Twilight zone at risk from climate change

Science

Lauren Walsh, Administrative Executive, dives into the impacts of climate change on life in the oceans' twilight zone

THE mesopelagic zone of the ocean describes the area 2001000m beneath the sea’s surface. Home to an abundance of life, it is one of the earth’s largest habitats and makes up one quarter of the ocean’s volume. Due to the depth, there is very little sunlight, hence its usual denomination of ‘the twilight zone’.

fellow at the University of Exeter.

However, very little is known about this part of the ocean, and as such, the Natural Environment Research Council recently funded research into the impact of climate change on the twilight zone. The study, titled ‘What the geological past can tell us about the future of the ocean’s twilight zone’, was led by Dr Katherine Crichton, who is now a postdoctoral research

The study used evidence from preserved microscopic shells in ocean sediments, as their chemical and isotopic composition can reveal valuable information about the life environment of the twilight zone in the past. Two time periods were focused on: the early Eocene (about 50 million years ago) and the mid Miocene (about 15 million years ago). The first as an example of oceans much warmer than ours, and the second as an example of the temperature our oceans may be in the near future, if greenhouse gas emissions continue as they are. Earth System Model simulations were also used to predict what is happening now and what may happen in the future in the twilight zone, with three scenarios: a low emission-, medium emission-, and high emission-future.

It was found that life in the twilight zone could decrease by 20-40 per cent by 2100 as a result of climate change. This is because the warmer the ocean is, the faster organic matter is degraded, meaning that less food reaches the lower depths (like those of the twilight zone). Therefore, in these warmer periods, organic life was less abundant and found closer to the surface. This in turn also affects the size of the store of carbon in the deep ocean in the shells of organisms, that are eventually broken down into seafloor sediment, as part of the process known as the Biological Carbon Pump, which sequesters carbon from the atmosphere to the ocean store. sions, this could lead to the disappearance or extinction of much twilight zone life within 150 years, with effects spanning millennia thereafter.”

Furthermore, the impacts of climate change on the twilight zone are hard to predict due to the complexity and fragility of the ocean ecosys- tem, how even in this day and age, our oceans are poorly understood. Regardless, decisive action must be taken to reduce our greenhouse gas emissions, if we want to protect the unique and valuable environment that is our oceans’ twilight zone.

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Unfortunately, according to the study, significant changes to the twilight zone are already underway, and Dr Crichton says, “Unless we rapidly reduce greenhouse gas emis-