Global temperatures reached record levels over the 2000s, but it wasn’t until 2010 that the record for the hottest single year, set in 1998, was matched. This was despite the fact that greenhouse gas emissions continued to rise during that period and, according to satellite measurements, the discrepancy between the amount of incoming sunshine and outgoing radiation from the Earth actually increased, implying that heat must be building up somewhere on the planet.

In the present study, which was published online in Nature Climate Change, researchers from the US National Center for Atmospheric Research (NCAR) and the Bureau of Meteorology in Australia investigated the possibility that the oceans could be storing the missing heat. They used powerful software to portray the interactions among the atmosphere, land, oceans and sea ice, using five simulations of global temperature based on projections of future greenhouse gas emissions.

In each of the simulations, they observed temperatures rising by several degrees during the century, but there were also periods during which temperatures stabilised for about a decade before climbing again. Closer examination showed that during these hiatus periods, extra energy entered the oceans. Changes in oceanic circulation led to deeper layers absorbing a disproportionate amount of the heat, with the area below about 300 metres warming by about 18 per cent more during hiatus periods than at other times.

‘This study suggests the missing energy has indeed been buried in the ocean,’ said Kevin Trenberth of the NCAR. ‘The heat has not disappeared, and so it cannot be ignored. It must have consequences.’

November 2011