Deep ocean temperatures are steadily rising

A new study led by the University of Exeter has found that much of the excess heat stored in the subtropical North Atlantic is in the deep part of the ocean, below 700 meters. The scientists estimated that, over the next 50 years, the deep ocean’s temperatures will increase by a further 0.2°C. 

Until now, oceans have absorbed about 90 percent of the warming caused by human activities. By using temperature records and “chemical tracers” – compounds whose composition can be used to discover past changes in the oceans – the researchers have found that, in the subtropical North Atlantic, 62 percent of the warming from 1850 to 2018 is held in the deep ocean. This warming will most likely have a negative impact on our planet, leading to sea-level rise, changing ecosystems, ocean currents and chemistry, and deoxygenation.

“As our planet warms, it’s vital to understand how the excess heat taken up by the ocean is redistributed in the ocean interior all the way from the surface to the bottom, and it is important to take into account the deep ocean to assess the growth of Earth’s ‘energy imbalance,’” said Dr. Marie-Jose Messias, a senior lecturer in Oceanography at the University of Exeter. 

“As well as finding that the deep ocean is holding much of this excess heat, our research shows how ocean currents redistribute heat to different regions. We found that this redistribution was a key driver of warming in the North Atlantic.”

According to Dr. Messias and her colleagues, the main cause of North Atlantic deep ocean warming is the system of currents known as the Atlantic Meridional Overturning Circulation (AMOC). This system works like a conveyor belt, carrying warm water from the tropics north, where colder and denser water sinks into the deep ocean, slowly spreading southward. Thus, excess heat from the Southern Hemisphere oceans is constantly transferred to the North Atlantic by AMOC.

Further research is needed to clarify the mechanisms of heat transferred from the Southern to the Northern Hemisphere oceans, and the role that AMOC plays in these processes.

The study is published in the journal Nature Communications: Earth & Environment. 

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By Andrei Ionescu, Earth.com Staff Writer