OUR actions today will change the world’s oceans for thousands of years. That is the conclusion of a study simulating a little-discussed consequence of climate change: it could choke entire ecosystems by cutting oxygen levels in the ocean. In the most extreme scenarios, with the planet warming by almost 10°C, the oceans could be starved of oxygen for 8000 years.
Oxygen-poor waters have always existed in the sea, but in the last 50 years these “oxygen minimum zones” have grown. Climate change is one cause: the sea is warming, and warmer water can dissolve less of the gas.
Marine life is sensitive to these anoxic conditions, so a fall in oxygen of just a few per cent is enough to put enormous stress on ecosystems. The 360 million years ago, one of the biggest die-offs ever, unfolded largely in the oceans. It wiped out a fifth of all families in the tree of life, and anoxia was a key contributing factor.
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Previous studies suggested the oceans will lose 7 per cent of their oxygen by 2100. But many effects of climate change, like rising seas, play out over millennia, and the same is true of deoxygenation.
Future Earth
and at the University of Bern, Switzerland, simulated changes in ocean oxygen levels between now and the year 10,000. They looked at temperature stabilising at four different levels above pre- industrial conditions: 1.5°C (the key target set by the Paris Agreement), 1.9°C, 3.3°C and 9.2°C.
Average oxygen levels fell 6 per cent when warming met the Paris target, whereas 9°C of warming led to a 40 per cent fall ().
The study is awaiting peer review, but the implications are stark.
“What is startling is the timescale over which they predict such dramatic reductions in oxygen,” says at the University of Plymouth, UK. “They are considerably shorter than anoxic events in the fossil record.” The rapidity may make it even harder for animals to cope.
Vulnerable spots
The findings agree with those of a 2009 simulation that concluded oxygen levels could fall 40 per cent (). The new study uses a more detailed model, though, allowing the team to look at different ecosystems.
In the 1.5°C scenario, the biggest oxygen falls were in the depths, home to a profusion of life that we are only now discovering, from anglerfish to yeti crabs. This is now at risk.
Low oxygen is particularly hard on fast-swimming animals with correspondingly high metabolic rates, like tuna and sharks, says at the University of Oxford. “We know that declining oxygen levels in the tropical Atlantic have already resulted in compression of habitat for fast- swimming large pelagic fish, such as marlin,” he says.
The Bay of Bengal is particularly vulnerable, says Rogers. “That entire ocean basin is on a knife- edge, in terms of flipping over to a completely anoxic state,” he says. “That’s going to have a major impact on the marine fauna.”
Fixing the seas
Ultimately, oxygen levels will recover – but not necessarily within the simulated time frame. In all the scenarios, Earth stopped warming in 2300, yet average oxygen fell for another 1000 years before bottoming out. Even then the dissolved gas still had to spread before things stabilised. “It takes several thousand years for the ocean to mix once through,” says Battaglia.
With 1.5°C or 1.9°C of warming, oxygen levels eventually began to rise. But in the hotter scenarios, oceans remained depleted of oxygen by about 8 per cent.
This emphasises “the extreme sensitivity of oxygen levels to the climate emissions pathway we actually follow”, says Rogers. While some impacts can’t be avoided, the worst effects can.
“I think that’s a really critical message that has to go out to the people that have the power to change these things,” he says.
This article appears in print under the headline “Our oceans are set to suffocate”
