| Is the sun to blame? Bitter winters blighted much of the northern hemisphere for decades in the second half of the 17th century, which was known as the Little Ice Age. The French army invaded the Netherlands by using frozen rivers as roads. Citizens of New York walked across the frozen harbour from Manhattan to Staten Island. Sea ice surrounded Iceland for miles and the island's population decreased by fifty percent. lt wasn't the first time temperatures had plummeted: a couple of hundred years earlier, between 1420 and 1570, a worsening of climate destroyed the Viking colonies on Greenland, turning them from fertile farmlands into arctic wastelands. Could the sun have been to blame? We now know that both these mini ice ages coincided with prolonged calm periods in the sun's activity - the sunspots and dramatic flares that are driven by its powerful magnetic field. Now some astronomers are predicting that the sun is about to begin another quiet period. With climate scientists warning that global warming is approaching a tipping point, beyond which rapid and possibly irreversible damage to our environment will be unavoidable, a calm sun and a resultant cold snap might be exactly what we need to give us breathing space to agree and implement pollution controls. Global average temperatures have risen by about 0.6 degrees Celcius in the past century and, until recently, almost all of this has been put down to human activity. But that may not be the only factor at work. A growing number of scientists believes that there are clear links between the sun's activity and the temperature on Earth. While solar magnetic activity cannot explain away global warming completely, it does seem to have a significant impact. "A couple of years ago, I would not have said that there was any evidence for solar activity driving temperatures on Earth," says Paula Reimer, a palaeoclimate expert at Queen's University, Belfast, in the UK. "Now I think there is fairly convincing evidence." What has won round Reimer and others is evidence linking climate to sunspots. These blemishes on the sun's surface appear and fade over days, weeks or months, dependent on their size. More than a mere curiosity, they are windows on the sun's mood. They are created by contortions in the sun's magnetic field and their appearance presages massive solar eruptions that fling billions of tonnes of gas into space. Fewer sunspots appear when the sun is calm and, historically, these calm periods have coincided with mini ice ages. The number of sunspots, and solar magnetic activity in general, normally wax and wane in cycles lasting around 11 years. However, every 200 years or so, the sunspots virtually disappear as solar activity slumps. For the past 50 years, however, the sun has been particularly active. "If you look back into the sun's past, you find that we currently live in a period of abnormally high solar activity," says Nigel Weiss, a solar physicist at the University of Cambridge. The concentrations of rare isotopes locked into tree rings and ice cores provide an indirect record of the sun's activity, stretching back thousands of years. While the sun is magnetically calm, its field extends around 12 billion kilometres into space. But the field puffs up to 15 billion kilometres when the sun is active. Cosmic rays (the high-energy particles from deep space that are constantly hurtling towards us) are deflected by the field so, at active times, far fewer of them reach the Earth. The rays that do reach our planet leave traces in the form of carbon-14 and beryllium-6, isotopes that are created only when cosmic rays slam into the Earth's atmosphere. Plants and trees then absorb carbon-14, while beryllium-6 settles onto the polar ice sheets and becomes incorporated into that year's ice layer. So, by measuring the levels of the isotopes in tree rings and polar ice cores, we can work out how many cosmic rays were reaching Earth when the rings or ice layers were formed, and thus estimate how active the sun was at those times. Sam Solanki and his team at the Max Planck Institute for Solar System Research in Katlenburg-Lindau, Germany, have looked at the concentrations of carbon-14 in wood and beryllium-6 in ice as far back as back 11,000 years ago. The similarity of the fluctuations in both isotopes convinced them that they were seeing effects due to the sun. The peaks and slumps showed a recognisable pattern: "Periods of high solar activity do not last long, perhaps 50 to 100 years, then you get a crash," says Weiss. "It's a boom-bust system, and I would expect a crash soon." Although another crash is likely, predicting the sun's activity with any certainty is difficult because of the chaotic way in which the solar magnetic field is generated. If anyone can do it, though, it's solar physicist turned computer programmer Leif Svalgaard, from Stanford University in California, who has been forecasting solar activity for nearly three decades. In the 1970s, he pioneered the best forecasting method yet devised, which uses the strength of the magnetic field at the sun's poles to predict future levels of solar activity. He, also, expects a crash. The sun's polar field is now at its weakest since measurements began in the early 1950s and, to Svalgaard, the latest figures indicate that the sun's activity will be weaker during the next decade than it has been for more than 100 years. Sunspot numbers will fall in the next decade, he predicts. He expects fewer than six new sunspots per month, less than half the average number seen over the past decade. This does not compare with the sunspot crash that observations from 1645 to 1715 suggest. Back then, the appearance of even a single sunspot was major astronomical news, sparking hurriedly penned communications from one observatory to another. Nevertheless, it's a sign of things to come. "Sunspot numbers will be extremely small, and when the sun crashes, it crashes hard," says Svaalgard. So what does the sun's magnetic activity have to do with the climate on Earth? To pin down the connection, Solanki and his colleagues compared records of solar activity derived from tree rings with meteorological records from 1856 to the present day. They found that the temperature of the Earth's atmosphere changed in step with sunspot numbers until 1970. This is the evidence that has done more than anything else to convince climatologists to take the link seriously. What's more, the most recent calculations by Solanki's team suggest that the sunspot crash could lead to a cooling of the Earth's atmosphere by 0.2 °C. It might not sound much, but this temperature reversal would be as big as the most optimistic estimate of the results of restricting greenhouse-gas emissions until 2050 in line with the Kyoto protocol. There is still a big puzzle, though. Astronomers and climate scientists have always struggled to understand exactly how solar activity could influence the temperature on Earth. Whatever the variations in the sun's magnetic activity, the total energy it emits changes by only 0.1 per cent - too small a change to have any direct effect. As a result, the sun's role in climate change is highly controversial. "People have been arguing over this for years," says Reimer. What other factor is at work? Important clues have emerged recently from solar observatories, including the SOHO spacecraft operated by NASA and the European Space Agency for the past 10 years. Although the change in overall solar energy is small, measurements made by SOHO and other solar observatories have revealed much greater variation in the levels of ultraviolet radiation, which can peak at up to 100 times its minimum level. "This means that there is scope for ultraviolet to have a much larger effect on our atmosphere," says Haigh, who for the past decade has been studying the impact of the sun's variability on climate. According to computer models that she has developed, ultraviolet radiation heats the upper reaches of the Earth's atmosphere by energising atoms and molecules there. This drives chemical reactions involving ozone and other molecules, which can release still more heat. This heating changes the temperature structure of the atmosphere at all altitudes, although the details are unclear because of the sheer complexity of Haigh's model. "By varying the amount of ultraviolet radiation, solar activity changes the circulation of the whole atmosphere," she says. Change the circulation, and you change the weather. Haigh's work may help to explain one of the most puzzling aspects of the Little Ice Age: "Europe was badly hit, but other parts of the world may not really have noticed it," says Solanki. This might have been due to the different distribution of land masses in the northern and southern hemispheres. While Antarctica is surrounded by a wide belt of ocean, the distribution of land and oceans in the northern hemisphere is much less regular. This means that the interaction between the circulating atmosphere and the ground is more complex in the northern hemisphere. It gives rise to the North Atlantic Oscillation, an interplay of low and high pressure that dictates the movement of storms across the continents bordering the north Atlantic. Haigh has found that at times of low solar activity the air pressure over the North Pole is higher than normal and forces storms south, funnelling colder weather to lower latitudes. What happens in the southern hemisphere is less well known, but Haigh says she wouldn't be surprised if the reaction there to changes in solar activity is different. Solar activity might also influence climate through its effect on cosmic rays. In another study, Solanki has found an intriguing correlation between the temperature of the Earth's atmosphere and the number of cosmic rays striking it, with lower temperatures in periods of high numbers of cosmic rays. How could cosmic rays lead to cooler temperatures? Consider a theory proposed by Henrik Svensmark and Eigil Friis-Christensen of the Danish Meteorology Institute in Copenhagen almost a decade ago. They suggested that cosmic rays create an electric charge in particles in our atmosphere that then act as seeds for the formation of clouds at low altitudes. A spell of low solar activity would mean more cosmic rays and therefore more clouds and lower temperatures. Svensmark and Friis-Christensen's idea is controversial, however. Most climatologists accept that more low clouds would reflect more radiation back into space, thus lowering temperatures. But many dismiss Svensmark and FriisChristensen's evidence of a link between cosmic rays and cloud cover as coincidence. Others want the theory investigated, if only to rule it out. To this end, an international group of more than 50 scientists have proposed an experiment at the CERN particle physics laboratory near Geneva, Switzerland, to begin in 2008. The coming years could settle the sun's role on temperatures once and for all. If the expected sunspot crash does takes place, Solanki's work could receive dramatic confirmation. "Having a crash would certainly allow us to pin down the sun's true level of influence on the Earth's climate," says Weiss. None of this means that we can stop worrying about global warming caused by emissions into the atmosphere. "The temperature of the Earth in the past few decades does not correlate with solar activity at all," Solanki says. He estimates that solar activity is responsible for only 30 per cent, at most, of the warming since 1970. The rest may be the result of man-made greenhouse gases, and a crash in solar activity won't do anything to get rid of them. What might happen is that the sun gives the planet a welcome respite from the alleged ravages of man-made climate change - though for how long, nobody knows. During the Little Ice Age, the fall in average global temperature is estimated to have been less than 1 degree Celcius and lasted 70 years. The one before that persisted for 150 years, but a minor crash at the beginning of the 19th century lasted barely 30. For now, we will have to keep watching for falling sunspot numbers. "The deeper the crash, the longer it will last," Weiss says. There is a dangerous flip side to this coin. If global warming does slow down or partially reverse with a sunspot crash, industrial polluters and reluctant nations could use it as a justification for turning their backs on pollution controls altogether, making matters worse in the long run. There is no room for complacency, Svalgaard warns: "If the Earth does cool during the next sunspot crash and we do nothing, when the sun's magnetic activity returns, global warming will return with a vengeance." Stuart Clark is a science journalist based in the UK Cloud Cover In 1997, meteorologists Henrik Svensmark and Eigil Friis Christensen of the Danish Meteorology Institute in Copenhagen analysed weather satellite records from 1979 to 1992. This was long enough for the sun's activity to complete one of its regular 11 -year cycles. The researchers found that the Earth was 3 per cent cloudier when the sun's activity was at a minimum than when it was at its peak. They also noted the influx of cosmic rays at five experiments across the globe and found that it was as much as 25 per tent higher at the solar minimum. They called their discovery a "missing link in solar-climate relationships" and argued that cosmic rays were responsible for increasing cloud formation by electrically charging the lower atmosphere. Intriguing as this link is, it is far from proof that solar activity and cloud cover are connected. "You have to demonstrate such an effect with an experiment, otherwise it is not physics," says Robert Bingham, a physicist at the UK's Rutherford Appleton Laboratory in Didcot, Oxfordshire. Robert Bingham Is part of an international collaboration building an experiment called CLOUD to test the idea that cosmic rays seed clouds. CLOUD will start up in 2008 using a particle accelerator at the CERN laboratory near Geneva as a source of simulated cosmic rays. The researchers will fire charged particles through a chamber holding a mixture of gases similar to the Earth's atmosphere to determine how often the particles trigger cloud formation. "CLOUD will go a long way towards understanding the microphysics of droplet formation," says Bingham. 
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