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Wave near San Diego: Until now, the focus of observation of ocean heat waves has mostly been on near-surface water layers

Photo: K.C. Alfred / dpa

Heat episodes also leave their mark on the world's oceans. On the contrary – especially in deeper water, marine heat waves can be more intense and last longer, as a study in the journal Nature Climate Change shows. Organisms are therefore also increasingly threatened there, because climate change is increasing phases with persistently high temperatures.

So far, the focus of the observation of sea heat waves has mostly been on near-surface water layers. Previous studies have already shown that their frequency and duration are increasing as oceans absorb much of the heat generated by man-made greenhouse gases. Now, an international research group has brought together global data sets from deeper layers of the oceans, evaluated them and combined them with data on biodiversity. The study uses worldwide measurements from the EU Copernicus Marine Service from 1993 to 2019 and looks at temperature changes in water layers down to a depth of 2000 meters.

The result: organisms in the oceans are also exposed to the influence of sea heat waves in the deeper layers. These even last longer and more intensely there than on the sea surface. The team defines marine heat waves as phases in which the water temperature is higher than 90 percent of the values from 1993 to 2019 in the respective region for at least five days. Warming can span several million square kilometers and last for weeks or months.

The Arctic is the hardest hit

The team led by climate researcher Eliza Fragkopoulou from the University of Faro in Portugal found that the highest heat wave intensity can be found at a depth of between 50 and 200 meters. While the intensity then decreases with increasing depth, the average duration of warming actually increases compared to the duration of marine heat waves at the sea surface. With an average of around 40 days at a depth of 2000 metres, it is about twice as long as on the surface – albeit with great spatial variability. The researchers suspect that the reason for this is that the layers mix less and less with increasing depth. The duration of deep marine heat waves in the Arctic was extended most sharply – up to three years.

This could have serious ecological consequences, as the ecosystems of the deep water layers may be particularly sensitive to thermal stress because they are adapted to constant temperatures, according to the authors of the study. They identified high-risk ecological zones at various depths and regions, including large areas in the Indian Ocean and the North Atlantic, where high heatwave intensity encounters species communities that are very sensitive to temperature changes. Overall, biodiversity is most endangered by heat in the upper 250 meters, they write.

Both near-surface and deep marine heat waves can affect biodiversity and thus change processes in the ecosystem, according to the study. The consequences can be massive: mobile species such as fish may migrate, site-bound organisms such as corals, seaweed or seagrass will be severely damaged or die. In addition, there are phenomena such as toxic algal blooms, lack of oxygen and acidification of the oceans.

Researchers from Germany also emphasize the importance of the study and see the need for further investigations. Christian Wild, head of the Marine Ecology working group at the University of Bremen, told the Science Media Center: "For the first time, the current study brings together a number of existing global data sets and long-term measurement series analytically." However, it remains "very vague about the effect of heat waves at greater depths on the various organisms in the sea."