Does a Weaker Sun Mean a Warmer Earth?

The sun controls Earth's climate, bathing us in light ranging from ultraviolet to visible that warms the planet and drives the heat engines we know as weather systems and ocean currents. The sun is changeable, cycling from maximum to minimum outputs over a roughly 11-year cycle, increasing or decreasing the amount of light that reaches Earth as a result of the poorly understood aspects of the sun's seething nuclear fusion. Now new satellite measurements reveal that from 2004 to 2007—the declining phase of an unusually low and prolonged solar minimum—the sun put out even less ultraviolet light than expected but compensated by putting out more visible light.

 "The amount of visible radiation entering the lower atmosphere was increasing, which implies warming at the surface," says atmospheric physicist Joanna Haigh of Imperial College London, who led the research, published in Nature on October 7. (Scientific American is part of Nature Publishing Group.) "The solar radiative forcing of climate increased by 0.1 [watt per square meter]." That means the sun, at least for those three years, played a larger role in ongoing climate change than previously thought.

Global climate change—average temperatures have risen by roughly 0.6 degree Celsius since the beginning of the 20th century—is caused by greenhouse gases in the atmosphere. These gases, chief among them carbon dioxide (CO2), act as a blanket, trapping the sun's heat that would otherwise be radiated back into space. Rising greenhouse gas levels in the atmosphere means rising average temperatures for the planet, causing climate change.

But the change from 2004 to 2007 in the sun's output of visible light, and the attendant warming at Earth's surface of 0.1 watt per square meter, is roughly equivalent to the overall forcing of the sun on the climate over the past 25 years—estimated by the U.N. Intergovernmental Panel on Climate Change to be an additional 0.12 watt per square meter. That suggests scientists may have overestimated the sun's role in climate change.

Regardless, the solar change is dwarfed by the impact from the extra heat trapped by CO2 alone since 1750: an additional 1.66 watts per square meter, an effect that other greenhouse gases, such as methane, strengthen further. In other words, whereas the new satellite measurements call into question computer models of solar output, it does not change the fundamental physics of human-induced global warming.

Still, the finding suggests that scientists' understanding of solar cycles and their impact on climate needs more work. "The result reverses understanding of solar cycle climate effects," which had been that the sun generally warms the climate on the way up from minimum to maximum and generally cools the climate on the way down from maximum to minimum, explains atmospheric scientist Piers Forster of the University of Leeds in England. "But the opposite seems to have been true of the last solar cycle."

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In addition, the larger than expected loss of UV light meant less stratospheric ozone up to 45 kilometers above the surface, but more above that line. That distinguishes this solar cycle from the preceding two and "suggests that the declining phase of solar cycle 23 is behaving differently to previous solar cycles," the team wrote.

Of course, solar irradiance measurements from just three years of one solar cycle cannot be applied to any other period than the one measured by the Spectral Irradiance Monitor on NASA's Solar Radiation and Climate Experiment (SORCE) satellite. "We cannot extrapolate to a 250-year period," Haigh says. "While this increase is similar to that produced by greenhouse gases, it may well turn round with the 11-year cycle so it can't be used to imply any long-term forcing."

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In fact, the solar minimum for the last cycle was reached in 2009, and the sun's activity has picked up in the intervening months. It remains to be seen if that will bring a decline in the sun's output of visible light—and therefore a decline in the sun's contribution to a warming climate during this upward part of the present solar cycle. The sun "was thought to be having a cooling effect over the last few years," Forster notes, a thought now shown likely to be mistaken. "Perhaps the sun has been trying to warm the Earth after all."