Average temperatures across North America dropped in 2008—which may seem to contradict global warming theory.



Not so, scientists say. The cooling, caused by natural changes in global air circulation, temporarily masked the effects of global warming, which is getting worse, a new study says.



New computer-model simulations suggest that the continent-wide dip resulted from an unusually long cooling of the Pacific Ocean, driven by the La Niña phenomenon.



During a La Niña, event, the sea-surface temperature in the eastern tropical Pacific Ocean drops, sometimes as much as 7 degrees Fahrenheit (4 degrees Celsius) below normal.



La Niña, conditions recur every few years and typically last about one year. The one that began in 2007, however, lasted about two years, said study leader