared under glaciers. So should we expect another freeze?

Those who claim the rise in temperatures we've seen over the last century are predominantly the result of intense solar activity might argue that we should, but they're in the minority. Most scientists believe humans are the main culprit when it comes to global warming, and Weiss is no exception. He points out that the ice remained in Europe long after solar activity picked up from the Maunder minimum. Even if we had another, similar low, he says, it would probably only cause temperatures on Earth to drop by the order of a tenth of a degree Celsius - peanuts compared to recent hikes. So don't pack your suncream away just yet.