While climate events like El Nino -- when warm water spreads over much of the tropical Pacific Ocean --affect global temperatures, the increasing role of human-made pollutants plays a big part.

"There has been a strong warming trend over the past 30 years, a trend that has been shown to be due primarily to increasing greenhouse gases in the atmosphere," said James Hansen of NASA's Goddard Institute for Space Studies, based in New York.

The warmest year on record was 1998, with 2002 and 2003 coming in second and third, respectively.

Short-term factors like large volcanic eruptions that launched tiny particles of sulfuric acid into the upper atmosphere in 1963, 1982 and 1991 can change climates for periods ranging from months to a few years.

Last year was the fourth-warmest recorded, with a global mean temperature of 57 degrees Fahrenheit (14 C), which was about 1.5 degrees warmer than the middle of the century, NASA scientist Drew Shindell said in an interview.

Average temperatures taken from land and surfaces of the oceans showed 2004 was 0.86 degrees Fahrenheit (0.48 C) above the average temperature from 1951 to 1980, according to Hansen.

The spike in global temperatures in 1998 was associated with one of the strongest El Ninos of recent centuries and a weak El Nino contributed to the unusually high global temperatures in 2002 and 2003, NASA said.

Carbon dioxide, emitted by autos, industry and utilities, is the most common greenhouse gas. Hansen also said that the Earth's surface now absorbs more of the sun's energy than gets reflected back to space.

That extra energy, together with a weak El Nino, is expected to make 2005 warmer than 2003 and 2004 and perhaps even warmer than 1998, which had stood out as far hotter than any year in the preceding century, NASA said in a statement.

The U.S. National Oceanic and Atmospheric Administration said on Thursday the current weak El Nino will diminish and end during the next three months.