The ocean has become significantly more stratified over the last half century as the climate has warmed, inhibiting the ability for heat, oxygen, and carbon dioxide from the surface to be transported deeper into the ocean, according to a new study.

The decrease in ocean mixing could amplify warming due to human-caused climate change at the Earth’s surface, intensify ocean acidification, and impact the marine food chain, among other impacts.

The study, published this week in the journal Nature Climate Change, found that stratification in the upper 200 meters (656 feet) of the ocean increased by about 7% between 1960 and 2018. 

“The ocean has absorbed the majority of excess heat due to climate change,” said Kevin Trenberth, a distinguished scholar at the National Center for Atmospheric Research (NCAR) and co-author of the new paper. “If that heat remains trapped at the surface and cannot easily be locked away deeper into the ocean, global warming and its impacts will be intensified, including the possibility of more vicious hurricanes feeding off of an increasingly warm sea surface.”

The study was led by Guancheng Li and Lijing Cheng of the Institute of Atmospheric Physics in China. Other co-authors include Michael Mann of Pennsylvania State University and John Abraham of the University of St. Thomas.

The research was funded by the Chinese Academy of Sciences and the National Key R&D Program of China. NCAR is sponsored by the National Science Foundation. 

Reduced mixing between ocean layers

The ocean is stratified due to differences in density, with warmer, lighter, less salty water layering on top of heavier, colder, saltier water. Mixing between layers occurs as heat slowly seeps deeper into the ocean and by the action of current, winds, and tides. But the greater the difference in density between the layers, the slower and more difficult the mixing and the more stable the ocean becomes.

A warming climate increases ocean stability by making the surface ocean less dense, first by warming the water itself, which expands its volume, and also by melting ice, which adds freshwater into the ocean and decreases surface salinity.

The resulting increase in stratification further drives global warming. Warmer water on the surface can absorb less carbon dioxide from the atmosphere, which increases the atmospheric concentration of carbon dioxide and in turn further warms the Earth’s surface, including the upper layer of the ocean.

Increasing the temperature of the upper ocean has a number of other negative effects. Warmer water can absorb less oxygen, and the oxygen that is absorbed cannot mix as easily with the cooler ocean waters below, making it difficult for marine life to thrive. Warmer ocean water also leads to increased bleaching of coral reefs and more favorable conditions for intense long-lasting hurricanes to develop.

“Oceans are becoming more stable. That might sound good, but it’s not,” said Mann. “It has rather profound and troubling implications.”

A new look at stratification

Scientists have known for some time that the ocean is becoming more stratified, but quantifying the degree of stratification has been difficult due in large part to a lack of ocean observations, both across the surface and especially at different depths. Because of these constraints, past estimates of ocean stratification have tended to compare conditions at the surface with the conditions 200 meters below. However, the thickness of the ocean’s layers vary over the globe, meaning that sometimes 200 meters is not deep enough — or too deep — to get an accurate understanding of how stratification is changing. 

For the new study, the research team took advantage of a recently produced data set of temperature and salinity that has a much more robust coverage of the ocean in space and time, going back to 1960. The scientists validated the data with the help of a detailed set of ocean observations taken since 2000 by Argo floats, which profile conditions in the ocean down to 2,000 meters.

This produced a more precise picture of ocean conditions, revealing a sharper increase in stratification over time than did previous studies. Globally, ocean stratification has increased 5.8% when looking from the surface down to 2,000 meters, and 7.3% when looking at just the top 200 meters, according to the new study. Stratification, however, has not increased uniformly across ocean basins. The largest increase has been in the Southern Ocean (9.6%), followed by the Pacific Ocean (5.9%), the Atlantic Ocean (4.6%), and the Indian Ocean (4.2%).

“These numbers reveal the distinct and significant impact humans are having on the oceans as we continue to emit greenhouse gases,” Trenberth said. “The impact of these changes will not be limited to the oceans but will affect the entire Earth system and our day-to-day lives.”

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