Scientists warn that climate change could reduce life in the ocean’s “twilight zone“, which is 200 to 1,000 meters deep, by 20-40% by the end of the century. They also said that in a future with high emissions, life in the twilight zone would be severely depleted within 150 years, without recovery for thousands of years.
The ocean twilight zone receives little light, but is home to a wide variety of organisms and billions of tons of organic matter. Scientists from universities in Great Britain described their findings in the journal Nature Communications.
The researchers said that while relatively little is known about the ocean’s twilight zone, what may happen in the future can be understood from past experience. A team of paleontologists and ocean modelers looked at records from preserved microscopic shells in ocean sediments and studied the abundance of life in the twilight zone in past warm climates.
Professor Paul Pearson of Cardiff University, who led the research says “We looked at two warm periods in Earth’s past, about 50 million years ago and 15 million years ago, during these warm periods, there were far fewer organisms living in the twilight zone because there was far less food coming from the surface waters”.
Organic matter sunk from ocean floor and fed on by animals in the twilight zone
The study showed that in earlier warmer seas, organic matter sunk from the ocean floor and fed on by animals in the twilight zone was degraded much more quickly by bacteria meaning less food made it into the twilight zone.
“The rich diversity of life in the twilight zone evolved over the past few million years when the ocean waters cooled enough to act more like a refrigerator, preserving food longer and improving the conditions for life to thrive,” said Katherine Crichton from the University of Exeter and lead author of the study .
To understand what will happen in the coming decades, centuries and millennia, they combined evidence of past warm periods with Earth model simulations.
“If we don’t reduce greenhouse gas emissions quickly, it could lead to the disappearance or extinction of much life in the twilight zone within 150 years, with consequences lasting millennia thereafter.
The study’s three emissions scenarios 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.
Emissions were close to 40 billion tons annually from 2010-22, so most of the carbon dioxide, about 500 billion tons, under the study’s “low” scenario has already been emitted. At the current rate, scientists said the “medium” scenario would be reached in 50 years and the “high” scenario in just over a century.
“The twilight zone plays an important role in the ocean’s carbon cycle because most of the carbon dioxide absorbed by phytoplankton ends up there as their remains sink from the surface ocean,” said Jamie Wilson of the University of Liverpool.
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