The deep oceans have been cooling for the past two decades and it is not possible to say whether changes in ocean heat adequately explain the “pause” in global warming, two of the world’s leading ocean scientists have said.
Warmer oceans have been a key explanation for the “missing” heat. Global average surface temperatures have not increased dramatically for more than a decade despite steadily rising carbon dioxide levels in the atmosphere.
A paper by Carl Wunsch from Harvard University and Patrick Heimbach from MIT, accepted for publication with the Journal of Physical Oceanography, says more work is needed.
“Direct determination of changes in ocean heat content over the past 20 years are not in conflict with estimates of the radiative forcing, but the uncertainties remain too large to rationalise, e.g. the apparent ‘pause’ in warming,” Professor Wunsch and Dr Heimbach say.
They conclude that much less heat is being added to the oceans than has been claimed in previous studies.
Professor Wunsch and Dr Heimbach say trends showed a warming in the upper ocean and a net cooling below 2000m. Below 3600m, the cooling is about 0.01C over 19 years.
“As with many climate-related records, the unanswerable question here is whether these changes are truly secular, and/or a response to anthropogenic forcing, or whether they are fragments of a general red noise behaviour,’’ the paper says.
Some climate scientists claim the deep oceans are not significant because of the long timeframes over which temperature changes occur.
Professor Wunsch and Dr Heimbach say shifts in deep ocean properties “may indeed be so slight that their neglect in discussions of heat uptake and sea level change is justified”.