The oceans are important actors in the earth’s climate system. But climate change is also severely affecting the seas. Now a study reveals that the stratification of the oceans has increased significantly in recent decades. In particular, the warming of the upper layers of water increasingly hinders an exchange of the water masses. This is intensified locally by the influx of fresh water from thawing glaciers. As a result, the oceans are less able to dissipate heat, oxygen and nutrients into the depths – with consequences for marine ecology but also for the terrestrial climate, as the researchers explain.
The water of the seas is not evenly mixed vertically, but typically forms stratified zones. Warmer water with a lower salt content collects near the surface of the water because it has a lower density. In contrast, in deeper layers there is mostly cold, salty deep water. There is only a limited exchange of water between these layers – this usually occurs in strong winds or in winter when the surface temperature is lower. “This stratification acts like a barrier for the mixing of the water and it also inhibits the vertical exchange of heat, carbon, oxygen and other components,” explain Guancheng Li from the Chinese Academy of Sciences in Beijing and his colleagues. “The extent of this stratification is therefore a central element in the earth’s climate system.”
Marine water stratification in sight
But how the stratification of the oceans is changing as a result of climate change has only been roughly known so far. The increasing warming of the water surface suggests that this also affects the water stratification and strengthens it. But the increased influx of fresh melt water from the thawing glaciers in the Arctic and Antarctic could also contribute. Because the fresh water is less dense and therefore forms a relatively stable layer on the salty sea water. However, so far there has been insufficient data on the extent to which these processes have already affected the stratification of the oceans. The reason: “Observations on ocean temperatures and salinity are sometimes sparse and spatially unevenly distributed,” explain Li and his team. Often only data on the upper 200 meters of the seas have been taken into account and gaps have been bridged with rough estimates.
In order to remedy this, the researchers have now used new measurement data on water temperatures and salinity that go down to a depth of 2000 meters. They also used measurements of the nitrogen dissolved in the water to indirectly obtain information about the stratification. Because its amount depends on the density of the water. In addition, the scientists used a computer model that can close any data gaps on the basis of physical principles.
Warming is the main driving force
The evaluations confirm the trend towards more stable stratification of the ocean water. “We have found that the stability of these layers is increasing faster than we thought and also faster than current models predict – that is a worrying prospect,” says co-author Michael Mann of Pennsylvania State University. According to the data, the stratification in the upper 2,000 meters has increased by an average of 5.3 percent since 1960 – this corresponds to around 0.9 percent per decade. In the upper 150 meters, the differences between the water layers have even increased by 18 percent. These changes can be observed in all four large sea basins, as the researchers explain. “The rate is highest in the Southern Ocean, followed by the Pacific, Atlantic and then the Indian Ocean,” said Li and his colleagues. However, there are geographical differences with regard to the water depths that are most affected: in the tropics, the uppermost 200 meters of the oceans have changed most significantly, while water depths below 500 meters are most affected in the middle and higher latitudes.
The researchers identified global warming of the air and thus of the upper water layers as the main cause of this development. 97 percent of the observed changes in ocean stratification are due to this, say the scientists. “Global warming makes the atmosphere more unstable, but at the same time the oceans more stable,” explains Mann. “Because the water near the surface warms up faster than the greater depths, this makes the stratification more stable.” On a local scale, however, the influx of melt water and the associated decrease in salinity also play a role. As the researchers point out, their observations are more than just basic oceanographic research. “Increasing stratification has significant effects on the climate,” say Li and his team. “The associated decrease in mixing can impair the absorption of heat and carbon dioxide by the oceans and also influence the formation of tropical storms and their intensity.” In addition, insufficient mixing of the seawater also hinders the distribution of dissolved oxygen in the water. This can intensify low-oxygen “dead zones” on the sea floor.
Source: Guancheng Li (Chinese Academy of Sciences, Beijing) et al., Nature Climate Change, doi: 10.1038 / s41558-020-00918-2