EXETER, United Kingdom — The twilight zone is in danger, and we’re not talking about the old TV show. Stretching all over the globe, the ocean’s “twilight zone” lies anywhere from 600 to 3,300 feet below the ocean surface. That twilight zone resides just beyond the reach of sunlight and contains many bioluminescent fish, other organisms, and billions of tons of organic matter. Now, researchers from the University of Exeter report that even aquatic life far below the surface could decline dramatically due to climate change.
The study estimates that climate change could cause a 20 to 40-percent reduction in twilight zone life by the end of this century. Even worse, in a future with high emissions, life in the twilight zone could be severely depleted within as little as 150 years — with no conceivable recovery for thousands of years!
“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,” says Dr. Katherine Crichton, lead author of the study, in a university release.
The research team behind this work, consisting of paleontologists and ocean modelers, examined how abundant life was in the twilight zone during prior warm climate periods. They accomplished this by using records from preserved microscopic shells in ocean sediments.
“We looked at two warm periods in the Earth’s past, about 50 million years ago and 15 million years ago,” explains Professor Paul Pearson of Cardiff University, who led the research. “We found that the twilight zone was not always a rich habitat full of life. In these warm periods, far fewer organisms lived in the twilight zone, because much less food arrived from surface waters.”
Less food reaches the twilight zone in warmer oceans
Animals in the twilight zone primarily eat particles of organic matter that have drifted down from the surface. This research shows that in the warmer seas of the past, bacteria degraded organic matter much faster than in cooler periods. So, less food reached the twilight zone.
“The rich variety of twilight zone life evolved in the last few million years, when ocean waters had cooled enough to act rather like a fridge, preserving the food for longer, and improving conditions allowing life to thrive,” Dr. Crichton adds.
This finding led the researchers to ask themselves what will happen to life in the twilight zone in a future, warmer world.
By combining the evidence on past warm periods with Earth System Model simulations, study authors were able to simulate what might be happening now in the twilight zone as our planet heats up, as well as what could happen in future decades, centuries, and millennia due to greenhouse gas emission-driven climate warming.
“Our findings suggest that significant changes may already be under way,” Dr. Crichton continues. “Unless we rapidly reduce greenhouse gas emissions, this could lead to the disappearance or extinction of much twilight zone life within 150 years, with effects spanning millennia thereafter. Even a low-emissions future may have a significant impact, but that would be far less severe than medium- and high-emissions scenarios. Our study is a first step to finding out how vulnerable this ocean habitat may be to climate warming.”
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Carbon dioxide emissions are trending in the wrong direction
The three emissions scenarios described by the study are based on total carbon dioxide emissions after 2010; “low” is 625 billion tons, “medium” is 2,500 billion tons, and “high” is 5,000 billion tons. For reference, the Global Carbon Budget (led by the University of Exeter) estimates total global carbon dioxide emissions in 2022 alone were 40.6 billion tons. Annual emissions have hovered around 40 billion tons for roughly the past decade (2010-2022). Thus, most of the carbon dioxide (about 500 billion tons) for the study’s “low” scenario has already been emitted.
At the current rate we’re on, the “medium” scenario will be reached 50 years from now, and the “high” will be reached in a little more than 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 their remains sink down from the surface ocean. One of the challenges of predicting how this movement of carbon might change in the future is that there are many processes to disentangle in the modern ocean,” says Dr. Jamie Wilson, from the University of Liverpool.
“By looking back at the twilight zone in past warm periods we can identify the most important processes and use those to predict the future. We found that this natural cycling of carbon is likely already changing and may be perturbed long into the future.”
The UN program JETZON has been established to help expand modern science’s understanding of the ocean twilight zone.
“It is poorly understood from almost any perspective. However, it contains possibly the world’s largest and least exploited fish stock and recycles ~80% of the organic material that sinks out of the productive surface waters,” the program’s website reads.
The study is published in Nature Communications.