THE upper layers of Earth's oceans have cooled significantly over the past two years, even though the planet as a whole is warming up. While this may just be part of the natural variation of oceans, climatologists are still confounded by the massive, unaccountable loss of heat.

Earth's oceans can hold 1000 times as much heat as the atmosphere, and sea temperatures have climbed almost without pause over the past 50 years. This agrees with climate models that show that the oceans mitigate atmospheric warming by absorbing much of the heat.

Now measurements by John Lyman of the National Oceanic and Atmospheric Administration in Seattle, Washington, and colleagues have put a wrinkle in that trend. The researchers used data from a network of 3000 free-floating buoys, called ARGO, which monitors the oceans worldwide. They found that the upper 750 metres of ocean lost enough energy between 2003 and 2005 to cause an overall drop in water temperature of about 0.02