National Geographic Daily News
The sun, seen in ultraviolet light.
Swirling plasma envelops the sun in September 2010.

Image courtesy SDO/NASA

Ker Than

for National Geographic News

Published October 6, 2010

A decline in the sun's activity can actually lead to a warmer Earth—not a cooler one as previously suspected, according to a new study.

The sun is currently at a low point in its 11-year solar cycle, when the sun's total output of light, called solar irradiance, is relatively reduced.

(See "Sun Oddly Quiet — Hints at Next 'Little Ice Age'"?)

Because solar irradiance contributes to atmospheric warming on Earth, scientists had previously thought that less total solar irradiance results in cooler global temperatures.

But a new study, in this week's issue of Nature, shows this isn't necessarily the case.

The study uses data collected by the NASA-sponsored SORCE satellite, which allows scientists to fully break down the sun's light output into its different wavelength components for the first time.

"Previously we only had measurements of the total energy coming out of the sun and measurements of part of the ultraviolet spectrum," explained study leader Joanna Haigh, an atmospheric physicist at the U.K.'s Imperial College London.

Sun's Role in Climate Change Overestimated?

The SORCE data reveals that even though total solar irradiance declined from 2004 to 2007, the amount of visible light output from the sun was actually increased.

Unlike other wavelengths of light, visible light cuts through different atmospheric layers to warm the surface of the Earth directly, creating an overall warming effect.

If low solar activity can lead to a warmer Earth, the opposite might also be true, the team speculates.

It's possible that rising solar activity tends to cool—rather than warm—Earth, because visible light output is actually reduced.

If this is true, it would mean that climate scientists have been overestimating the contributions of the sun on climate change and underestimating the effect of human activity, according to Michael Lockwood, a climate scientist at the U.K.'s University of Reading.

(Related: "Mars Melt Hints at Solar, Not Human, Cause for Warming, Scientist Says.")

"This is not good news for skeptics" of the idea that humans largely cause global warming, said Lockwood, who was not involved in the study.

Some such skeptics "have been proposing that because the solar variation is not what we thought, that means we just got it wrong [all] along, so how can we trust our climate models?" he added.

"But that's rather like saying that because I found one word spelled wrong in Shakespeare, none of Shakespeare is worth reading. It's about on that level."

Sun's Impact on Global Warming Still Cloudy

Study author Haigh stressed, however, that it's still too early to jump to conclusions about the long-term implications of her team's findings.

Only three years worth of SORCE data are currently available, and scientists will have to monitor solar activity for a complete sun-activity cycle to test their ideas. (See sun pictures.)

"At the moment, we have no evidence that the sun does behave like this on longer periods," she said. "It may just be that the sun is unusual at the moment."

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