An image taken in February by NASA’s Solar Dynamics Observatory revealed just a few small sunspots on the sun’s surface. ( NASA VIA REUTERS)

The sun’s activity is in free fall, according to a leading space physicist. But don’t expect a little ice age.

“Solar activity is declining very fast at the moment,” Mike Lockwood, a professor of space environmental physics at Britain’s Reading University, said, “we estimate faster than at any time in the last 9,300 years.”

Lockwood and colleagues are reassessing the chances of this decline’s becoming the first “grand solar minimum” in four centuries. During a grand minimum, the normal 11-year solar cycle is suppressed and the sun has virtually no sunspots for several decades. This summer should have seen an increase in the number of sunspots, but it didn’t happen.

Lockwood thinks there is now a 25 percent chance of a repetition of the last grand minimum, the late-17th-century Maunder Minimum, when there were no sunspots for 70 years. Two years ago, Lockwood put the chances of this happening at less than 10 percent.

The Maunder Minimum coincided with the worst European winters of the little ice age, a period lasting centuries when several regions around the globe experienced unusual cooling.

But Lockwood says we should not expect a new grand minimum to bring on a new little ice age. Human-induced global warming, he says, is already a more important force in global temperatures than even major solar cycles. Temperatures have risen by about 1.5 degrees since 1880, with more warming expected, according to the most recent assessment of the Intergovernmental Panel on Climate Change.

Still, there may be noticeable consequences. For instance, less solar activity can slow the jet stream, triggering extreme weather events such as the Russian heat wave of 2010 and the devastating floods in Pakistan that same year.

There have been 24 grand solar minimums in the past 10,000 years. Their history is reconstructed by looking for isotopes, including carbon-14, that cosmic rays generate in the atmosphere. Solar activity boosts the solar wind, which deflects cosmic rays coming at Earth, so less solar activity means more cosmic rays and more of these isotopes.

How the isotopes vary over time can be measured by looking at such things as tree rings, which absorb carbon-14, and ice cores, which accumulate beryllium-10.

The current long-term decline in solar activity began after the last grand solar maximum peaked in 1956, Lockwood says. The decline has accelerated recently, and the absence of sunspots this summer has set alarm bells ringing.

The extent to which solar cycles influence global temperatures is still debated, as is the question of whether the recent decline has helped cause the current hiatus in the pace of global warming.

“Mike is probably right that there is a chance of the sun returning to a level of activity similar to the Maunder Minimum,” says atmospheric physicist Joanna Haigh of Imperial College London. But she adds: “Even under the most optimistic scenario [of minimal global warming and a deep solar minimum] the solar cooling would only just offset greenhouse gas warming. So no ice age.”

It is more likely that it will simply reduce the warming a little, and set us up for greater warming if it receded.

New Scientist