It’s not just the land that groans in the heat, the ocean also suffers from heat waves. In the Mediterranean Sea, along the Italian and Spanish coasts, for example, water temperatures are currently up to 5°C above the long-term average for this time of year. Scientists have been studying marine heat waves for a few years now, for example at the University of Bern. However, relatively little is known about how marine heatwaves coexist with other extreme events in the ocean. Such events are known as compound events and considered a major climate change risk. While the processes that lead to extreme events on earth, such as floods, wildfires, heat waves or droughts and how they interact with each other, have been intensively studied in the past, the discovery that extreme weather and climate events can also occur in combination is relatively new.
A group of researchers from the Oeschger Center for Climate change research, led by Thomas Frölicher, has now investigated whether marine heatwaves coexist in combination with extreme events in other potential marine ecosystem stressors. In addition to the heat, potential stressors also include high levels of acidity in the ocean. “For the first time, we have quantified the frequency of compound events in which marine heat waves occur with extreme acidity,” says Friedrich Burger, postdoctoral researcher and first author of the study just published in the Nature Communications magazine. Extreme events of high ocean acidity are events where the concentration of protons in seawater is higher than normal.
Compound events particularly common in subtropical oceans
The main summary of the study, which is based on monthly observations of the surface ocean from 1982 to 2019, is that marine heat waves and extreme ocean acidity events occur relatively often together. This means that the negative impacts of past marine heat waves have been potentially exacerbated by extreme acidic situations. “We can show,” says ocean modeler Friedrich Burger, “that these compound events are most common in the subtropical oceans, but comparatively rare in high latitudes and the tropical Pacific.”
The co-occurrence of marine heatwaves and ocean acidity extremes in regions such as the subtropical oceans is caused by increased acidity at higher temperatures. However, if the
temperature increase also causes other effects, such as less mixing of the relatively more acidic groundwater with surface water, heat darkness can also reduce acidity and thus decrease the frequency of compound events. This occurs in the Southern Ocean or the tropical Pacific. “To determine the relative frequency of combined extreme events, it is important to understand the effects of heat waves on the circulation, biology and chemistry of the respective ocean region,” says study co-author Jens Terhaar.
Compound events in the ocean are increasing sharply
Due to climate change and carry emissions increase in CO2, extreme events such as marine heatwaves and ocean acidity extremes will continue to increase in frequency, which will exacerbate extreme marine heatwave and ocean acidity events. Earth system model simulations conducted by the Bernese researchers show that the number of days during which marine heat waves and high acidity events occur simultaneously increases by 22 times at 2°C global warming compared to pre-industrial situations. “This projected large increase could have serious impacts on marine ecosystems,” said Thomas Frölicher, co-author.
A team led by Frölicher had already shown the impact of sea heat waves in a Mother nature study by 2018. The conclusion was that ocean heat waves can irreversibly damage ecosystems and pose a threat to fisheries. Although there is evidence that marine organisms can be further harmed by the conjunction of hot and acidic seawater conditions, relatively little is still known about the biological effects of the co-occurrence of marine heatwaves and extreme weather conditions. acidity of the oceans.
This study was supported by the Swiss National Fund for Scientific Research and the Horizon research and innovation program 2020 of the European Union.