Oceans becoming more stratified due to global warming

A worrying new study, by a group of international researchers from China and the US, has found that our oceans are becoming more layered and resistant to vertical mixing, due to increased warming at the surface from climate change.

This extreme colour differentiation shows the horizontal stratification between two bodies of water with different properties, it highlights how the same thing can happen vertically

In the ocean, stratification occurs between water with different physical properties, such as temperature, salinity and density, which act as a physical barrier for mixing and results in distinct layers of sea water. This is a natural phenomenon that occurs in all oceans, usually vertically but also occasionally horizontally as well. As a general rule, warmer and less dense water forms layers on top of colder and denser water which sinks to the bottom. If you have ever been scuba diving or snorkelling in a tropical country, this may even be something you have experienced yourself as stratification can occur very close to the surface. Stratification is something that has long been understood to play a crucial role in ocean systems, but monitoring how it changes over time is something that is much more challenging. However, a ground-breaking new international study has revealed that increased surface temperatures, caused by global warming, are increasing stratification between layers with some potentially devastating results for marine life and the rest of the planet.  

New study

The main problem in estimating changes in stratification over time, is the sparse distribution of ocean observations, both horizontally and vertically, which make it very hard to collect enough data to make reliable predictions. As a result previous quantification of stratification changes have been limited to a simple index which neglects the spatial complexity of ocean density change. The new study, by a group of researchers from the Institute of Atmospheric Physics (IAP) in China and various universities in the US, is the first to overcome this problem. Using the IAP’s new temperature and salinity data set, containing a range of measurements at much greater depths than ever before, they were able to calculate a much more accurate index of stratification. This new index, known as the squared buoyancy frequency, has allowed them to make much more accurate estimates of ocean stratification and its changes over the last 60 years.

This graphic shows how temperature decreases with depth, as well as how the surface layer sits on top of colder water beneath it

Increased stratification

Unfortunately the results of their study, recently published in a new paper in the journal Nature Climate Change, show that ocean stratification has increased much more than we previously believed. Their new data shows a 5.3% increase in stratification in the upper 2000m of the ocean since 1960. As well as an even stronger increase, as high as 18%, in the upper 150m during the same time. The reason for these increases is undoubtedly human caused climate change, for two main reasons. Firstly, and most importantly, rising global temperatures have meant that of our oceans have absorbed more heat, so the temperature difference between warmer surface waters and colder deep waters is greater than ever before. Secondly a reduction in wind driven currents over the last half a century, also caused by climate change, has reduced the amount of natural mixing between the layers. The result is that the physical barriers between the layers are much stronger and the layers themselves are more distinct.

What does it mean?

You may be wandering why differences in the stratification between layers in our oceans are anything to really worry about, but it is already creating serious problems for marine life and the planet which are only going to get worse. The main issue with increased stratification is that it prevents the mixing of surface and deep waters, which in turn means essential resources cannot be exchanged between the two. The most important of which are oxygen from the atmosphere and nutrients, including plankton and decaying matter known as detritus, which can get trapped in the surface layer. This has already resulted in large dead zones in the ocean, where a lack of oxygen and food means most marine creatures cannot live below the surface layer at all. These dead zones are only just starting to be discovered, but already scientists are alarmed at their growth in size and effect on marine life.

This map shows known dead zones in the ocean (red) and areas where oxygen levels are low (blue). The high concentration of dead zones around coastlines highlights how nutrient run-off from land can trigger the formation of dead zones

The other main problem with a lack of ocean mixing between layers is that it creates a positive feedback loop that superheats surface waters. This means that as the surface layer absorbs more heat, the stratification increases which means more of the atmospheres’ heat which the ocean absorbs remains in the surface layer, which in turn increases stratification and so on. This surface superheating is believed to be a main cause of increased El Nino heatwave events in recent decades, which in turn has damaging effects on coral reefs and other ecosystems. It also means the problem is only likely to get worse in the future as well. Another worrying trend is that stratification may also be limiting the ocean’s ability to absorb carbon at its current rate and may interfere with their ability to help stabilize the already changing climate.

All in all increased ocean stratification is a big problem that is only starting to get the recognition it deserves. Our activities above the surface are starting to seriously alter the physical properties of the oceans below it, which in turn is causing worrying ecological shifts. With no solution in sight, other than drastically reducing our global carbon output (which alarming doesn’t seem to be happening anytime soon), it is also a problem that will only get worse throughout rest of this century.

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