The environment in the oceans is now also being affected by heat waves. But these often do not come alone: In some marine areas, these heat extremes are increasingly coinciding with other exceptional events such as extreme acidification, as researchers have found. Such a double whammy is particularly devastating for marine ecosystems. In the course of climate change, however, this will increase: by a factor of 22 at just two degrees warming.
Not only the country groans from the heat - the ocean also suffers from heat waves. This is currently the case in the Mediterranean, for example, where the water temperatures on the Italian and Spanish coasts are sometimes up to five degrees higher than normal for this time of year. Such marine heat waves have been known and researched for a long time, but so far relatively little is known about their side effects: do such heat waves in the ocean occur together with other extremes such as strong acidification, a lack of oxygen or other factors?
Heat and acid at the same time
When two extreme events coincide in one place and at the same time, climate researchers speak of so-called compound events. For example, a common example on land is periods of intense heat coinciding with unusual droughts. Because these "double blows" are particularly difficult for nature and humans to cope with, they are considered to be particular risks of climate change. While on land it has been studied for some time how the processes that lead to floods, forest fires, heat waves or droughts interact with each other, such combined events in the oceans are still comparatively little researched.
For this reason, Friedrich Burger and his colleagues from the Oeschger Center for Climate Research at the University of Bern have now investigated in more detail how often and why marine heat waves occur together with extreme acidification of the ocean. This combination of abnormally warm water with high levels of acidity is particularly harmful to many marine organisms. There is a suspicion that some of the mass extinctions triggered by marine heat phases could have been such double extremes. Burger and his team therefore first used marine measurement data from the period 1982 to 2019 to investigate where and how often both occur together.
No coincidence
The evaluations showed that "double strikes" from marine heat waves and extreme acidification occur more frequently than previously thought. Such a compound event occurs somewhere in the world's oceans on average 1.8 months out of 100. "We can show that these composite events occur most frequently in the subtropical oceans, but are comparatively rare in the high latitudes and in the tropical Pacific," reports Burger.
The reason for these regional differences lies in the mechanisms underlying these dual events, the team determined. Accordingly, warmer surface water promotes the dissolution of salts and the appearance of free protons. This in turn increases the acidity of the water. The warmer the seawater, the more likely it is that acidification will also get worse - as is the case in the subtropical ocean regions. Conversely, warming can also reduce acidity. This is the case, for example, where it reduces the mixing of relatively acidic deep water with surface water, such as at higher latitudes. "In order to determine the relative frequency of combined extreme events, it is therefore crucial to understand the effects of heat waves on the circulation, biology, and chemistry of the ocean region under study," says co-author Jens Terhaar.
Climate change makes double whammy more common
The research team has also studied how the frequency of dual marine heat wave and acid surge events will evolve in the wake of climate change. Their model simulations found that the number of days that the ocean experiences heatwaves and acidification extremes combined could increase 22-fold with two degrees of global warming compared to pre-industrial conditions. "This strong predicted increase can have serious impacts on marine ecosystems," says co-author Thomas Frölicher.
Source: University of Bern; Specialist article: Nature Communications, doi: 10.1038/s41467-022-32120-7