Increased solar output contributed to at least 10 to 30 per cent of global warming from 1980 to 2002, two Duke University physicists report.

The study does not discount the theory that greenhouse gases contribute to global warming, stressed Nicola Scafetta and Bruce West, whose findings were published online in the research journal Geophysical Research Letters.

"Those gases would still give a contribution, but not so strong as was thought," Scafetta said in a written statement.

The new study is based in part on Columbia University research from 2003 in which scientists found errors in how data on solar brightness is interpreted.

After the 2003 Columbia disaster grounded the U.S. shuttle fleet, the data from sun-observing satellites was replaced by less accurate data from other satellites, Scafetta says.

In the absence of standardized data, Scafetta and West wrote, other researchers reported there was no increase in heating from the sun, thus concluding that all global warming increases observed between 1980 and 2002 were caused by greenhouse gases.

According to Scafetta, records of sunspot activity suggest that solar output has been rising slightly for about 100 years.

Using a 22-year interval to examine solar changes, the physicists filtered out shorter range effects that can influence surface temperatures but are not related to global warming.

These shorter-range effects include volcanic eruptions -- which can temporarily cool the climate -- and ocean current changes that affect global weather patterns.

Many questions remain, however.

"We don't know what the sun will do in the future," Scafetta says.

"For now, if our analysis is correct, I think it is important to correct the climate models so that they include reliable sensitivity to solar activity. Once that is done, then it will be possible to better understand what has happened during the past hundred years."