Extreme marine heatwaves and ocean acidification can have serious consequences for marine ecosystems. For the first time, researchers have determined the frequency and drivers of these compound events and projected them into the future.
Heatwaves are causing problems not only on land, but also in the ocean. Water temperatures in the Mediterranean Sea along the Italian and Spanish coasts, for example, are currently up to 5 °C higher than the long-term average for this time of year. Scientists have been studying marine heatwaves for several years, including at the University of Bern.
However, relatively little is known about how marine heatwaves co-occur with other extreme events in the ocean. Such events are known as compound events and are considered to be a major risk of climate change. While the processes that lead to extreme events on land, such as floods, forest fires, heatwaves, or droughts, and how they interact with each other have been intensively studied in the past, the finding that ocean weather and climate extreme events can also occur in combination is relatively new.
For the first time, we quantified the frequency of compound events involving marine heatwaves and extreme acidity. Extreme events of high ocean acidity occur when the proton concentration in seawater exceeds normal levels.Friedrich Burger
A team of researchers led by Thomas Frölicher at the Oeschger Center for Climate Change Research has now investigated whether marine heatwaves co-occur with extreme events in other potential marine ecosystem stressors. Aside from heat, other potential stressors include high acidity levels in the ocean.
“For the first time, we quantified the frequency of compound events involving marine heatwaves and extreme acidity,” says Friedrich Burger, postdoctoral researcher and first author of the study, which was recently published in the journal Nature Communications. Extreme events of high ocean acidity occur when the proton concentration in seawater exceeds normal levels.
Compound events especially frequent in the subtropical oceans
The main finding of the study, which is based on monthly observations from the surface open ocean from 1982 to 2019, is that marine heatwaves and extreme ocean acidity events occur relatively often together. This means that the negative impacts of past marine heatwaves were potentially exacerbated by extreme acidic conditions. “We can show,” says ocean modeler Friedrich Burger, “that these compound events are most common in the subtropical oceans, but comparably rare in the high latitudes and tropical Pacific.”
In regions such as the subtropical oceans, the co-occurrence of marine heatwaves and ocean acidity extremes is caused by an increase in acidity at higher temperatures. However, if the temperature increase has other consequences, such as less mixing of relatively more acidic subsurface water with surface water, a heatwave can reduce acidity and thus reduce the frequency of compound events. This occurs in the southern or tropical Pacific oceans. “Understanding the effects of heatwaves on the circulation, biology, and chemistry of the respective ocean region is critical to determining the relative frequency of combined extreme events,” says Jens Terhaar, co-author of the study.
Compound events in the ocean are increasing strongly
Extreme events such as marine heatwaves and ocean acidity extremes will become more common as a result of climate change and ongoing CO2 emissions, as will compound marine heatwave and ocean acidity extreme events. According to the Bern researchers’ Earth system model simulations, the number of days when marine heatwaves and high acidity events co-occur increases 22-fold at a 2 °C global warming compared to pre-industrial levels. “This large projected increase may have severe consequences for marine ecosystems,” said co-author Thomas Frölicher.
In a 2018 Nature study, a team led by Frölicher demonstrated the impact of marine heatwaves. The study concluded that ocean heatwaves can irreversibly damage ecosystems and may endanger fisheries. Although there is evidence that the co-occurrence of warm and acidic seawater conditions can harm marine organisms, little is known about the biological effects of marine heatwaves and ocean acidity extremes.