Puzzle of deep ocean cooling

IT is not possible to say whether changes in ocean heat adequately explain the “pause” in global warming, two leading scientists have said.

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”.

“The history of exploration suggests, however, that blank places on the map have either been assumed to be without any interesting features and dropped from further discussion, or at the other extreme filled with ‘dragons’ invoked to explain strange reports,” they say.

The paper says that, given the combination of the high stakes for society in the accurate ­estimation of global heating rates and sea level rise, and the fundamental science questions of understanding of oceanic variability, direct confirmation or refutation of the existing hypothesis was essential.

Andy Hogg from ANU said while there was uncertainty about temperatures in the deep ocean, shallower regions were well understood, and the findings of the Wunsch paper were “consistent” with warming oceans. He said cooling of the deep ocean was not necessarily significant. “Most parts of the abyssal ocean take a very long time (centuries to millennia) to come into equilibrium with surface forcing,” he said. “So if cooling has occurred over large parts of the abyssal ocean, it is unrelated to global warming of the atmosphere over the last century.”

He said there were key parts of the abyss, which had a closer connection with the surface. “The paper indicates that these regions have indeed been consistent with the expected heat uptake of the ocean in a warmer world,” Dr Hogg said.

A recent paper by Matthew England, executive director of the climate change research centre at the University of NSW, said the global surface temperature “hiatus” could be explained by increased winds in the Pacific Ocean. The paper claims the strong trade winds, which pushed heat deeper into the ocean, explained why climate models had not matched physical observations on global temperatures, a key area of dispute between climate scientists and sceptics.