Global warming not without cooler periods

ASHEVILLE, N.C., May 7 (UPI) -- U.S. scientists say people denying global warming due to short-term periods of cooling don't understand the statistical insignificance of such periods.

David Easterling of the National Climatic Data Center in Asheville, N.C., and Michael Wehner of the Lawrence Berkeley National Laboratory in Berkeley, Calif., said some Web sites and media articles have noted the average global temperature has not risen since 1998 and cite that as evidence against anthropogenic, or human-caused, global warming.

The researchers said although most climate scientists recognize the statistical insignificance of such short-term trends, some segments of the public pay attention to them. To quell such objections, they argue such short periods are not meaningful in the context of long-term climate change.

Easterling and Wehner said they have determined decade-long periods of cooling can occur even within a strong overall warming trend. The scientists said that due to natural variability, they expect some multi-year periods of cooling or stable temperatures to occur during the 21st century within longer-term periods of anthropogenic global warming.

The research appears in the journal Geophysical Research Letters.