4 WILL THE CLIMATE CRISIS CHANGE THE DEEP SEA?
Climate change is already reaching down into the deep ocean. A 2020 study confirmed the average global temperature between the surface and 2,000 metres has been rising year on year. The increase may seem small — in 2019, it was 0.075°C above the average between 1981 and 2010 — but due to the volume of water, the heat absorbed is equivalent to the energy of 3.6 billion atomic bombs exploding.
And there are greater changes on the way. By the end of the century, it’s predicted temperatures in the twilight and midnight zones, down to 1,000 and 4,000 metres respectively, will rise to 8°C. This will come as a hot shock for deep-sea organisms that are adapted to around 4°C.
Other climate impacts will accompany the rising temperatures. Ocean acidification is expected to hit hardest between 200 and 3,000 metres down, where deep-sea corals will find it increasingly difficult to make their exoskeletons. Warming seawater will lose its ability to hold oxygen. In the northeast Pacific, off Vancouver Island, oxygen levels down to 3,000 metres have already declined by 15 per cent over the last 60 years.
Human impacts are likely to reduce the ability of the deep to buffer against rising carbon concentrations and temperature.
A recent study estimates that trawling disrupts seabed carbon stores and causes emissions similar to the aviation industry. There are also plans to fish the open waters of the twilight zone for lanternfish, thought to be the world’s most abundant vertebrates. Each night, huge shoals of the fish migrate from the twilight zone to feed in the shallows, before fleeing back to the deep at dawn, bringing masses of carbon with them. Hunting these fish in large numbers could cut off a critical pathway of carbon into deeper waters.
Plans to begin mining the abyss likewise come with worrying predictions. Mining could disturb seabed carbon stores, potentially on a larger scale than trawling. Contaminated wastewater extracted from the mined slurry could be disposed of by pumping it into the twilight zone, where it would choke gelatinous midwater animals such as jellyfish and siphonophores, all of which are important in the drawdown of carbon into the deep.
A great unanswered mystery is whether seabed mining would help solve the climate crisis by providing metals to make green technologies like electric car batteries, or make the situation a great deal worse.