The Ocean's Role in Regulating Global Temperatures

An international team of scientists from China and the USA recently published a joint study in Science Advances. The scientists solved the three-dimensional deglacial ocean temperature change by combining state-of-the-art deglacial simulations with proxy-based reconstructions and discovered that strong warming in intermediate-depth waters in response to deglacial forcing significantly increases the deglacial ocean heat storage efficiency of ~0.1.

Over 90% of the excess energy from continuous anthropogenic warming is absorbed by the global ocean, one of the largest heat reservoirs in the climate system. The upper 500 m of the ocean has warmed up the most over the past century, while the deep ocean has warmed up much less, meaning that the ocean's heat storage efficiency is only about 0.1.

However, paleoceanographic observations indicate that, over extended periods, the warming of the deep ocean may be equivalent to or greater than that of the surface ocean, with the efficiency of ocean heat storage during the last deglaciation being roughly ten times that of today.

Our simulations and proxy reconstructions demonstrate that the three-dimensional ocean warming during the last deglaciation was strongly nonuniform with strongest warming occurring at intermediate depths, in striking contrast to contemporary observations.

Dr. Chenyu Zhu, Study Co-First Author, Institute of Atmospheric Sciences, Chinese Academy of Sciences

The study used sensitivity experiments to show that the significant warming of intermediate waters can be significantly amplified by changes in oceanic circulation linked to meltwater forcing and can be linked to surface warming at mid-to-subpolar latitudes through ventilation in response to greenhouse gas and ice sheet forcing.

The unique ocean warming structure facilitates a large ocean heat storage efficiency. In particular, this resolves the paradox suggested by the conventional view that warming occurred at sites of deep-water formation that remained covered by sea ice.

Zhengyu Liu, Professor and Study Corresponding Author, The Ohio State University

“These results have valuable implications. For example, if strong surface warming and strong ventilation are collocated like in our simulations, then the ocean will absorb more heat from the atmosphere, potentially slowing the rate of atmospheric warming,” said Peter U. Clark, Professor and study Co-Corresponding Author at Oregon State University.

The study emphasizes how crucial changes in oceanic circulation and surface warming patterns are to long-term changes in ocean heat storage and suggests that “the ocean can serve as a far greater reservoir of energy in the climate system than implied by contemporary observations,” according to the study.

Journal Reference:

Zhu, C., et al. (2024) Enhanced ocean heat storage efficiency during the last deglaciation. Science Advances. doi.org/10.1126/sciadv.adp5156