Solar cycles of magnetic fields and sunspots have become a popular foothold for climate change skeptics. A new study in the journal Geophysical Research Letters, however, shows that even if predictions of an extended minimum of solar activity are accurate, it will have only a tiny effect on the Earth's climate in comparison to the current track of human-caused warming.
"There is a lot of hysterical stuff out there," said Gavin Schmidt, a climatologist at NASA's Goddard Institute for Space Studies. "For some reason, solar effects seem to attract more than their fair share of cranks. There are always people with these statistical models claiming that it would have a big effect, but mostly that's just nonsense."
The new study, conducted by Georg Feulner and Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany, modeled what might happen to global temperatures if the sun enters a period of low magnetic and sunspot activity resembling that of the Maunder Minimum.
The Maunder Minimum was the last extended period of decreased solar activity, from about 1650 to 1710. (Normally, solar magnetic cycles last 11 years.) At the time, it was associated with markedly cooler temperatures, but that was before the Industrial Revolution and the beginning of modern emissions of greenhouse gases from widespread fossil fuel use.
Feulner and Rahsmtorf reported two versions of their model. They found that the greatest impact on temperature in the year 2100 from a solar minimum like the Maunder Minimum would be 0.26 degrees Celsius lower than without the effect. Using two models from the Intergovernmental Panel on Climate Change, they also showed that with normal solar activity plus current human activities, global temperature is on pace to rise by 3.7 or 4.5 degrees Celsius above pre-industrial times by the year 2100.
Even with the quarter-degree impact from the lower solar activity factored in, global temperatures will still be on pace to rise more than 3 degrees — well above the 2-degree limit that scientists and world leaders consider a threshold for dangerous climate change.
"A new Grand Minimum of solar activity cannot save us from global warming," Feulner said in an e-mail to SolveClimate. "Moreover, the slight temperature decrease due to a prolonged solar minimum would be temporary, since the historic Grand Minima have lasted for a few decades to a century at most. There is no way around climate mitigation measures to prevent dangerous climate change."
Feulner did point out a few limitations of the study. "Our model accounts for the variation of the total incoming solar radiation only, but does not take into account variability in the sun's ultraviolet radiation," he said. Schmidt, who was not involved in the research but has done work on solar effects on climate, said that the ultraviolet changes can be of greater magnitude; this can have an impact on ozone in the atmosphere, which could slightly increase the effect on temperature.
"This is one of the reasons why we — conservatively — say that the temperature offset in 2100 could be as large as -0.3 degrees, although the most likely value is -0.1 degrees only," Feulner said.
Schmidt agreed, saying: "You'll get slightly different sensitivities with different models and different levels of complexity, but the results are clear enough that it wouldn't make any difference."
Matt Penn, an associate astronomer at the National Solar Observatory in Tuscon, said the discussion of whether or not a grand minimum in solar activity is on its way is justified based on recent periods of low activity, but it is a very difficult thing to predict.
"I wish I could be definitive, but it is research at the edge of what we know," he told SolveClimate.
"We know that the new solar cycle has started, the magnetic cycle has started, but if the sunspots continue on this trend with weakening magnetic fields, then we're going to enter the same type of Maunder Minimum."
Penn added that Feulner and Rahmstorf's analysis appears solid and that it does clearly show greenhouse gases play a much larger role than the sun's activity in driving warming.
That possibility of a new solar minimum has produced strong statements from climate change skeptic camps in recent years. Marc Morano, the former communications director for Sen. James Inhofe's Republicans on the Senate's Environment and Public Works Committee, has repeatedly harped on the sun's impact on climate, both on Senate web sites and now on the conservative blog ClimateDepot. The focus of many of even his claims, though, is on where the sun's activity is heading and less on the climatic result. This new study shows that even with that solar minimum, the impact on climate is minimal compared to human activity.
Schmidt notes that it is still unlikely to end the solar-related skeptic claims even though "serious consideration in the literature ended a while ago."
He added that this was a novel experiment in that it looked forward and assumed the sun's diminished activity.
"It is good to quantify things," Schmidt said. "Instead of just sitting around waving your arms and saying 'well I don't think it will be a big factor,' you actually throw it into the model and you demonstrate that its not going to be a big factor, so that's useful."
(Photo: TRACE/Stanford-Lockheed Institute for Space Research)