Climate-change sceptics have suggested that solar cycles may be more responsible than human activity for increasing global temperature. But Lean said her findings showed "the sun is a factor of 10 less than the anthropogenic."

Scientists at the forum listed causes for climate change including alterations in the atmosphere's makeup, changes in forests or ice covering the land, volcanoes, man-made greenhouse gases, solar cycles and factors yet undiscovered.

"We're not in an either-or world," said Kurt Cuffey, geography department chairman at the University of California at Berkeley. "It's all of these thrown together."

The AAPG, whose members work in the oil industry, has an official position backing more study of climate change. AAPG also says human fossil fuel use should be more efficient and greenhouse gases should be cut, but only at reasonable cost.

Eric Barron, geosciences dean at the University of Texas, argued that causal factors in climate change may be interacting in ways not yet imagined. For example, whether carbon dioxide build-up is a cause or effect of temperature rise depends on whether one is talking 30 years or a million years, he said.

In the short run, human activity is putting more carbon dioxide into the air, but in the long run, the impact might be mitigated or made worse by climate responses not yet understood or predicted, he said.

Lean said small changes do not occur in isolation but as part of a larger, ever-changing climate system, and that makes forecasting difficult.

"You've got to be careful, because everything's changing," she said.

Barron called for deeper study of the geologic record to better calibrate computer models predicting the future.

"If you do that, you might find we have even more to worry about," Barron said, adding that the Earth's geologic history has an important climate story to tell. "It shows the climate is very sensitive to small changes," he said.