It is known as the Little Ice Age. Bitter winters blighted much of the northern hemisphere for decades in the second half of the 17th century. The French army used frozen rivers as thoroughfares to invade the Netherlands. New Yorkers walked from Manhattan to Staten Island across the frozen harbour. Sea ice surrounded Iceland for miles and the island’s population halved. It wasn’t the first time temperatures had plunged: a couple of hundred years earlier, between 1420 and 1570, a climatic downturn claimed the Viking colonies on Greenland, turning them from fertile farmlands into arctic wastelands.
Could the sun have been to blame? We now know that, curiously, both these mini ice ages coincided with prolonged lulls 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 enter 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 enact pollution controls. "It would certainly buy us some time," says Joanna Haigh, an atmospheric physicist at Imperial College London.
Global average temperatures have risen by about 0.6 °C 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 believe 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, depending 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 foretells massive solar eruptions that fling billions of tonnes of gas into space. Fewer sunspots pop up when the sun is calm, and historically these 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, but every 200 years or so, the sunspots all but disappear as solar activity slumps (see "Field feedback"). For the past 50 years, on the other hand, the sun has been particularly restless. "If you look back into the sun’s past, you find that we live in a period of abnormally high solar activity," says Nigel Weiss, a solar physicist at the University of Cambridge.
Fortunately, an indirect record of the sun’s moods stretching back thousands of years has been preserved on Earth in the concentrations of rare isotopes locked into tree rings and ice cores. The story begins way out beyond the orbit of Pluto, at the boundary of the sun’s magnetic field. 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.
For the full article: New Scientist