A new study warns that life in the oceans’ “twilight zone” could be reduced by 20-40% by the end of the century due to climate change. The twilight zone is a region between 200 and 1,000 metres deep, where little light penetrates, but billions of tonnes of organic matter and a wide variety of organisms are found. The study, published in the journal Nature Communications, was conducted by researchers from universities in the UK and combined evidence on past warm periods with Earth System Model simulations.
The researchers found that in warm seas of the past, organic matter sunk down from the ocean surface and was degraded faster by bacteria, resulting in fewer organisms living in the twilight zone. The study’s three emissions scenarios are based on total carbon dioxide emissions after 2010, with the “low” scenario already surpassed. The “medium” scenario is expected to be reached in 50 years, and the “high” in just over a century if emissions continue at the current rate.
The study warns that if greenhouse gas emissions are not rapidly reduced, the disappearance or extinction of much twilight zone life could occur within 150 years, with effects lasting for millennia. The twilight zone plays a crucial role in the ocean’s carbon cycle because most of the carbon dioxide taken up by phytoplankton ends up there as their remains sink down from the surface ocean. The study was led by Professor Paul Pearson of Cardiff University, who stated that past experience could help understand the future of the ocean twilight zone, although relatively little is known about it.
Katherine Crichton, from the University of Exeter and lead author of the study, added that the rich variety of twilight zone life evolved in the last few million years when ocean waters had cooled enough to act like a fridge, preserving food for longer and improving conditions for life to thrive. The researchers urge the rapid reduction of greenhouse gas emissions to avoid severe depletion of life in the twilight zone.