Global warming: checking out the sun.

Why has the globe's surface warmed so dramatically in the last century? Scientists and policymakers are anxious to determine whether this climate change stems from greenhouse-gas pollution or from entirely natural processes. In the past, several researchers have suggested that solar changes underlie the 20th-century warming, but their evidence left many experts unconvinced. Now, two Danish geophysicists present intriguing data that revive the sun-climate theory.

Eigil Friis-Christensen and Knud Lassen of the Danish Meteorological Institute in Copenhagen chose a new method to compare solar activity and temperature records. Scientists had previously used sunspot frequency as an indication of the sun's energy output; the Danish duo instead looked at the length of the solar cycle, which varies from 10 to 12 years. They based their technique on evidence that solar radiation is slightly more intense during shorter cycles.

When they plotted solar-cycle lengths against land temperatures recorded in the Northern Hemisphere since the late 1800s, the two curves looked almost identical, they report in the Nov. 1 SCIENCE. They also compared cycle lengths with Iceland sea ice records since 1740. Again, a correlation between cycle length and climate appeared. "I think what we have found is that the global warming up until now has been forced by variations in the activity of the sun," Lassen says.

While the findings have renewed interest in a possible link between the sun and climate change, most scientists remain wary of drawing conclusions. The connection may be merely coincidental; in the past, many impressive correlations have crumbled under closer scrutiny.

Confirmation of the Danish findings would indicate that greenhouse-gas pollution has not significantly raised Northern Hemisphere temperatures to date. But that could change, Lassen notes. "We cannot tell what will happen in the future, but we think there's a good possibility in years to come you will feel the greenhouse warming," he says.

Lassen suggests that other types of pollution in the heavily industrialized Northern Hemisphere may have compensated for greenhouse gases by cooling this half of the blobe. In the Southern Hemisphere -- for which temperature records are less reliable -- greenhouse gases may have had a greater impact than in the north. Scientists say such differential warming could alter the climate worldwide.