Since the turn of the millennium, devastating heat waves – called blobs – have repeatedly occurred in the northeast Pacific off the west coast of North America. The most recent event lasted three years from 2019 to 2021 and cost the lives of tens of thousands of whales, seabirds and seals.

A team led by Armineh Barkhordarian from the University of Hamburg has investigated this phenomenon. In “Nature Communications Earth and Environment” it presents the cause of the accumulating hot water bubbles in the region: man-made climate change.

The blob, which can be up to three million square kilometers in size, is due to the human-caused increase in greenhouse gases in the atmosphere, which favors extreme heat waves in this part of the sea, the working group writes. The water temperatures are then up to six degrees Celsius above the long-term average, which is fatal for many cold-loving animal species or forces them to migrate.

The warm water also encourages toxic algae blooms, which, when they die, remove oxygen from the water and thus harm or kill many living beings.

According to the analysis by Barkhordarian and Co, the probability that such a heat wave as between 2019 and 2021 would have taken place without human influence is less than one percent. There is a 99 percent probability that the increase in greenhouse gases in the atmosphere and the resulting global warming are responsible.

According to this, the water temperature in the northeastern Pacific has increased by an average of 0.05 degrees Celsius per year over the past 25 years. At the same time, the region cools less in winter due to less cloud formation and the period of summer conditions increased by 37 days.

In winter, the high pressure areas over the water have intensified, making cloud formation more difficult. The undisturbed solar radiation can thus warm the sea during the day. This effect is obviously stronger than the nocturnal heat radiation that would otherwise be impeded by the clouds.

As a result, since 2000, 31 marine heatwaves have occurred in this part of the ocean, while between 1982 and 1999 there have been only nine blobs. “More frequent and extreme heat waves in the ocean are dramatically stressing ecosystems. This not only harbors enormous dangers for biodiversity. It can also cause marine ecosystems to cross a threshold beyond which recovery is not possible,” says Barkhordarian.

Previous studies had already suggested that climate change made events like the blob 20 times more likely. Similar warm-water bubbles have also appeared in other sea areas such as the South Pacific or the Atlantic in recent years.

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