The Oceans Appear to Be Stabilizing. Here's Why it's Very Bad News | Opinion

As we continue to warm the planet through fossil fuel burning and other activities releasing carbon pollution, the surface and lower atmosphere is warming faster than the air aloft. That favors heavy colder air on top of lighter warmer air, and a less stable atmosphere. It means more turbulence and more energetic storms. And it's part of the reason we expect more extreme weather events in a warmer world.

With the oceans, we have the opposite. The warm, lighter surface waters are warming faster than the cold deeper water, since heat penetrates slowly down into the depths of the ocean. Global warming is consequently tending to make the oceans more stable. Sea water density depends not just on temperature but saltiness ("salinity") too. Fresh water is lighter than salty water, and the melting of ice is leading to the accumulation of fresh, light water at the surface, especially at higher latitudes. It's a double whammy, with both heating and reduced salinity leading to a more stable ocean.

My colleagues and I have just published an article in the journal Nature Climate Change showing that the oceans are not only becoming more stable, but are doing so faster than was previously thought. Led by Guancheng Li of the Institute of Atmospheric Physics in China, our team (which also includes Lijing Cheng, Jiang Zhu, Kevin Trenberth and John P. Abraham) analyzed a quantitative measure of stability known as "stratification". We found that the stratification of the world oceans is not only increasing, but is doing so at a greater rate than estimated in previous studies. Our study uses more comprehensive data and a more sophisticated method for estimating stratification changes, and we found a nearly 6 percent increase in the stratification of the upper 200 meters (~650 feet) of the world oceans over the past half century.

This seemingly technical finding has profound and troubling implications. The more stable the upper ocean, the less vertical mixing that takes place. This mixing is a primary means by which the ocean buries warming surface waters. So the surface warms up even faster. It's what we call a "positive feedback"—a vicious cycle.

That's bad for a number of reasons. As we currently watch the most active Atlantic hurricane season on record play out, a key underlying factor is the anomalous surface warmth of the tropical Atlantic. The increasingly intense and damaging hurricanes we've seen in recent years have fed off warmer surface waters. More stably stratified waters tend to inhibit the wind-driven mixing up of cold deeper waters that often serves as a sort of release-valve, shutting off the source of energy at the surface that intensifies these storms. A more stably stratified ocean potentially favors more intense, destructive hurricanes.

Warmer waters absorb less atmospheric carbon dioxide (just as warm soda loses its carbonation faster when you open the top.) Less ocean mixing also means that less of the atmospheric carbon dioxide gets buried beneath the ocean surface. So carbon pollution accumulates even faster in the atmosphere, causing yet more warming. Another "positive (i.e. bad) feedback" and another double whammy.

Finally, warmer upper ocean waters hold less dissolved oxygen. And less ocean mixing means less oxygen (and nutrients,) since oxygen and nutrients depleted by sea life are less likely to be replaced from rising, colder more oxygen-rich waters. That's bad for marine productivity. It means the potentially interruption of food webs and fish populations that provide the main source of protein for more than three billion people.

Our findings also have important implications for how much additional warming we may see in the future. State-of-the-art climate models used today to predict future climate change tend to underestimate the stratification of the ocean. They consequently bury heat (and carbon) too easily beneath the ocean surface. For that reason, they are likely underestimating the impact that the increased stratification (and decreased mixing) found in our study is having on rising atmospheric carbon dioxide levels and surface warming.

Our study suggests that key positive feedbacks (amplifying factors) related to reduced ocean heat might lead to more rapid surface warming in the decades ahead than many of the models predict. Decreased ocean carbon burial resulting from the reduced upper ocean mixing, moreover, could cause atmospheric carbon dioxide concentrations to build up faster in the atmosphere. Our findings lend credence to a recent study arguing we cannot rule out "worst case" scenarios (that some dismiss as "exceedingly unlikely") where carbon dioxide concentrations reach nearly triple pre-industrial levels—and the planet warms up by perhaps as much as 8F—by the end of this century.

In short, it's unwise to be complacent given the accumulating scientific evidence that climate change and its impacts may well be in the upper end of the range that climate scientists currently project. There is ever-greater urgency when it comes to acting on climate. But there is agency as well. Our actions make a difference—something to keep in mind as we head into a presidential election whose climate implications are monumental.

Michael E. Mann is Distinguished Professor of Atmospheric Science at Pennsylvania State University. His forthcoming book, The New Climate War: The Fight to Take Back our Planet is due out in January 2021.

The views expressed in this article are the author's own.