for National Geographic News
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 Judith Perlwitz of the National Oceanic and Atmospheric Administration (NOAA).
The two-year La Niña affected the patterns of jet streams and and so-called storm tracks across North America.
"If you have colder sea-surface temperatures in the tropical Pacific, they generate circulation patterns in the atmosphere that cause cold air to move into North America," Perlwitz said.
David Easterling, a NOAA climatologist who was not involved in the new work, added that while 2008 temperatures in North America were cooler than average, globally 2008 was still among the warmest on record.
"People often only consider the weather in their locality and not the global picture," Easterling said.
The team used real-world 2008 sea-surface temperatures to model the atmospheric response and resulting annual surface air temperature in North America in 2008.
SOURCES AND RELATED WEB SITES