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Sun fuels climate change

The recipe for global warming has changed, according to a new statistical analysis of solar output. A team of scientists says that recent increases in Earth’s surface temperatures result not only from greenhouse gases and climate variability, but also from rises in solar output — a contribution that researchers say climate models have previously neglected.

The sun, pictured here over Antarctica, may be increasing its output and contributing to global warming more than previously thought. Image courtesy of Commander John Bortniak, NOAA Corps.

In 2003, Richard Willson of Columbia University in New York City interpreted more than two decades of satellite data and concluded that solar output has increased. Using that data, Nicola Scafetta and Bruce West, researchers in the physics department at Duke University in Durham, N.C., conducted a statistical analysis to find out how the increasing solar output affects climate change. The results, published online Sept. 28 in Geophysical Review Letters, found that from 1980 to 2002, rises in solar output contributed a minimum of 10 to 30 percent to the total increases in Earth’s surface temperatures.

The results, Scafetta says, show that solar activity cannot be ignored and that current climate models need to be changed. “The climate seems to respond to solar changes much more strongly than what present climate models have predicted.”

But Willson’s solar data reconstruction, from which Scafetta’s study is based, is a topic of heated debate among solar physicists. Of the satellites that monitored solar output since the late 1970s, three consecutive ACRIM experiments produced some of the most reliable data, which are widely used in solar research. The Challenger space shuttle disaster in 1986, however, prevented immediate replacement of the first satellite, leading to a two-year gap, from 1989 to 1991, in the solar data.

Different methods of bridging the gap, based on data from less accurate satellites, led to two different interpretations. Although the sun’s energy fluctuates naturally within an 11-year cycle, one method shows no change in that cycle over time, while the other, which Willson and Scafetta used, shows an overall increase.

Judith Lean, a solar physicist at the U.S. Naval Research Laboratory in Washington, D.C., and colleague Claus Fröhlich dispute Willson and Scafetta’s claim of an upward trend in solar output and its purported influence on climate. Lean says that Willson filled the gap with data from a satellite that did not account for instrument sensitivity changes over time, which led to his measured jump in solar output over the two-year gap. “He says that this is a secular trend, but it isn’t,” Lean says. “It’s just a jump.”

But the debate about the missing data will not end anytime soon. “You can’t prove it one way or the other,” Lean says.

Measurements of solar output continue via missions such as the NASA-sponsored Solar Radiation and Climate Experiment, launched in January 2003, which is providing more data about the sun’s effect on Earth and climate. Regardless, however, of whether or not the sun is increasing its solar output, Scafetta says that the contribution from greenhouse gases alone is sufficient to keep Earth warming.

Kathryn Hansen

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