New international climate data show that 2005 is on track to be the hottest year on record, continuing a 25-year trend of rising global temperatures.
Climatologists at NASA's Goddard Institute for Space Studies calculated the record-breaking global average temperature, which now surpasses 1998's record by a tenth of a degree Fahrenheit, from readings taken at 7,200 weather stations scattered around the world.
The new analysis comes as government and independent scientists are reporting other dramatic signs of global warming, such as the record shrinkage of the Arctic sea ice cover and unprecedented high ocean temperatures in the Gulf of Mexico.
Late last month, a team of University of Colorado and NASA scientists announced that the Arctic sea ice cap shrank this summer to 200 million square miles, 500,000 square miles less than its average area between 1979 and 2000. And a scientist at the National Oceanic and Atmospheric Administration determined that sea surface temperatures in the Gulf of Mexico were higher in August than at any time since 1890, which may have contributed to the intense hurricanes that struck the region this year.
"At this point, people shouldn't be surprised this is happening," said Goddard atmospheric scientist David Rind, noting that 2002, 2003 and 2004 were among the warmest years on record.
Many climatologists, along with policymakers in a number of countries, believe the rapid temperature rise over the past 50 years is heavily driven by the burning of fossil fuels and other human activities that have spewed carbon dioxide and other "greenhouse gases" into the atmosphere. A vocal minority of scientists say the warming climate is the result of a natural cycle.
Rind compared the warming trend to what happens when a major league baseball team owner spends lavishly on players' salaries. Pumping heat-trapping gases into the atmosphere, he said, produces the same kind of predictable results as boosting a team's payroll.
"When they get into the playoffs, should we be surprised?" he asked. "We're putting a lot more carbon dioxide into the atmosphere, and we're getting a lot higher temperatures."
Global temperatures this year are about 1.36 degrees Fahrenheit (0.75 Celsius) above the average between 1950 and 1980, according to the Goddard analysis. Worldwide temperatures in 1998 were 1.28 degrees Fahrenheit (0.71 Celsius) above that 30-year average. The data show that Earth is warming more in the Northern Hemisphere, where the average 2005 temperature was two-tenths of a degree above the 1998 level.
Climate experts say such seemingly small shifts are significant because they involve average readings based on measurements taken at thousands of sites. To put it in perspective, the planet's temperature rose by just 1 to 1.5 degrees Fahrenheit over the past century.
Rind, who said it would probably take a major event such as a massive volcanic eruption to keep this year from setting a record, said that scientists expect worldwide temperatures to rise another degree Fahrenheit between 2000 and 2030, and an additional 2 to 4 degrees by 2100.
From that perspective, this year's higher temperatures are "really small potatoes compared to what's to come," he said.
But one skeptic, state climatologist George Taylor of Oregon, said it is difficult to determine an accurate global average temperature, especially since there are not enough stations recording ocean temperatures.
"I just don't trust it," Taylor said of the new calculation, noting that Goddard's findings are "mighty preliminary."
Several scientists said yesterday that Earth's rapid warming could become self-perpetuating as the buildup of heat in the air, on land and in the sea accelerates. Ted A. Scambos, the lead scientist at the National Snow and Ice Data Center in Boulder, Colo., said the shrinkage of sea ice in the Arctic makes it more likely that the region will warm faster, because open water absorbs much more heat from the sun than snow and ice.
"Change is really happening in the Arctic. We're going to see this again and again," Scambos said. He added that, because the Arctic helps keep global temperatures down, any warming there can mean "you're going to change [the world's] climate significantly."
In response to recent warming in the Arctic, a coalition of environmental groups said it plans to sue the Interior Department to force it to list polar bears as threatened under the Endangered Species Act because the sea ice they depend on is disappearing. The Natural Resources Defense Council, the Center for Biological Diversity and other groups petitioned for the listing in February, but they say Interior Secretary Gale A. Norton has yet to respond.
"The polar bear's a harbinger of what's to come. It's the first animal to be threatened with extinction by climate change, but it won't be the last," said NRDC attorney Andrew Wexler. He noted that polar bears cannot adapt well to rising temperatures because they are dependent on sea ice for survival.
U.S. Fish and Wildlife Service spokesman Chris Tollefson said the agency is analyzing the petition. "We haven't really reached a conclusion," Tollefson said.
The Bush administration has consistently advocated funding for technological research rather than requiring curbs in carbon dioxide emissions, saying that such limits could damage the economy.
William O'Keefe, chief executive of the George C. Marshall Institute, which is skeptical of global warming predictions, said policymakers should not rush to impose new rules on industry when it remains unclear whether the current warming worldwide reflects natural climate variability or a human-induced trend.
"It still remains very complicated," O'Keefe said.
But Rafe Pomerance, who served as deputy assistant secretary of state for the environment under President Bill Clinton and who now chairs the bipartisan Climate Policy Center, said a modest system to limit and trade carbon dioxide emissions could help curb global warming.
"We need to develop a range of very serious policies and put them in place," Pomerance said.