The mystery of Earth's missing heat may have been solved: it could lurk deep in oceans, temporarily masking the climate-warming effects of greenhouse gas emissions, researchers reported.
Climate scientists have long wondered where this so-called missing heat was going, especially over the last decade, when greenhouse emissions kept increasing but world air temperatures did not rise correspondingly.
The build-up of energy and heat in Earth's system is important to track because of its bearing on current weather and future climate.
The temperatures were still high -- the decade between 2000 and 2010 was Earth's warmest in more than a century -- but the single-year mark for warmest global temperature was stuck at 1998, until 2010 matched it.
The world temperature should have risen more than it did, scientists at the National Centre for Atmospheric Research reckoned.
They knew greenhouse gas emissions were rising during the decade and satellites showed there was a growing gap between how much sunlight was coming in and how much radiation was going out. Some heat was coming to Earth but not leaving, and yet temperatures were not going up as much as projected.
So where did the missing heat go?
Computer simulations suggest most of it was trapped in layers of oceans deeper than 1,000 feet during periods like the last decade when air temperatures failed to warm as much as they might have.
This could happen for years at a time, and it could happen periodically this century, even as the overall warming trend continues, the researchers reported in the journal Nature Climate Change.
"This study suggests the missing energy has indeed been buried in the ocean," NCAR's Kevin Trenberth, a co-author of the study, said in a statement. "The heat has not disappeared and so it cannot be ignored. It must have consequences."
Mr. Trenberth and the other researchers ran five computer simulations of global temperatures, taking into account the interactions between the atmosphere, land, oceans and sea ice, and basing the simulations on projected human-generated greenhouse gas emissions.
These simulations all indicated global temperature would rise several degrees this century. But all of them also showed periods when temperatures would stabilize before rising. During these periods, the extra heat moved into deep ocean water due to changes in ocean circulation, the scientists said.