The deep oceans can absorb enough heat to mask the effect of global warming for a decade at a time, say scientists at the National Center for Atmospheric Research (NCAR).
'Missing heat' has been something of a mystery during periods such as the last ten years, when global air temperatures have shown little sign of change. This is despite the fact that satellite measurements show that the discrepancy between incoming sunshine and outgoing radiation from Earth actually increased.
The implication was that the heat was building up somewhere other than in the atmosphere.
But the new study, based on computer simulations of global climate, indicates that the heat's to be found in ocean layers deeper than 1,000 feet. It also suggests that several more intervals like this can be expected over the next century, despite the trend towards overall warming continues.
"We will see global warming go through hiatus periods in the future," says NCAR's Gerald Meehl, lead author of the study. "However, these periods would likely last only about a decade or so, and warming would then resume. This study illustrates one reason why global temperatures do not simply rise in a straight line."
Meehl and his colleagues used the Community Climate System Model, developed by scientists at NCAR and the Department of Energy.
The simulations, which were based on projections of future greenhouse gas emissions from human activities, indicated that temperatures would rise by several degrees during this century. But each simulation also showed periods in which temperatures would stabilize for about a decade before climbing again.
During these hiatus periods, simulations showed that extra energy entered the oceans, with deeper layers absorbing a disproportionate amount of heat due to changes in oceanic circulation. The periods appear to coincide with La Niña events.
"This study suggests the missing energy has indeed been buried in the ocean," Trenberth says. "The heat has not disappeared, and so it cannot be ignored. It must have consequences."