Study finds high efficiency of ocean heat storage during last deglaciation

The ocean, one of the largest heat reservoirs in the climate system, absorbs more than 90% of the excess energy from ongoing anthropogenic warming. Over the past century, the greatest warming of the ocean has occurred in the upper 500 m, with relatively little warming in the deep ocean, corresponding to a low ocean heat storage efficiency of about 0.1.

Paleoceanographic observations, however, suggest that over long periods, warming of the deep oceans may be comparable to or greater than that of the surface, with the efficiency of ocean heat storage during the last deglaciation being about ten times higher than its present value. This raises the question: what mechanisms are responsible for ocean heat absorption/storage, and how efficient can they be?

Recently published in Scientific progressA joint study by an international team of Chinese and American scientists has provided insight into this issue. By combining state-of-the-art deglaciation simulations and proxy-based reconstructions, they resolved the three-dimensional change in ocean temperature during deglaciation and found that the ocean heat storage efficiency during deglaciation is significantly enhanced to ≥1 by strong warming in intermediate-depth waters in response to deglaciation forcing.

“Our simulations and proxy reconstructions demonstrate that three-dimensional ocean warming during the last deglaciation was strongly non-uniform, with the strongest warming occurring at intermediate depths, in stark contrast to contemporary observations,” said Dr Chenyu Zhu from the Institute of Atmospheric Sciences, Chinese Academy of Sciences, co-first author of the study.

Using sensitivity experiments, the study found that the large warming of intermediate waters can be linked to surface warming in mid- to subpolar latitudes by ventilation in response to greenhouse gases and ice sheet forcing, and significantly enhanced by the change in ocean circulation associated with meltwater forcing.

“The unique warming structure of the oceans facilitates high efficiency of ocean heat storage. In particular, it resolves the paradox suggested by the conventional view that warming occurred at deep-water formation sites that remained covered by sea ice,” said Professor Zhengyu Liu of Ohio State University, one of the corresponding authors of the study.

“These results have interesting implications. For example, if strong surface warming and strong ventilation are co-located as in our simulations, then the ocean will absorb more heat from the atmosphere, potentially slowing the rate of atmospheric warming,” said Professor Peter U. Clark of Oregon State University, another corresponding author of the study.

The study highlights the important role of surface warming patterns and changes in ocean circulation in the long-term change in ocean heat storage, and suggests that “the ocean may serve as a much larger reservoir of energy in the climate system than contemporary observations suggest.”