Mixing of the planet’s ocean waters is decreasing, and that is speeding up global warming, study finds

The layers of the world’s oceans aren’t mixing like they used to due to climate change, potentially speeding up how fast the planet will warm in the coming decades. This new finding, contained in a study published Monday in the journal Nature Climate Change, finds that the reduction in the mixing of ocean layers is piling up warm water near the surface while cutting back on the circulation of cold, deep water.

The reduced up and down mixing is expected to have sweeping implications beyond just accelerating global warming. It is projected to increase energy available to hurricanes and other storms, reduce essential nutrients for fish in upper ocean layers and diminish the oceans' ability to store carbon, among other impacts.

The study assesses how the separation of seawater layers, known as stratification, has changed based on new temperature, salinity and density data. It finds substantial shifts have occurred as the ocean has absorbed more heat in the upper 6,500 feet of water.

The study, from researchers in China as well as the United States, found stratification has increased by about 5.3% during the 1960 to 2018 period, for a rate of 0.9% per decade.

The way the ocean layers are separated is similar to a basic vinaigrette salad dressing, where lighter oil sits at the top, and more dense vinegar sits near the bottom. Once shaken, however, the layers mix, forming the familiar dressing. What’s happening in the oceans now is that there’s less shaking going on.

The ocean trends are especially significant since computer models used to simulate how human-caused climate change is likely to play out don’t include such a rapid uptick in ocean stratification, which could be causing them to downplay future warming rates.

The majority of the stratification trends, the study finds, are due to the rapidly warming topmost 700 feet of water, which is growing fresher and lighter over time. Ocean heat content in the upper ocean has reached record levels, as the oceans continue to absorb the vast majority of added heat from global warming.

The results also suggest a reduced ability of the oceans to act as a massive carbon savings account, otherwise known as a carbon sink. The ocean absorbs huge amounts of carbon dioxide annually, and it is circulated through mixing into the deep ocean, to remain there for decades or longer.

A more divided ocean, with less exchange between layers, means there may be less carbon absorption over longer time periods. This could lead to more carbon dioxide remaining in the atmosphere that will lead to greater and faster global warming.

The results are also important for understanding changes in the ability of ocean layers to support marine life, since the overturning of water helps bring nutrients up from the deep, and some parts of the globe have already been seeing decreases in the amount of oxygen available in the upper layers of ocean waters.

The study also spells trouble for some of the most important ocean currents in the world, which are powered by the exchange of lighter surface waters and deeper, colder and saltier ocean layers.

One such current is the global ocean conveyor belt, formally known as the Atlantic Meridional Overturning Circulation, of which the Gulf Stream is a part. This circulation sends warm, salty waters northward within the upper layers of the Atlantic, where the water becomes saltier and more dense, and sinks as it flows southward into the Southern Hemisphere.

The Gulf Stream helps moderate the climate of Europe and plays a major role in powering storms along the U.S. East Coast.

This current helps distribute heat around the world and regulate the burying of carbon within the oceans. Studies show that as the Greenland ice sheet melts and dumps lighter freshwater into the North Atlantic, this current is slowing down.

Hurricane season implications

The results indicate that there may be more energy available to power future hurricanes and other storms that draw their strength in part from warm ocean waters.

“So many key impacts (ocean biological productivity, carbon absorption and burial, surface warming and effects on e.g. hurricane intensities) are impacted by ocean stratification and changes therein,” said study co-author Michael Mann, who directs the Earth and Environmental Systems Institute at Pennsylvania State University, in an email.

Mann said the results show that warm waters are getting bottled closer to surface waters, which may be resulting in more frequent hurricanes and more intense and water-laden storms.

“I do think that this may be playing a role in the trend toward greater hurricane activity in regions of the ocean like the tropical North Atlantic that are warming rapidly. The anomalous warmth in the tropical Atlantic this year was the main ingredient in our preseason prediction of a hyperactive season,” Mann said.

Paul Durack, a research scientist at Lawrence Livermore National Laboratory in California who has published studies on climate change’s effects on ocean currents, said the study “fills a void” in the scientific literature by quantifying the rate of change. But, he said, the new estimates may still be too low.

“I would not be surprised if this study underestimates the observed change,” he said via email. Durack was not involved in the new study.