Sun fuels debate on climate change

New Scientist vol 178 issue 2390 - 12 April 2003, page 14


IS THE Sun contributing to global warming? It's a simple but crucial question and, despite making detailed measurements of the Sun's brightness for more than two decades, researchers are still arguing over the answer.

In the latest instalment of the controversy, solar astronomer Judith Lean of the Naval Research Laboratory in Washington has attacked claims that the Sun is getting hotter. There has been no overall change in the Sun's output, she told the UK/Ireland National Astronomy Meeting in Dublin.

Her main adversary, Richard Willson of the Institute of Climate Research at Columbia University, California, created headlines last month when he announced that he had detected a 0.1 per cent increase in the Sun's brightness since 1980. The Sun's output varies by this much in a short, 11-year cycle which is not thought to appreciably affect climate.

But Willson compared successive low points of the cycle. He says the change may represent a much longer-term trend in the Sun's activity. If he is right, the increase in brightness could account for half of the 0.3-degree increase in average global temperatures recorded since 1980.

The claim is good news for those who say the greenhouse effect is not responsible for global warming. But Lean has now hit back, saying there is no evidence the Sun is getting brighter.

Most researchers, including Lean, agree that long-term trends in the Sun's brightness can affect the Earth's climate. A 1500-year cycle has been identified by measuring isotope levels in seabed deposits, which may be responsible for generating mini ice ages. Yet the received wisdom is that the Sun's overall activity is currently constant, a fact that clearly frustrates Willson.

"The role of the Sun [in global warming] has been minimised or ignored," he says. "Most of the climate modellers have a mindset about the Sun being a constant, but they have only grudgingly admitted that there might be something outside the atmospheric box they need to think about." But Lean argues that there simply is not enough evidence for Willson's claims. "The bottom line is that no other solar observation shows an increasing trend," she says.

Willson and Lean studied the same satellite data, from NASA's ACRIM1 and ACRIM2 missions, so how did they reach such different conclusions?

The ACRIM programme, of which Willson is principal investigator, measures the Sun's brightness or "total solar irradiance". This is done by allowing sunlight to fall onto a black surface, then measuring the amount of electrical energy needed to cool it again. But it's a delicate operation. The satellite's own microclimate can affect readings, and the black coating often degrades in space.

ACRIM1 was launched in 1980 and operated until 1989. ACRIM2 should have been launched before ACRIM1 finished operating, but its launch was delayed until 1991 by the Challenger space shuttle disaster. That left what Willson calls the "ACRIM gap" - a critical two-year period when no data was collected. It means researchers have no way of cross-checking readings from the two satellites.

Both Willson and Lean used data from other satellites to fill the gap. Willson turned to Nimbus7, one of the longest-serving Sun-watching satellites, operating from 1978 until 1993. Meanwhile Lean and her colleague Claus Frohlich of the World Radiation Data Centre in Davos, Switzerland, used data from NASA's Earth Radiation Budget Satellite (ERBS), launched in 1984.

Lean says that while Willson claims an increase in brightness of 0.1 per cent, the satellites were only designed to be accurate to plus or minus 0.2 per cent. Willson counters that the data from ERBS is itself flawed. He says that during the ACRIM gap, the ERBS satellite dropped in sensitivity by 0.1 per cent. "It's too close to be a coincidence. Our interpretation is most likely to be correct," he says.

With the data so finely balanced, it is easy for opinions to become more heavily influenced by politics than science. The Bush administration, unconvinced by greenhouse warming, is likely to embrace Willson's findings.

Harvard solar physicist Willie Soon, who is also a senior scientist at the Marshall Institute, a US think tank closely allied to the hydrocarbon and defence industries, supports Willson too. "Lean and Frohlich make too many assumptions," he says. Soon has spent the past 30 years observing nearby Sun-like stars, and says he has seen changes in brightness similar to that claimed by Willson. However the Intergovernmental Panel on Climate Change, which reports to the UN, relies mainly on Lean and Frohlich's work for its assessment of solar trends.

Peter Foukal, an astrophysicist who has been tracking the Sun since the 1970s, is sceptical of strong claims made on either side. "It is a very perilous exercise," he says. "The data are marginal, perhaps not good enough for the kind of statement Willson is trying to make." But he adds: "Frohlich and Lean may be overstating the case also - some of us are concerned that they pay too much attention to the theory of what the Sun is supposed to be doing."

The answer looks set to remain tantalisingly out of reach for the next few years. The best hope for resolving the controversy lies with NASA's newest satellite for measuring the Sun's brightness, SORCE1, which was launched in January. It's the first mission to contain self-calibrating instruments.

But history may yet repeat itself. No successor to SORCE1 is scheduled to launch until 2011, three years after the first craft is expected to expire. "We need to launch something around 2007, otherwise the SORCE gap will be just as controversial as the ACRIM gap," says SORCE's project manager, Robert Cahalan.

Gerry Byrne