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Extreme weather events on land are well known, but there has been little research into those that occur in the ocean. A study led by ETH Zurich shows for the first time that marine heatwaves and extremes with high acidity or low oxygen can occur together, with difficult to predict consequences for marine life.
In the summer of 2021, heavy rains and flooding caused by anthropogenic climate change, became noticeable in Switzerland. It has been known for a long time that global warming is causing more intense heatwaves and more severe weather in different parts of the world. Extreme weather events increasingly occur in combination.
Extreme events in the world's oceans have not been studied. In the early 2000s, the first scientific studies pointed out the significance of marine heatwaves. A wake-up call came in the form of a persistent marine heatwave off the west coast of Australia that destroyed the species-rich kelp forests there.
The "Blob," a giant bubble of warm water that spread in the northeast Pacific Ocean and along the US West Coast from Alaska to the equator, is probably the most prominent example of a marine heatwave. Millions of birds, fish, and other creatures were killed by it.
Researchers at the University of Bern and the University of Tasmania used a high-resolution ocean model to analyze the extreme weather event. The international team concluded that the mass die-off was probably caused by a combination of extreme events, but not solely by the high water temperatures.
Extreme events are dangerous.
The researchers used their model to reconstruct the Blob's development over time, and in doing so, they analyzed for the first time the combination of temperature, acidity and oxygen concentration of the ocean water. Extremes in acidity and low oxygen spread throughout the affected region in the northeast Pacific at the peak of the heatwave in July 2015, according to their simulations.
The researchers from the ETH concluded that what happened off the coast of Oregon, Washington and British Columbia was a compound extreme event. When marine life is confronted with multiple stressors at once, it has difficulty acclimatizing. An oxygen deficiency can mean death for a fish species that is already living at the upper end of its optimal temperature range.
In their study, the researchers called on the scientific community to pay more attention to extreme events in the ocean. "To assess the risks of these kinds of events, we need to study the chain of different environmental factors leading to such extremes more closely, and also at the global level."
For the first time, global distribution was analyzed.
The first step in this direction was taken by the authors of this study. They used a global climate model to investigate where and how often extreme events occur, and how severe they are.
The researchers compared the current situation with pre-industrial times and simulations of extreme events for the period from 1861 to 2020 to demonstrate the impact of climate change. The number of hot days on the ocean surface has increased tenfold in the last decade. The number of days with low oxygen in the ocean has increased fivefold.
The situation is graver with regard to extremes. Compared with pre-industrial times, what has now established itself is a permanent extreme situation. The study shows how far climate change has advanced in the ocean.
The researchers show on a world map which ocean regions have the most intense extreme events. 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.
We don't know much about marine species communities.
The effect of extreme events on ocean life is generally stronger than with climate change, despite the fact that the researchers cannot assess the ecological consequences of extreme events in detail. Many adaptation strategies are not possible because of the sudden occurrence of environmental changes.
Current model simulations can only replicate the response to extremes, and they can't do justice to the complexity of biological and ecological processes. "Our models are limited in their ability to distinguish between different groups of zooplankton," says Meike Vogt, a senior researcher in the group. Different species have different ability to survive extreme conditions.
"We know from Swiss forests that beech trees are not as tolerant of the weather as pines," he says. Far too little is known about the marine environment. We don't have a good understanding of the function and structure of the ocean. The impact of climate change and extremes will only be determined by this foundation.
Extreme events will increase if climate change continues. A better data basis and extensive research work would lead to more suitable action to protect the ocean. There are already international protected areas on the high seas, so we could establish a fishing ban to protect areas affected by extreme events. The case of the Blob has already been done. Further measures are needed, says the professor. "Time is short."
Nicolas Gruber and his team of scientists have studied the ocean's biogeochemical extremes and compound events. There is a DOI of 10.1038/s41586-021-03984-7.
Nature journal information.
Climate change is intensifying in the oceans, even in December.
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