What would happen to global warming if the sun quit producing sunspots for a few decades?
The question is of more than passing interest to climate scientists as they ponder the prospect that the sun may be about to enter such a period.
The coming flip of the sun's magnetic field in a few months marks the peak of the current sunspot cycle – one that has produced the fewest sunspots in at least 100 years, and perhaps the last 200 years.
Peering at trends – or in some cases, the lack of them – in the sun's behavior during the run-up to the current cycle's peak, some solar physicists increasingly are considering the possibility that the sun may be on the verge of a "grand solar minimum," comparable to a 70-year period running from the early 17th century into the early 18th century, when the sun produced no sunspots at all.
That period, known as the Maunder Minimum, coincided with the Little Ice Age, when the climate in the Northern Hemisphere cooled significantly.
In the most detailed look yet at the impact a similar event might have on global warming, researchers from the US and Australia have concluded that a 50-year grand minimum in sunspot activity likely would reduce global average temperatures during the period by a few tenths of a degree Celsius, but that the warming trend would resume once solar activity returns to normal.
"What if we went into another Maunder Minimum? Would that actually stop global warming"? asks Gerald Meehl, a researcher at the National Center for Atmospheric Research in Boulder, Colo., who led the team that conducted the study. "The short answer is: No. It slows it down for a while. But the minute the sunspots come back and the solar output goes back up, the temperature pops back up" close to where it would have been if the sun spots hadn't taken a powder, and the warming trend resumes.
The notion that the sun could be heading for a grand minimum hit the headlines two years ago, when three research teams using independent measures suggested that the next sunspot cycle's activity could be substantially lower than the current cycle's.
One sign: A fairly steady decline in the strength of the spots' magnetic fields over a 13-year-period. If the trend is to continue, scientists said, they anticipated a spotless sun by around 2022.
"We still see a decrease in the sunspot magnetic fields," says Matt Penn, a researcher with the National Solar Observatory in Tucson, who took part in the study. "The results are consistent with what we presented in 2011. It seems the trend is continuing along that line" leading to a cut-off in sunspot production.
More recently, research has suggested that the strength of the suns' magnetic field during one solar minimum – when the field is at its strongest – is a harbinger of the size of the peak for the next sunspot maximum. Over the past three sunspot cycles, those fields at solar minimum have been getting weaker, with the weakest appearing during the most recent minimum.
Given these trends, "I don't see how we're going to get fields any stronger this time around than they were" prior to the current solar maximum, known as cycle 24, says David Hathaway, a solar physicist at NASA's Marshall Space Flight Center in Hunstville, Ala. "It looks like cycle 25 might be smaller yet."
"My suspicion: If you think this cycle's bad, wait for then next one," he says.
Meanwhile, over the past decade climate scientists have evolved a better understanding of how – between the valleys and peaks of the sunspot cycle – a tiny increase in the energy the sun radiates toward Earth can affect climate.
Two mechanisms have emerged that can have a measurable effect, especially regional climate. The center of action for both is the tropical Pacific, and to a lesser extent, the North Atlantic, Dr. Meehl explains.
Most of the change in the sun's output is in the form of ultraviolet radiation. A top-down mechanism warms the stratosphere, as increased UV radiation stimulates the production of ozone, which releases heat. This heating changes circulation patterns in the stratosphere, which in turn alter circulation patterns in the troposphere below, where weather happens.
The other process is bottom up, where even smaller changes in visible light reach the sea in the relatively cloudless subtropics to set off a chain of changes in rainfall and wind patterns.
In each case, these changes can have effects far beyond the tropical Pacific, leading to changes in regional climate that are more pronounced than the direct affect of the slight increase in the sun's output. And they work in tandem.
During solar minimum, when the sun's output declines, these processes shut down, cooling the climate by a few tenths of a degree.
For the first time, Meehl and colleagues explored the impact of a grand solar minimum with a model that encompasses the top-down and bottom-up processes in the same model.
They assumed a 50-year sunspot hiatus presumed to run from 2020 to 2070. They used the solar cycles between 1965 and 2008 as their "normal" scenario, and temperature data from 1986-2005 as their temperature base. And both approaches used atmospheric greenhouse-gas concentrations that reach twice preindustrial levels by about 2070, then stabilize.
The team found that between 2026 and 2035, global average temperatures in the experiment would increase 0.80 degrees Celsius (1.4 degrees Fahrenheit) with normal solar activity, but only 0.64 degrees C. assuming sunspots went into a long hibernation.
By 2040, the pace of warming begins to pick up in the grand-minimum scenario. Between 2065 and 2080, after the grand minimum ends, warming has reached 1.47 degrees C above 1986-2005 levels with normal solar activity and 1.32 degrees C in the grand-minimum scenario.
Other researchers have performed similar experiment with similar models with similar results, the team acknowledges. But this work appears to capture the full extent of the effects, from initial cooling to the resumption of warming.
The results were published in May in the journal Geophysical Research Letters.