A record-breaking stretch of warmth in recent years -- with 2001 now going down as the third-warmest year on record and 1998 still holding the all-time record -- has scientists and climate experts concerned that greenhouse gases are warming the planet more quickly than previously expected.
"Studying these annual temperature data, one gets the unmistakable feeling that temperature is rising and that the rise is gaining momentum," said Lester R. Brown, an economist and president of the Earth Policy Institute in Washington.
The Earth's average temperature during the 2002 meteorological year was 58.35 degrees Fahrenheit, more than one degree warmer than the long-term average of 57.2 degrees, said James E. Hansen, a climate scientist at the NASA Goddard Institute for Space Studies who analyzes surface temperatures collected from several thousand weather stations around the world.
The meteorological year runs from Dec. 1 to Nov. 30. During that period, 2001 temperatures averaged 58.12. In 1998, the average global temperature rose to 58.41 degrees, the highest seen since temperature records were first compiled in the late 1800s.
The string of warmer years provides strong evidence that humans are in large part to blame for changing the climate, said Peter Frumhoff, an ecologist and senior scientist with the Union of Concerned Scientists in Cambridge, Mass.
"It's important we pay attention to this drumbeat of evidence as the signal of human impact starts to emerge from the noise of natural climate patterns," he said.
The warm temperatures of 2001 and 2002 are especially significant when they are considered in the light of El Nino weather patterns that alter global climate, Hansen said.
Some of the heat of 1998 can be attributed to a large El Nino event that year, which warmed the waters of the Pacific Ocean. But 2001 saw a La Nina event, a spell of cooler conditions, which kept temperatures that year from soaring even higher. There is a weak El Nino developing now, but it is not generating nearly as much heat as the 1998 event.
"The fact that 2002 is almost as warm as the unusual warmth of 1998 is confirmation that the underlying global warming trend is continuing," Hansen said.
Most of the warmth of 2002 was seen in Alaska, Siberia and throughout the Arctic, usually frozen areas that have experienced a massive loss of ice and thawing of permafrost.
The ice in those regions appears to be responding almost immediately to the warmer temperatures.
Scientists at the National Snow and Ice Data Center said Arctic sea ice was at a record low this summer.
Most of the world saw temperatures slightly warmer than average this year, Hansen said. Temperatures in California and the western U.S. remained normal.
Many groups oppose curbing greenhouse gases because it would be costly and harm the coal, oil and automobile industries. They say estimates of how much warming will occur in the future are uncertain, flawed and exaggerate the problem.
Richard B. Alley, an expert on abrupt climate change at Penn State University, said such arguments are wrong.
A new analysis of how the planet's climate has responded to sharp changes in the past suggests that "either the models are accurate or they underestimate the changes we will see in the future," he said.
Groups that are concerned about climate change point out that the rate of warming is steeply increasing. The excess heat could unleash deadly heat waves, raise the sea level as more ice caps melt, and cripple agriculture, warned Brown of the Earth Policy Institute.
"Farmers may now be facing higher temperatures than any generation of farmers since agriculture began 11,000 years ago," said Brown, who added that higher temperatures are already resulting in lower grain yields around the world.
The Union of Concerned Scientists is calling for the U.S. government to reduce greenhouse gas emissions and invest in cleaner energy technologies such as wind and solar power. The union is also urging consumers to drive less and use cars that are more fuel efficient.
"We know we're responsible for warming," Frumhoff said. "Now we have to start acting responsibly."