One of Earth’s largest habitats could see its rich diversity of life reduced by the end of the century due to the climate crisis. The ocean’s mesopelagic zone, also called the “twilight zone,” is located between 656 feet and 3,280 feet (200 meters to 1,000 meters) below the surface.
The Marine Region
The marine region, which accounts for approximately a quarter of the ocean’s volume, is home to billions of metric tons of organic matter and some of Earth’s most stunning biodiversity, despite being beyond the reach of sunlight. The twilight zone is also a crucial habitat for marine life that dives in search of prey, like sharks, or lanternfish that hide in the twilight zone during the day and swim to the surface waters to feed at night.
Ancient Warm Waters
Paleontologists and ocean scientists teamed up to study the impacts on the ocean’s twilight zone during previous ancient warming events in order to predict how the habitat may respond in the future due to global warming. The research team studied cores taken from the seafloor that included evidence of preserved microscopic shells from plankton. Over time, the calcium carbonate shells accumulate on the seafloor, 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.
A study detailing the findings was published Thursday in the journal Nature Communications. “We still know relatively little about the ocean twilight zone, but using evidence from the past we can understand what may happen in the future,” said lead study author Dr. Katherine Crichton, a postdoctoral research fellow at the University of Exeter in the United Kingdom, in a statement. The researchers focused on two warm periods that occurred 15 million years ago and 50 million years ago, where even ocean temperatures were “markedly warmer than today,” according to the study. Luiz A. Rocha, curator and Follett Chair of Ichthyology at the California Academy of Sciences, worries that changes are underway that haven’t been detected because the twilight zone is so understudied, largely due to a disconnect between funding and the costs of exploring this region.
Predicting The Future
Based on what they discovered from the ancient warming events, the researchers combined that data with Earth system mode simulations — modeling of Earth’s carbon cycle as it moves through the land, sea and atmosphere. “Our findings suggest that significant changes may already be under way,” Crichton said. “Unless we rapidly reduce greenhouse gas emissions, this could lead to the disappearance or extinction of much twilight zone life within 150 years. The low estimate was 625 billion metric tons, medium was 2,500 billion metric tons and high was 5,000 billion metric tons.
The Global Carbon Budget estimates that the total global carbon dioxide emissions in 2022 was 40.6 billion metric tons. Yearly emissions have been close to that number each year since 2010, so the researchers noted that the low-estimate scenario they used has already been emitted. “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 their remains sink down from the surface ocean,” said study co author Jamie Wilson, a postdoctoral researcher at the UK’s University of Liverpool, in a statement. Protecting the twilight zone will be difficult since typical conservation efforts, such as preventing fishing or deep-sea mining, can’t apply there, Rocha said.
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