It has long been known that global warming is causing not only longer and more intense heatwaves, but also, depending on the region, more severe droughts, rains and storms. Moreover, these kinds of extreme weather events increasingly occur in combination, compounding each other.
However, there has been little research into how extreme events develop in the world’s oceans. Beginning in the early 2000s, first scientific studies pointed out the significance of marine heatwaves and their impact on ecosystems. A wake-up call came in 2011 in the form of a persistent marine heatwave off the west coast of Australia that destroyed the species-rich kelp forests there. Probably the most prominent example of a marine heatwave is the “Blob,” as it is known — a giant bubble of warm water that spread in the northeast Pacific Ocean and along the US West Coast from Alaska to the equator from 2013 to 2015. It killed millions of marine birds, fish and other creatures.
Researchers at ETH Zurich, the University of Bern and the University of Tasmania used a high-resolution ocean model to analyze this extreme weather event from a new perspective. And concluded that it was not solely the high water temperatures that caused the mass die-off, but probably a combination of extreme events that occurred simultaneously.
In addition to the Blob, they used a global climate model to investigate where and how often extreme events — separated into heatwaves and situations involving anomalously high acidity and low oxygen — occur and how severe they are. Click here
Globally from 1861 to 2020, comparing the current situation with pre-industrial times, the number of hot days on the ocean surface each year has increased tenfold, from around 4 days to 40. The number of days on which the ocean depths are characterized by anomalously low oxygen has increased fivefold. With regard to acidity extremes, the situation is even graver. Compared with pre-industrial times, what has now established itself is almost a permanent extreme situation. “This shows how far climate change has already advanced in the ocean,” says Thomas Frölicher, Professor at the University of Bern and co-author of the study.