Global warming: What happens if the sun loses its spots?
Solar physicists increasingly say we could be entering a 'grand solar minimum' of no sunspot activity, the last one of which coincided with the Little Ice Age. Climate scientists are looking at how that could impact global warming.
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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."Skip to next paragraph
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"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.