The Financial Times acknowledges the Sun’s role in Earth’s climate; ends with sure, it’s cooling, but it would be a lot colder if it weren’t for global warming.
To add to the uncertainty, no one knows how long the Sun is likely to stay quiet. One extreme would be a continued period of inactivity, with very few sunspots or solar storms, that could last for decades. The last such suspension of the 11-year solar cycle occurred between 1645 and 1715, a period known by historians of astronomy as the Maunder Minimum, which coincided with the coldest period of the past millennium, known as the “little ice age”.
“If we had a repeat of the Maunder Minimum it would be very exciting [for science] but that is not likely to happen,” says Nancy Crooker, research professor at Boston University. Like the majority of astronomers, she expects solar activity to pick up soon, leading to the next maximum around 2012.
There is no agreement, however, on how disturbed the Sun will be during the next maximum. According to some predictions, it will be relatively calm, with fewer solar storms than during the last few maxima (in 2001-02, 1990-91 and 1980-81). Others say that, despite the low starting point, the Sun could still build up to an exceptionally intense maximum over the next four years.
“Predicting the next maximum now is rather like forecasting next summer’s weather in the middle of winter,” says Jim Wild, a space scientist at Lancaster University.
The 11-year cycle is driven by the changing magnetism of the “solar dynamo”. The faster rotation of the Sun’s equatorial regions than the poles amplifies the magnetic field until it bursts through to the surface, causing sunspots and shooting billions of tonnes of solar material into space – a “coronal mass ejection.” [B]ut there is no good model to explain the timing or intensity of these events. . . .
One certain effect of a quiet Sun is that more high-energy cosmic rays from elsewhere in the universe can beat their way through the weakened solar wind and reach Earth. Although these cosmic rays do not arrive in a sudden eruption, like a solar storm, their steady cumulative impact may shorten the lifetime of satellites. . . .
The relationship between solar variability and climate remains a mystery, says Jamie Casford, a climate researcher at Durham University. While the Sun’s magnetic field and the solar wind change remarkably over the years – the Ulysses satellite measurements show that they are 20 to 30 per cent weaker now than at the last solar minimum in 1996-97 – the accompanying changes in the Sun’s total energy output are tiny.
When the Sun is very quiet, the amount of energy that reaches Earth is only 0.1 per cent less than when it is very active – a change too small to produce significant global cooling on its own. “I would say that solar variability does feed into the climate system but we really do not know what the mechanism is,” Dr Casford says.
There are several theories. A controversial one comes from Henrik Svensmark, a physicist at the Danish National Space Centre in Copenhagen; he believes increased cosmic radiation, hitting the atmosphere when the Sun is quiet, stimulates cloud formation – which cools the planet. Paul Mayewski, director of the University of Maine Climate Change Institute, says the primary impact of solar variability is on atmospheric circulation, which then affects temperature.
Although some people who are sceptical about the human influence on global warming like to emphasise the link between solar variability and climate, Prof Mayewski turns their argument on its head: “The fact that we are not in conditions like the little ice age today shows that the atmosphere is being perturbed by human activities,” he says.
If the Sun stays quiet for the next few years, it may temper the effects of man-made global warming for a while but most experts believe that rising levels of carbon dioxide in the atmosphere will eventually push temperatures higher again.