One of the largest habitats on Earth, the climate crisis could see a decline in the rich diversity of life by the end of the century.
The mesopelagic zone of the ocean, also called the “twilight zone”, is located between 656 feet and 3,280 feet (200 m to 1,000 m) above the surface.
The oceanic region, which accounts for about a quarter of the ocean’s volume, is home to billions of metric tons of organic matter and some of Earth’s most spectacular biodiversity, despite being out of reach of sunlight.
The twilight zone is also a critical habitat for marine life that dives in search of prey, such as sharks or lanternfish that hide in the twilight zone during the day and swim to surface waters to feed at night.
New research warns that the climate crisis could reduce life in the twilight zone by between 20% and 40% by the end of the century. And if greenhouse gas emissions continue, researchers estimate that ocean life could be severely depleted within 150 years—and recovery may not be possible for thousands of years.
Paleontologists and ocean scientists teamed up to study the effects on the ocean’s twilight zone during previous ancient warming events to predict how future habitats might respond to global warming. The research team studied cores taken from the sea floor that contained evidence of microscopic shells preserved from plankton.
Over time, calcium carbonate shells accumulate on the ocean floor, preserving information about what the environment was like during their lifetime. The tiny shells effectively create a timeline of how the ocean has changed over millions of years.
There was a study detailing the findings Published Thursday in the journal Nature Communications.
“We still know relatively little about the ocean’s twilight zone, but using evidence from the past we can understand what might happen in the future,” lead study author Dr. Catherine Crichton, a postdoctoral research fellow at the University of Exeter in the United Kingdom. Kingdom, in a statement.
The researchers focused on two warm periods 15 million years ago and 50 million years ago, where ocean temperatures were also “significantly warmer than today”.
“We found that the Twilight Zone is not always a rich habitat full of life,” study co-author Paul Pearson, an emeritus professor at Cardiff University in the United Kingdom, said in a statement. “In this warm period, very few organisms lived in the twilight zone, because very little food came from the surface water.”
Particles of organic matter drift downward from the surface of the ocean and serve as one of the main food sources for life in the twilight zone. But past warming events caused the material to be degraded more quickly by bacteria, so less of it reached the ocean floor.
According to the study, warmer ocean temperatures also increase the metabolic rate of organisms, leading to increased food demand and oxygen consumption.
“The rich diversity of twilight zone life evolved over the past few million years, when ocean waters cooled enough to act like a refrigerator, preserving food longer and improving the conditions that allowed life to flourish,” Crichton said.
Louise A. Rocha, curator and Follett Chair of Ichthyology at the California Academy of Sciences, worries that there are changes going on that go undetected because the Twilight Zone is so understudied, largely because of the disconnect between funding and the cost of exploring it. territory
Rocha, who was not involved in the study, researches the twilight zone and the mesophotic zone above it, located between 98 and 492 feet (30 and 150 m) below the surface.
“There’s no baseline data to compare what we’re measuring against, so this study looking at the structure of the fossil record over time is one way we can try to understand how we’re changing the twilight zone,” Rocha said.
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Based on what they discovered from ancient warming events, the researchers linked that data to Earth System Mode Simulation – Modeling the Earth’s carbon cycle as it passes through the land, ocean, and atmosphere.
The team’s work revealed what is currently happening in the Twilight Zone and how it might change decades, centuries, and even millennia into the future as the world warms.
“Our findings suggest that significant changes may already be occurring,” Crichton said. “Unless we rapidly reduce greenhouse gas emissions, this could lead to the disappearance or extinction of most life in the twilight zone within 150 years, with impacts lasting thousands of years. Even a low-emissions future could have significant impacts, But it would be far less severe than under moderate and high emissions scenarios.Our study is a first step in exploring how vulnerable these ocean habitats may be to climate warming.
The researchers used three emission scenarios based on total carbon dioxide emissions after 2010.
The low estimate was 625 billion metric tons, the medium was 2,500 billion metric tons, and the high estimate was 5,000 billion metric tons.
The Global Carbon Budget estimates that total global carbon dioxide emissions in 2022 were 40.6 billion metric tons. Annual emissions have been close to that number every year since 2010, so the researchers note that the low-estimate scenario they used has already been emitted.
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The team thinks that moderate emissions conditions may have been reached In about 50 years, and the high estimate in just over a century.
“The twilight zone plays an important role in the ocean’s carbon cycle because most of the carbon dioxide taken up by phytoplankton ends up there as its residues sink down from the surface ocean,” said study coauthor Jamie Wilson, a postdoctoral researcher at the University of the UK. of Liverpool, in a statement.
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“One of the challenges in predicting how this movement of carbon might change in the future is that there are many dissipation processes in the modern ocean. By looking at the twilight zone in past warm periods we can identify the most important processes and use them to predict the future. We found that this natural cycling of carbon is likely already changing and could be disrupted for long periods of time in the future.
According to the study, the climate crisis has affected the Earth’s oceans in the form of pollution, warming, deoxygenation, acidification, and overfishing. These effects have led conservationists to consider various protective efforts such as restricting harmful activities in the oceans.
Rocha said protecting the twilight zone would be difficult because normal conservation efforts, such as preventing fishing or deep-sea mining, cannot be applied there.
“A marine protected area for (the twilight zone) makes very little sense because the impacts on it are global in nature,” he said. “What we really need to protect (the twilight zone) is to stop or at least slow down the high rate of change that we are subjecting our planet’s climate to.”
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