Climate Change Poses a Risk to the Twilight Zone

A new study finds that life in the ocean’s “twilight zone” could be greatly diminished by climate change. The twilight zone (200m to 1000m) gets very little light but is home to a variety of organisms and billions of tons of organic matter.

The new study warns that climate change could cause a 20-40% drop in life in the Aurora Zone by the end of the century.

And in a high-emitting 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 aurora zone, but using evidence from the past we can understand what might happen in the future,” said Dr Catherine Crichton, from the University of Exeter and lead author of the study.

The research team, made up of paleontologists and ocean modellers, looked at the abundance of life in the twilight zone in past warm climates, using records from microscopic shells preserved in ocean sediments.

“We looked at two warm periods in Earth’s past, about 50 million years ago and 15 million years ago,” said Professor Paul Pearson of Cardiff University, who led the research.

“We found that the Twilight Zone wasn’t always a rich, life-filled home.

“In these warm periods, the number of organisms living in the twilight zone was much lower, because much less food arrived from the surface waters.”

Animals in the twilight zone feed primarily on particles of organic matter that have sunk from the ocean’s surface.

The study showed that in the warmer seas of the past, this organic matter decomposed faster due to bacteria – which meant less food reached the twilight zone.

“The rich diversity of the aurora zone has evolved in the last few million years, when ocean water cooled enough to act like a refrigerator, preserving food for longer, and improving conditions that allow life to thrive,” said Dr. Crichton.

This led the researchers to wonder what would happen to life in the Twilight Zone in a warmer future world.

Integrating evidence about past warm periods with Earth system model simulations, they simulated what might happen now in the aurora zone, and what might happen decades, centuries and thousands into the future due to a warming climate caused by greenhouse gas emissions.

“Our findings suggest that significant changes may already be underway,” Dr Crichton continued.

“Unless we quickly reduce greenhouse gas emissions, this could lead to the disappearance or extinction of much of the life of the Twilight Zone within 150 years, with effects extending for thousands of years after that.

“Even a low-emissions future could have a significant impact, but it would be much less severe than in medium- and high-emissions scenarios.

“Our study is the first step to discovering how vulnerable this ocean habitat is to climate warming.”

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.

For context, the global carbon budget (led by the University of Exeter) has estimated total global carbon dioxide emissions at 40.6 billion tons in 2022 alone.

Emissions were close to 40 billion tons per year from 2010-22, so most of the carbon dioxide (about 500 billion tons) for the study’s “low” scenario was already emitted.

At the current rate, it will hit the “medium” scenario 50 years from now, and it will reach the “high” scenario in just over a century.

Dr Jamie Wilson, from the University of Liverpool, said: “The aurora zone plays an important role in the ocean carbon cycle because most of the carbon dioxide absorbed by phytoplankton ends up as their remnants sink from the ocean surface.

“One of the challenges of predicting how this movement of carbon might change in the future is that there are many processes that need to be separated in the modern ocean.

“By looking at the aurora zone of past warm periods, we can identify the most important processes and use those processes to predict the future.

“We found that this natural carbon cycle has already changed and may be disturbed long into the future.”

To increase our knowledge of the ocean twilight zone, a United Nations program (Jetzone) has been formed. It states: “It is poorly understood from almost any perspective. However, it probably contains the world’s largest and least exploited fish stock and recycles nearly 80% of the organic matter that sinks into produced surface water.”

The new study was funded by the Natural Environment Research Council and includes researchers from the Universities of Exeter, Liverpool, California Riverside, Bremen, Cardiff and University College London.