THE upper layers of Earth鈥檚 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鈥檚 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 掳C (Geophysical Research Letters, vol 33, p L18604).
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That may not sound like much, but trying to account for the missing energy is proving to be an enormous task, says team member Josh Willis of NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California. 鈥淚t鈥檚 enough to melt all of the sea ice in the world many times over,鈥� he says. 鈥淲e know it鈥檚 not [doing] that.鈥�
The researchers speculate that the missing heat is being radiated back into space, possibly because of aerosols injected into the atmosphere by volcanic eruptions or because of annual variations in cloud cover. No matter what the cause, though, over the long term we鈥檙e still very much in hot water. 鈥淭he 50-year trend is still much warmer than this small bit of cooling,鈥� says Willis.
Indeed, blips in temperature are to be expected and should be studied, says Tim Barnett at the Scripps Institution of Oceanography in La Jolla, California. 鈥淲hen you鈥檙e in the global warming business, the first thing you want to know is 鈥榳hat鈥檚 the natural variability?鈥�,鈥� he says. That鈥檚 because we need to be able to distinguish what鈥檚 natural from what鈥檚 caused by humans. 鈥淲e鈥檙e used to these bumps and wiggles in the ocean and atmosphere.鈥�
A similar cooling of oceans occurred between 1980 and 1983, and such aberrations are normally chalked up to natural changes in the ocean鈥檚 behaviour. The researchers believe their discovery is just the first step in a whole new level of understanding of how the seas interact with global climate. 鈥淏y measuring temperature, and soon salinity through ARGO, we鈥檒l really be able to nail down a lot of unanswered questions about climate,鈥� says Willis.
