However, paleoceanographic observations suggest that over long time scales, deep ocean warming can match or even exceed surface warming, with heat storage efficiency during the last deglaciation being around ten times higher than present levels. This raises questions about the mechanisms behind ocean heat uptake and storage and the potential limits of this efficiency.
A recent study published in 'Science Advances' by an international team of scientists from China and the USA offers new insights into this issue. By combining advanced deglacial simulations with proxy-based reconstructions, the researchers uncovered the three-dimensional ocean temperature changes during the last deglaciation, revealing that ocean heat storage efficiency was significantly enhanced, with values close to 1. This was driven by substantial warming in intermediate-depth waters in response to deglacial forces.
"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," explained Dr. Chenyu Zhu from the Institute of Atmospheric Sciences at the Chinese Academy of Sciences, the study's co-first author.
Through sensitivity experiments, the study connected intermediate water warming to surface warming at mid-to-subpolar latitudes. This was facilitated by ventilation in response to greenhouse gases and ice sheet forces and was amplified by changes in ocean circulation linked to meltwater forcing. "The unique ocean warming structure facilitates a large ocean heat storage efficiency, resolving the paradox posed by the conventional view that warming occurred at sites of deep-water formation that remained covered by sea ice," noted Prof. Zhengyu Liu from The Ohio State University, one of the study's corresponding authors.
"These results hold valuable implications. For instance, if strong surface warming and ventilation coincide, as shown in our simulations, the ocean could absorb more heat from the atmosphere, potentially slowing atmospheric warming," added Prof. Peter U. Clark, a corresponding author from Oregon State University.
The study emphasizes the critical role of surface warming patterns and changes in ocean circulation in shaping long-term ocean heat storage. It suggests that the ocean may serve as a much larger energy reservoir in the climate system than contemporary observations indicate.
Research Report:Enhanced ocean heat storage efficiency during the last deglaciation
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