The oceans will go on warming and rising for five centuries. Some creatures of the deep will have less room to breathe.

LONDON, 22 April, 2021 − Even if humans stopped all use of fossil fuels immediately, and drastically reduced greenhouse gas emissions, the oceans would go on warming. And as the waters warmed, their burden of dissolved oxygen would continue to dwindle, stifling many creatures of the deep.

This could continue for another 500 years, at the end of which oxygen loss in the seas would have multiplied fourfold. Since oxygen is vital to almost all complex life on Earth, and since the ocean − covering 70% of the globe and reaching in places to depths of almost 11 kilometres − provides by far the oldest and biggest breathing space for living things, that could commit many creatures to a slow, stifling end, according to a new study in the journal Nature Communications.

Both oxygen and carbon dioxide are soluble in seawater. The colder the water, the greater the capacity for dissolved gases, which ultimately is why polar seas are vastly and massively richer in life than tropical waters. But the latest study of the long-term consequences of carbon dioxide emissions offers a bleak picture for the future.

As the planet has warmed, so have the seas. As the greenhouse gas burden of the atmosphere has increased, so has the acidity of the ocean. And as the ocean waters have warmed, the levels of dissolved oxygen have fallen.

In the last 50 years, the ocean has on average lost 2% of its dissolved oxygen. That’s an average figure. In some parts of the water column, the loss has been much higher, directly as a consequence of global warming. And this loss will continue until around 2650.

“The deep ocean appears committed to turning into an as-yet-unrecognised area where the slogan ‘If you can’t breathe, nothing else matters’ will become reality for many centuries to come”

Andreas Oschlies of the Geomar Helmholtz Centre for Ocean Research in Kiel in Germany used a climate model of the Earth system to work out what would happen to the ocean in the long term if all carbon dioxide emissions stopped right now.

He says: “The results show that even in this extreme scenario, the oxygen depletion will continue for centuries, more than quadrupling the oxygen loss we have seen to date in the ocean.”

Most of this loss will be at depths of 2000 metres or more, partly because ocean circulation is becoming more sluggish in response to climate change. So the deepest parts of the ocean could lose more than a tenth of all the oxygen it once held before the launch of the Industrial Revolution and the accelerated use of coal, oil and gas to drive national economies. And that would be bad news for the creatures that swim and replicate at depth: some of them could face a decline of up to 25%.

And if nations could achieve the impossible and halt all emissions now, surface air temperatures would stabilise rapidly. But the oceans would go on absorbing the extra carbon dioxide already in the atmosphere. Between now and 2650, according to the calculations of Professor Oschlies, the ocean would go on absorbing another 720 billion tonnes of the gas. This is larger than all the CO2 the oceans have taken up till now: an estimated 634 billion tonnes.

Too little air

But the atmospheric heat the oceans will absorb in the next five centuries is likely to be three times the heat already absorbed up till now. This warmth alone − because warm water is less dense than cold water − will mean another 16cms of unavoidable sea level rise. And as the waters warm, the oxygen levels in that water will continue to diminish: by 2650 it will have fallen by 7.4% compared with oxygen levels a century or more ago. And this is more than three times the loss that has already happened.

Those sea creatures that had adapted over a million years to one set of oxygen levels are going to face a problem: there won’t be enough oxygen dissolved in the deep seas to support all of them. Some regions of the ocean will slowly become “dead zones”.

Oceanography is a costly science, and most of the ocean is unexplored: humans have mapped the surface and plundered the coastal waters but have yet to explore the depths in much detail over vast tracts of the planet’s largest living room.

There’s a lot more research to be done, before researchers can be sure of the ways in which human action is about to irrevocably change the submarine world. But the outlook so far is ominous.

Professor Oschlies warns: “The deep ocean appears committed to turning into an as-yet-unrecognised area where the slogan of the American Lung Association − ‘If you can’t breathe, nothing else matters’ − will become reality for many centuries to come.” − Climate News Network