Study reveals that oceans stored heat efficiently during the last glacial period

Antarctic sea ice is melting. Photo by Jiuxin Shi
The Earth’s oceans, one of the largest heat storage sources in the climate system, absorb more than 90% of the excess energy from ongoing anthropogenic warming. Over the past century, the greatest ocean warming has occurred in the upper 500 meters, with relatively weaker warming in the deeper oceans, resulting in a small ocean heat storage efficiency of about 0.1.
However, paleoceanographic observations suggest that on long timescales, deep ocean warming was equal to or greater than that of the surface, and that the efficiency of ocean heat storage during the last glacial period was about 10 times greater than it is today. This raises the following questions: What mechanisms are responsible for ocean heat absorption/storage, and how efficiently?
A collaborative study by an international team of scientists from China and the United States, recently published in Science Advances, sheds light on this issue: by combining state-of-the-art deglacial simulations with proxy-based reconstructions, they unraveled three-dimensional changes in deglacial ocean temperatures and found that the heat storage efficiency of the deglacial oceans was significantly enhanced to above unity with significant mid-depth warming in response to deglacial forcing.
“Our simulations and proxy reconstructions show that the three-dimensional warming of the ocean during the last glacial period was highly uneven, with the strongest warming occurring at mid-depths, which is in stark contrast to modern observations,” said Dr Zhu Chenyu of the Institute of Atmospheric Sciences, Chinese Academy of Sciences, co-first author of the study.
Using sensitivity experiments, this study reveals that the significant warming of intermediate waters is associated with surface warming at mid- to subpolar latitudes through ventilation in response to greenhouse gas and ice sheet forcing, which may be significantly enhanced by changes in ocean circulation associated with meltwater forcing.
“This unique ocean warming structure increases the efficiency of ocean heat storage. In particular, it resolves the contradiction suggested by the conventional view that warming occurred in deep-sea 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 valuable implications. For example, when strong surface warming and strong ventilation occur simultaneously, as in our simulations, the oceans may absorb more heat from the atmosphere and slow the rate of atmospheric warming,” said Professor Peter U. Clark of Oregon State University, the study’s other corresponding author.
The study highlights the important role of surface warming patterns and changes in ocean circulation in changing long-term ocean heat storage, and suggests that “the ocean may act as a much larger energy reservoir in the climate system than current observations suggest.”
Further information: Chenyu Zhu et al., “Increased efficiency of ocean heat storage during the last glacial period,” Science Advances (2024). DOI: 10.1126/sciadv.adp5156. www.science.org/doi/10.1126/sciadv.adp5156
Courtesy of the Chinese Academy of Sciences
Source: Study Reveals Ocean’s Massive Heat Storage Efficiency During the Last Ice Age (September 20, 2024) Retrieved September 21, 2024 from https://phys.org/news/2024-09-reveals-large-ocean-storage-efficiency.html
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