Although global temperatures have been rising over the past century, a slowdown in the rate of warming in the past few years has left some scratching their heads over a seeming "global warming pause."
The suggestion that global warming has stopped is "nonsense," climatologist Richard Alley of Penn State University said last fall. The fact that the year 2012 was no warmer than 2002, he said, ignores the long-term trend of warming.
But scientists say that trend has been partially obscured by the ocean, which is likely absorbing the excess heat.
A paper published in the journal Nature in August 2013 by staff of the Scripps Institution of Oceanography in La Jolla, California, suggests the extra heat has been absorbed by the surface waters of the Pacific Ocean, aided by the warming and cooling cycles of weather patterns known as El Niño and La Niña. (See: "Is El Niño Back? Climate Scientists Forecast Its Arrival.")
Another theory is that the deep, cold ocean has been absorbing the excess heat, says Jerry Meehl, a senior scientist at the National Center for Atmospheric Research in Boulder, Colorado (see video, above, produced by Earth Vision Trust). That process couldn't continue forever, as eventually the air above the ocean would resume warming as well.
Yet another theory that doesn't discount ocean absorption of heat but may also help explain global warming pause was published in February the journal Nature Geoscience. The scientists suggested that 15 percent of the global warming pause could be attributed to the impact of active volcanoes, which spew ash and gas that can reflect the sun's heat back into space.