Climate Change Threatens Ocean’s Twilight Zone

The ocean’s twilight zone, a dimly lit region that lies between 200 and 1,000 meters below the surface, is home to a diverse array of marine life and a vital carbon sink. But a new study warns that climate change could dramatically reduce life in this zone by as much as 40% by the end of the century.

The study, led by researchers from the University of Exeter and published in Nature Communications, looked at two warm periods in Earth’s past, about 50 million and 15 million years ago, and examined records from preserved microscopic shells. They found that fewer organisms lived in the twilight zone during these periods, because bacteria degraded food more quickly in warmer water, meaning less of it reached the zone from the surface.

“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. Katherine Crichton, the lead author of the study.

The twilight zone is home to more fish than the rest of the ocean put together, as well as microbes, plankton and jellies. Some organisms spend their lives in its shadowy depths, while others travel to and from the surface. The zone also serves an important environmental function as a carbon sink, drawing planet-heating gas out of the atmosphere.

The researchers simulated what might be happening in the twilight zone now, and what could happen in future due to climate warming. They said their findings suggested that significant changes may already be underway.

“Our study is a first step to finding out how vulnerable this ocean habitat may be to climate warming,” said Dr. Crichton. “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.”

The study adds to the growing evidence of the impact of climate change on the ocean, which absorbs about a quarter of human-made carbon dioxide emissions and more than 90% of excess heat trapped by greenhouse gases. Previous research has shown that climate change is affecting ocean circulation, acidification, oxygen levels, sea level rise and marine biodiversity.

Reference：

• Climate change: 40% of ocean’s ‘twilight zone’ species at risk from global warming. BBC Newsround. 5/3/2023 2:58:00 PM¹
• Twilight zone at risk from climate change. Phys.org. 4/27/2023 9:00:00 AM²
• Climate change could wipe out life in the ocean’s ‘twilight zone’. New Scientist. 4/27/2023 8:00:00 AM³