Climate change could reduce life in the ocean's twilight zone by up to 40%, says new study

The study showed that organic matter in the warmer seas of the past was degraded more quickly by bacteria, meaning less food reached the twilight zone. Conversely, the rich variety of life in the twilight zone evolved in the last few million years as ocean waters cooled down and began to act like a refrigerator, preserving the food for longer and allowing life to thrive.

Combining past evidence with Earth System Model simulations, the researchers simulated what may be happening now and what could happen in the future in the twilight zone due to climate warming driven by greenhouse gas emissions. The findings suggest that significant changes may already be underway, and unless there is a rapid reduction in greenhouse gas emissions, the extinction of much twilight zone life could occur within 150 years. Although a low-emissions future may still have an impact, it would not be as severe as the medium and high-emissions scenarios, according to the study.

The study's three emissions scenarios are based on total carbon dioxide emissions after 2010, with "low" being 625 billion tonnes, "medium" being 2,500 billion tonnes, and "high" being 5,000 billion tonnes. To put things in perspective, the Global Carbon Budget predicted that in 2022 alone, carbon dioxide emissions will reach 40.6 billion tonnes. At the current rate, the "medium" scenario would be reached in 50 years, and the "high" scenario in just over a century.

Jamie Wilson, a researcher from the University of Liverpool, emphasised the crucial role that the twilight zone plays in the ocean's carbon cycle. According to Wilson, the majority of the carbon dioxide absorbed by phytoplankton eventually ends up in the twilight zone as their remains sink down from the ocean surface. However, predicting how this carbon movement will be altered in the future is a complex task due to the numerous processes that need to be disentangled in the current ocean. The study showed that the natural cycling of carbon is likely already changing and may continue to be disturbed long into the future.

The researchers emphasised that the ocean twilight zone remains poorly understood from almost any perspective, and more knowledge is needed to understand how vulnerable this ocean habitat may be to climate warming.

The United Nations established a programme, JETZON, to increase knowledge on the ocean twilight zone. The programme aims to exploit the fish stock in the twilight zone and recycle approximately 80 percent of the organic material that sinks out of the productive surface waters.

Researchers from the universities of Exeter, Liverpool, California Riverside, Bremen, Cardiff, and University College London participated in the project, which was funded by the Natural Environment Research Council.