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 Late Devonian extinction 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.
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.
Gianna Battaglia and Fortunat Joos 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