In the ocean depths: scientists track global warming's 'missing heat'
Scientists from the American National Centre for Atmospheric Research (NCAR) set themselves the task of explaining why as the emissions of greenhouse gases continue to rise there are breaks in the rise in global temperatures.
For example, while the first decade of this century was the warmest on record, the global single year record temperature, from 1998, wasn't broken until 2010 - global temperatures have not risen in a strict relationship with greenhouse gas levels.
"We will see global warming go through hiatus periods in the future," says NCAR's Gerald Meehl, who is the lead author of the study published in Nature Climate Change. "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."Observations from space showed that the difference in the amount of heat coming into the planet from the Sun and the amount of heat radiated out from Earth had actually gone up, so where is the 'missing heat'? The team from NCAR and the Australian Bureau of Meteorology used complex computer models to test the hypothesis that the deep oceans were storing this energy.
Already, sea surface temperature measurements had suggested that the heat was warming the waters. Meehl and his colleagues used the Community Climate System Model, to try and predict how the climate would change when the actions of the whole climate system - the atmosphere, oceans, sea ice and land - were considered.
Their work found, in one of five examples, that a global temperature rise of 1.4 during this century was accompanied by two pauses in warming of a decade each as the deepest parts of the ceans (below 300 metres) took on a much larger share of the extra heat, warming by close to 20%.
"This study suggests the missing energy has indeed been buried in the ocean," said Kevin Trenberth, who with fellow author of this report John Fasullo was behind the first work suggesting deep ocean warming. "The heat has not disappeared, and so it cannot be ignored. It must have consequences."
Those consequences could, the researchers believe, be related to the El Nino and La Nina phenomena, with La Nina corresponding to deep ocean warming periods. Both the Pacific warming of El Nino and the cooling of La Nina are associated with extreme weather events.
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