Cooler temperatures in North America last year do not mean global warming is easing, according to a new study announced Friday by the National Oceanic and Atmospheric Administration.
The study comes just days before President Barack Obama heads to Copenhagen, Denmark, to speak at a United Nations conference on climate change.
Rising temperatures over decades have prompted scientific concern, and the last decade has been the hottest in thousands of years, according to climate records. However, the warming eased over North America last year, and groups seeking to deny climate change seized on that in an effort to challenge the idea of overall warming.
North America wasn't as warm as expected because of cooler water in the North Pacific — a condition called La Nina — but the rest of the world continued to warm, researchers said Friday. The overall warming trend is expected to continue worldwide.
La Nina caused cold air from the Arctic to move south into North America, temporarily overwhelming the warming influence from climate change in the region, said Judith Perlwitz of the University of Colorado, lead author of the report being published next week in the journal Geophysical Research Letters.
While temperature readings in North America dropped back to about the level of 1996 last year, it would have been even colder without the underlying effects of human-induced climate warming, said co-author Martin Hoerling of the Earth System Research Laboratory of the government's National Oceanic and Atmospheric Administration.
"Our work shows that there can be cold periods, but that does not mean the end of global warming," Perlwitz said.
Last year "was not an extremely cold year; it was not an extreme event," Hoerling said, but it did "raise a considerable stir."
The scientists launched their study of conditions last year and compared them with complex computer climate models, leading to the conclusion that it was a case of natural variability rather than any change in global warming.
The work was funded by the NOAA Climate Program Office, and other co-authors were from NOAA's National Weather Service and National Climatic Data Service.