In descending order, the years with the highest global average annual temperatures were 2005, 1998, 2002, 2003 and 2004, NASA said in a statement.

"It's fair to say that it probably is the warmest since we have modern meteorological records," said Drew Shindell of the NASA institute in New York City.

"Using indirect measurements that go back farther, I think it's even fair to say that it's the warmest in the last several thousand years."

Some researchers had expected 1998 would be the hottest year on record, notably because a strong El Nino -- a warm-water pattern in the eastern Pacific -- boosted global temperatures.

But Shindell said last year was slightly warmer than 1998, even without any extraordinary weather pattern. Temperatures in the Arctic were unusually warm in 2005, NASA said.

"That very anomalously warm year (1998) has become the norm," Shindell said in a telephone interview.

"The rate of warming has been so rapid that this temperature that we only got when we had a real strong El Nino now has become something that we've gotten without any unusual worldwide weather disturbance."

Over the past 30 years, Earth has warmed by 1.08 degrees F (0.6 degrees C), NASA said. Over the past 100 years, it has warmed by 1.44 degrees F (0.8 degrees C).

Shindell, in line with the view held by most scientists, attributed the rise to emissions of greenhouse gases such as carbon dioxide, methane and ozone, with the burning of fossil fuels being the primary source.

The 21st century could see global temperature increases of 6 to 10 degrees F (3 to 5 degrees C), Shindell said.

"That will really bring us up to the warmest temperatures the world has experienced probably in the last million years," he said.

To understand whether the Earth is cooling or warming, scientists use data from weather stations on land, satellite measurements of sea surface temperature since 1982, and data from ships for earlier years.