Researchers calculated that 2005 produced the highest annual average surface temperature worldwide since instrument recordings began in the late 1800s, said James Hansen, director of NASA's Goddard Institute for Space Studies.
The result confirms a prediction the institute made in December.
In a telephone interview, Hansen said the analysis estimated temperatures in the Arctic from nearby weather stations because no direct data were available. Because of that, "we couldn't say with 100 percent certainty that it's the warmest year, but I'm reasonably confident that it was," Hansen said.
More important, he said, is that 2005 reached the warmth of 1998 without help of the "El Nino of the century" that pushed temperatures up in 1998.
Over the past 30 years, Earth has warmed a bit more than 1 degree in total, making it about the warmest it's been in 10,000 years, Hansen said. He blamed a buildup of heat-trapping greenhouse gases.
The global average surface temperature in 2005 was about 58.3 degrees, he said.
Jay Lawrimore of the federal government's National Climatic Data Center said his own center's current data suggest 2005 came in a close second to 1998, in part because of how the Arctic was factored in. But he said a forthcoming analysis "will likely show that 2005 is slightly warmer than 1998."