2005 Vying With '98 as Record Hot Year

Despite these differences, the numbers end up being so close that all groups said it was nearly impossible to distinguish 2005 temperatures from those in 1998.

"We're somewhat surprised by how similar [the analyses] are," Lawrimore said.

Much of the warming in 1998 was a result of the so-called "El Nino of the century," which contributed about a third of the planet's excess warmth. The fact that 2005, a year with a negligible El Nino effect, saw temperatures that rivaled those of 1998 is clear evidence of an "intense underlying global warming trend," said James Hansen, who compiles temperature data at the NASA Goddard Institute for Space Studies in New York.

"If you look at a time series, '98 just stands out so dramatically. Then you look at the past several years and you see it creeping back up. It's quite striking," Lawrimore said.

The new data on warming temperatures come amid other manifestations of a changing climate, including a record loss of sea ice in the Arctic this summer.

"The sea ice was a record minimum in the satellite era since 1979 and probably in the last century," said Mark Serreze, a senior researcher at the National Snow and Ice Data Center at the University of Colorado. "Compared to where it should have been, you've lost an area roughly twice the size of Texas."

Serreze said the Arctic is on track to be ice-free in summer by 2070. "What you're starting to see is the greenhouse effect starting to emerge," he said.

Scientists with the U.S. Minerals Management Service this week reported the apparent drowning of four polar bears in 2004, observed during agency flights over the Beaufort Sea. Scientists said they spotted an "unusually large" number of bears swimming farther offshore than usual, and subsequently saw floating polar bear carcasses. It concluded that drownings would probably increase as sea ice continued to diminish.

Three environmental groups Thursday sued the U.S. government, demanding threatened status for the polar bear under the Endangered Species Act, contending that the bears are victims of global warming. The bears' native habitat is shrinking as sea ice levels diminish, and human activities such as burning fossil fuels are to blame, the groups contend.

"Their sea-ice habitat, their hunting ground, is literally melting away," said Kassie Siegel of the Center for Biological Diversity, which had petitioned the government to consider listing the bear earlier this year.