Life in the ocean’s twilight zone “may disappear” amid warming seas

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Life in the ocean’s twilight zone is expected to face dramatic decline and even extinction as seas warm and less food reaches dimly lit waters, according to one study.

The twilight zone lies between 200 and 1,000 meters below the surface and is home to a variety of organisms and animals, including specially adapted fish such as lantern sharks and kitetip sharks, which have huge eyes and glowing, bioluminescent skin.

Twilight zone animals feed on billions of tons of organic matter, such as dead phytoplankton and fish poop, that washes down from the ocean’s surface. The drifting particles are known as marine snow.

Related: Record ocean temperatures put Earth in ‘uncharted territory’, say scientists

The warmer waters were in effect reducing the amount of food sinking into the area, meaning up to 40 percent of life in the twilight waters could be gone by the end of the century, according to the study, which was published in Nature. Recovery could take thousands of years.

“The rich variety of life in the twilight zone evolved over the past million years, as ocean waters cooled enough to act like a refrigerator, storing food longer and improving the conditions for life to thrive.” said Katherine Crichton, head of the study. author and postdoctoral researcher at the University of Exeter.

“According to the studies we’ve done, 15 million years ago there wasn’t all this life [in the twilight zone] and now, due to human activity, we may lose everything. It’s a huge loss of wealth,” Crichton told the Guardian. “Unless we reduce greenhouse gas emissions rapidly, this could lead to the disappearance or extinction of much life in the shadows within 150 years, with effects extending for millennia afterward.”

Warmer oceans have also reduced carbon storage, said Paul Pearson of Cardiff University, the study’s principal investigator. That’s because the “sinking carbon as part of marine snow” is mostly consumed by microbes closer to the surface, rather than falling further. Less sinking means faster carbon release.

Crichton said the good part about the study was that “it doesn’t look like we’ve reached an irreversible point. We can’t avoid some losses, but we can avoid the worst if we control emissions.”

Though little known, the twilight zone “contains perhaps the world’s largest and least exploited fish stock, and recycles [about] 80% of organic material sinks,” according to a United Nations program studying the region.

Crichton said: ‘We still know relatively little about the twilight zone of the ocean, but using evidence from the past we can figure out what might happen in the future.’ His team’s findings suggested that “significant changes may already be underway.”

The study offered three possible future scenarios for the twilight zone: a low-carbon scenario, allowing for a total of 625 billion tons of emissions from 2010 onwards; an average scenario, which foresees 2,500 billion tons; and one high, taking into account 5,000 billion tons. “If we go to the medium or high scenario, both are very bad news for the twilight zone,” Crichton said.

To put the emissions figures into context, the Global Carbon Budget led by the University of Exeter has estimated that in 2022 total global CO2 emissions reached 40.6 billion tons. Emissions were close to 40 billion tons each year from 2010-22, so most of the CO2 in the low-carbon scenario of the study it has already been issued.

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