A combination of extreme events is particularly dangerous
The researchers used their model to reconstruct the Blob’s development over time, and in doing so, they analysed for the first time the combination of temperature, acidity and oxygen concentration of the ocean water. Their simulations show that, at the peak of the heatwave in July 2015, extremes in acidity and low oxygen had also spread extensively throughout the affected region in the northeast Pacific.
From this, the ETH researchers concluded that what occurred off the coasts of Oregon, Washington and British Columbia was not merely a heatwave but a compound extreme event. “When marine life is confronted with multiple stressors at once, it has difficulty acclimatising,” Gruber says. “For a fish species that’s already living at the upper end of its optimal temperature range, an added oxygen deficiency can mean death.”
That’s why, in their study – which was just published in the journal Nature – the researchers called on the scientific community to pay greater attention to compound extreme events in the ocean. “To assess the risks of these kinds of events, we urgently need to study the chain of different environmental factors leading to such extremes more closely – and not only in individual regions, but also at the global level,” Gruber says.
Global distribution analysed for the first time
The authors of this study have already taken a first step in this direction. 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.
To demonstrate the impact of climate change, the researchers simulated the extreme events for the period from 1861 to 2020 and compared the current situation with pre-industrial times. The results speak for themselves: globally, 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.
The researchers also show on a world map which ocean regions see the most intense extreme events – both at the ocean surface and 200 metres below it. The spatial resolution of these events within the water column is important because this further limits the possibilities for the affected marine life to escape, as the study’s authors highlight.
