Thursday, February 28th 2013, 2:42 PM EST
Current solar cycle 24 (the curve on the right) shows fewer sunspot numbers than cycle 23 (on curve on the left) (NASA)
The current solar maximum, the high point in the sun’s activity over an 11-year cycle, is the lowest in over 100 years NASA says. Some scientists speculate solar activity may decline further in the coming decades. Could a quieting sun cause the climate to cool enough to stall or even reverse global warming from the build-up of greenhouse gases? Leading climate scientists say no, the effect of the sun is too small.
Every 11-year solar cycle is different, exhibiting stronger or weaker peaks and valleys. These variations have implications for climate and weather which researchers are trying to better understand. The general thinking is that less sunspot activity imparts a slight cooling effect on the climate but with significant regional differences.
The sinking number of sunspots (at the solar max) in recent decades has some solar scientists wondering: are we heading towards a state of solar quiescence not seen since the famed Maunder Minimum of the mid-1600s to the early 1700s? That historic low point in sunspot activity coincided with the heart of Little Ice Age which featured bitter cold winters in North America and Europe.[b]
Time series of sunspot numbers, from 1600 to present. (Judith Lean)
[b]“Indeed, the sun could be on the threshold of a mini-Maunder event right now,” NASA said last month. “Ongoing Solar Cycle 24 is the weakest in more than 50 years. Moreover, there is (controversial) evidence of a long-term weakening trend in the magnetic field strength of sunspots.”
But Judith Lean, a climate and solar scientist at the Naval Research Laboratory, said if we’re headed towards a Maunder Minimum-type event, its arrival is hardly imminent.
“I doubt that we are on the threshold of a Maunder Minimum event right now (as in, the next decade),” Lean said in an email. “The current modern maximum has indeed likely peaked, but the growth in activity from the Maunder Minimum (1634-1715) to the modern maximum (1976-1996) took a few centuries. So if solar activity is indeed in decline it will, I suppose, take a few centuries to reach a Maunder Minimum type event.”
Lean stressed forecasts of the sun’s activity are fraught with uncertainty and in their infancy.
“I don’t know what the sun will do - no one does because forecasting future solar activity even one solar cycle ahead is not at all reliable,” Lean said. “You may recall that initial forecasts for solar cycle 24 (our current cycle) were that it would be stronger than cycle 23 (which peaked around 2000-2002) whereas we are now probably near the peak of cycle 24 and it is very modest - weaker than 3 prior cycles.”
Let’s assume solar activity does drop in the coming decades. What might that mean for climate?
Michael Mann, professor of climate science at Penn State University, said the effect of the 11-year solar cycle on the global temperature is small.
“Studies that have looked at the longer-term impact of a potential 21st century grand solar minimum have found that the [temperature] response would be dwarfed by anthropogenic [manmade] warming [from the build-up of greenhouse gas concentrations],” Mann said.
For example, a recent study in the journal Geophysical Research Letters (Feulner and Rahmstorf, 2010) concluded a minimum in solar activity would shave just 0.3 degrees C off projected warming by 2100.
For her part, Lean agreed with the results of this study and with Mann.
“Given our current understanding of how the sun varies and how climate responds, were the sun to enter a new Maunder Minimum, it would not mean a new Little Ice Age - it would simply slow down the current warming by a modest amount,” Lean said.
While these scientists see just a minor impact on the average global temperature from a possible solar minimum, the effects on regional climate may be more profound, though incompletely understood.
A recent report from the National Research Council (NRC) “The Effects of Solar Variability on Earth’s Climate” finds small variations in solar output can have “major influence” on climate, particularly at the regional scale.
As an example of the sun’s possible effects on regional climate, consider the 2011 study in Nature Geoscience which found low solar activity is associated with the negative phase of the North Atlantic Oscillation (NAO). A negative NAO often results in cold, snowy conditions in eastern North American and northern Europe in winter.
Scientists say more research is needed to understand the possible effects of the sun’s evolving behavior on climate.
“If the sun really is entering an unfamiliar phase of the solar cycle, then we must redouble our efforts to understand the sun-climate link,” said Lika Guhathakurta of NASA’s Living with a Star Program, which helped fund the NRC study.
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