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Saturday, June 25, 2011

Sunspot Drop Won’t Cause Global Cooling

Sunspot Drop Won’t Cause Global Cooling

News that solar activity might fizzle for a few decades has prompted talk of a new “Little Ice Age,” even a quick fix for global warming. But that’s just not going to happen.
The cooling impact of the last prolonged solar lull “was probably only a couple tenths of a degree Celsius,” said climatologist Michael Mann of Penn State University. “It’s a tiny blip on the radar screen if you’re looking at the driving factors behind climate change.”
The possibility of imminent solar dormancy was raised by reports from the ongoing American Astronomical Society meeting of fading sunspots and dips in the sun’s magnetic patterns. Those are considered portents of solar inactivity, suggesting that the next solar minimum — a natural downturn in activity — would be especially pronounced, perhaps lasting for decades.

The frozen Thames, circa 1677. Artist unknown, Wikimedia Commons.
When that last happened, between the mid-17th and early 18th centuries, northern Europe experienced a period of unusually cold weather. Known as theMaunder Minimum, or more conversationally as the Little Ice Age, it’s a period historicized by accounts of ice skating on the Thames and seasonal inns built on Baltic Sea ice.
Press accounts of the new solar reports played up the Maunder Minimum angle, hinting that it might happen again. Some even implied that global warming might be counteracted.
In fact, the meaning of the latest sunspot reports is still being debated, as Andrew Revkin at Dot Earth has chronicled. But even if they really do portend a decades-long solar lull, studies already point to a minimal effect on climate.
Most Little Ice Age cooling appears to have been the result of coincidentally high volcanic activity that cloaked Earth in sunlight-blocking soot. As for the sun, a study published in 2001 in Science found that reduced solar activity produced a cooling effect of about 0.5 °F. In other estimates, the cooling is even more insignificant.
‘Global mean temperatures in the year 2100 would most likely be diminished by about 0.1°C.’
More recently, in a 2010 Geophysical Research Letters study, Georg Fuelner and Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research asked the question directly: What would happen if Earth experienced another 70-year-long solar minimum?
The answer can be seen in the image at the top of this post, which estimates the temperature difference between a solar minimum future under “middle-of-the-road” climate scenarios and the Maunder Minimum. In a nutshell: It’s going to be much, much hotter in the future, solar minimum or not.
“Global mean temperatures in the year 2100 would most likely be diminished by about 0.1 °C,” wrote Rahmstorf and Fuelner. And even if their models and assumptions were so uncertain as to be off by a factor of three, that still puts the cooling at just 0.3 °C, or about 0.5 °F.
Compared to the end-of-century global average temperature increase predicted by the Intergovernmental Panel on Climate Change — about 5 °F under mid-range scenarios, and 7 °F under more realistic scenarios — solar cooling would be negligible.
“A new Maunder‐type solar activity minimum cannot offset the global warming caused by human greenhouse gas emissions,” concluded Rahmstorf and Fuelner.
“The example I like to use is that greenhouse warming right now is the equivalent of 2 watts of power illuminating every square meter of the Earth’s surface. It’s like a Christmas tree light over every square meter. By mid-century, it will be closer to 4 watts,” said Mann, who was a co-author on that 2001 Science paper. “The maximum impact factor of the sun is 0.2 watts per meter squared.”
At most, a prolonged solar minimum would temporarily offset rising global temperatures for a few years, perhaps a couple decades, said NASA climatologist David Rind, who has also studied Maunder Minimum dynamics. But “when the sunspots return, the additional energy will cause additional warming,” he said.
“To point to this as something that could in any way ameliorate greenhouse gas warming is folly,” said Mann.
Image: Average global temperature differences, 2071-2100 and 1681-1710, assuming another period of Maunder Minimum-like solar inactivity. (Feulner & Rahmstorf, Geophysical Research Letters).

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