Deep oceans could be covering up the full extent of global warming, a study suggests. Heat absorption at depths of more than 1,000 feet may have the effect of flattening the rate of warming for decades at a time, say scientists. The "missing heat" can give the impression that greenhouse gas emissions are having less of an impact than some have claimed. But experts warn this is an illusion.

"We will see global warming go through hiatus periods in the future," said lead researcher Dr Gerald Meehl, from the US National Centre for Atmospheric Research (NCAR). "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." The US and Australian scientists investigated why despite steadily climbing emissions in the 2000s, the global temperature record set in 1998 was not matched until 2010. Satellite measurements revealed a discrepancy between incoming sunshine and outgoing radiation from the Earth.

This implied that heat was building up in some hidden location on the planet. Suspicion fell on the oceans, which were thought to be storing some of the heat energy that would otherwise be warming the atmosphere or land, or melting ice and snow.

Observations from a global network of buoys showed some warming in the upper ocean, but not enough to account for the anomaly. The new study, published in the journal Nature Climate Change, used powerful climate modelling software to simulate complex interactions between the atmosphere, land, oceans and sea ice.