Life in the ocean’s "twilight zone" could decline dramatically due to climate change, new research suggests.
The twilight zone (200m to 1,000m deep) gets very little light but is home to a wide variety of organisms and billions of tonnes of organic matter.
The new study warns that climate change could cause a 20-40% reduction in twilight zone life by the end of the century.
And in a high-emissions future, life in the twilight zone could be severely depleted within 150 years, with no 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," said Dr Katherine Crichton, from the University of Exeter, and lead author of the study.
The research team, made up of palaeontologists and ocean modellers, looked at how abundant life was in the twilight zone in past warm climates, 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," said Professor Paul Pearson of Cardiff University’s School of Earth and Environmental Sciences, 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."
Animals in the twilight zone mainly feed on particles of organic matter that have sunk down from the ocean surface.
The study showed that in warmer seas of the past, this organic matter was degraded much faster by bacteria - meaning 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," said Dr Crichton.
This led the researchers to ask what will happen to life in the twilight zone in a future, warmer world.
Combining the evidence on past warm periods with Earth System Model simulations, they simulated what might be happening now in the twilight zone, and what could happen in future decades, centuries and millennia due to climate warming driven by greenhouse gas emissions.
"Our findings suggest that significant changes may already be under way," Dr Crichton continued.
"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 mediumand high-emissions scenarios.
"Our study is a first step to finding out how vulnerable this ocean habitat may be to climate warming."
The study’s three emissions scenarios are based on total carbon dioxide emissions after 2010. "Low" is 625 billion tonnes, "medium" is 2,500 billion tonnes, and "high" is 5,000 billion tonnes.
The paper, published in the journal Nature Communications, is entitled: "What the geological past can tell us about the future of the ocean’s twilight zone."
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