Deep oceans put global warming on brief hiatus

The world is projected to continue warming over the century, however, the increase is not expected to be a smooth one because deep areas of Earth's oceans may absorb enough heat to stabilize the warming for periods as long as a decade before the warming continues, a joint U.S. and Australian study found on Monday. The study, conducted by scientists at the U.S. National Center for Atmospheric Research and Australia Bureau of Meteorology, based on computer simulations of global climate, points to ocean layers deeper than 300 meters as the main location of the "missing heat" during periods such as the past decade when global air temperatures showed little trend. The findings also suggest that several more intervals like this can be expected over the next century, even as the trend toward overall warming continues. "In our model we found that most of the heat is going into the deep ocean at those times when the net temperature of the surface is flat," study co-author Julie Arblaster, senior research scientist at Australia Bureau of Meteorology, told ABC Science on Monday. "These hiatus periods, or slow down periods, can happen from time to time even when there's additional energy coming into the system." Lead author Dr Gerald Meehl from U.S. National Center for Atmospheric Research said the study illustrates one reason why global temperatures do not simply rise in a straight line. "We will see global warming go through hiatus periods in the future, however, these periods would likely last only about a decade or so, and warming would then resume," Meehl said. The 2000-2010 was Earth's warmest decade in more than a century of weather records. But the single-year mark for warmest global temperature, which was set in 1998, remained unmatched until 2010. The study suggests the oceans might be storing some of the heat that would otherwise go toward other processes, such as warming the atmosphere or land, or melting more ice and snow. But few measurements were available to confirm that hypothesis. To figure out where the heat was going, Meehl and his colleagues ran five simulations on a computer model that portrays complex interactions among the atmosphere, land, oceans and sea ice. The simulations showed that during hiatus decades ocean temperatures above 300 meters were cooler than at other times, but waters below 300 meters warmed by around 18 percent more than at other times. A regional pattern appeared in the simulations, which was similar to what we observe in La Nina events -- lower sea surface temperatures in the tropical Pacific, and warmer temperatures in the oceans 35 degrees north and south of the equator. Arblaster said these patterns are related to changes in ocean circulation. The study was published in the latest issue of Nature Climate Change.