2009 may rank as the fifth-warmest year on record, the WMO said, although the final rank won't be available until next year. 1998 holds the rank as the hottest year. It was characterized by an unusually strong El Nino, a giant patch of warm water along the equator in the Pacific that appears periodically and can strongly affect the wind currents flowing over it.
The announcement was made at the U.N. Climate Change Conference in Copenhagen, Denmark, where delegates are meeting for the second day of a two-week session aimed at reaching agreement on an international climate treaty.
President Obama, who has promised the U.S. will make cuts in its emissions of greenhouse gases, is expected to attend the end of the Copenhagen meeting.
On Monday, the Environmental Protection Agency ruled that six greenhouse gases were threats to the health and welfare of Americans, and therefore, subject to regulation.
The WMO said above-normal temperatures this year were recorded in most parts of the world, and only North America had conditions that were cooler than average.
Researchers said the temperature analysis was based on three independent sets of data, one maintained by the United Kingdom's Hadley Climate Center and the Climate Research Unit at the University of East Anglia, as well as two other sets maintained in the United States by NASA and the National Oceanic and Atmospheric Administration.
E-mails hacked from the University of East Anglia have been trumpeted in recent weeks by climate change skeptics, who have argued that some of the e-mail discussions between scientists reveal a conspiracy to fudge or ignore data to drive an agenda.
Current Decade Warmest on Record
U.S. climate negotiator Jonathan Pershing defended the scientific work behind climate change and said the e-mail discussion would not affect talks at Copenhagen.
"What I think is unfortunate and, in fact, shameful, is the way some scientists are being pilloried. The science is incredibly robust. I worry much, much more about not acting urgently than I do about what ultimately will be a small blip," Pershing said.