Delegates at COP21 were today warned that in addition to the warming of the atmosphere the parallel trend toward the acidification of the oceans offers similarly profound challenges.

The progressive acidification of the seas is being caused by the same changes that are causing global warming. Absorption of carbon dioxide into the marine environment makes it more acidic.

This fundamental change in marine chemistry is causing a number of ecological impacts but is especially important in relation to the effect it has on a range of shell-forming organisms.

For example, the shells of pteropods – marine snails that live in the Southern Ocean – are being dissolved as waters acidify, according to a new study published last week in Nature Geoscience. These tiny animals are an important food species for fish and birds so their loss or diminished abundance could have far reaching effects.

There are precedents – but you have to go back a long way!

The massive injection of CO₂ that has followed the industrial age has already resulted in conditions that have probably not been not experienced on Earth for million of years, and perhaps not since the extinction of the dinosaurs some 65 million years ago.

Alexander MacDonald, Director at the US National Oceanic and Atmospheric Administration’s Earth System Laboratory in Boulder Colorado, has research interests that embrace ocean acidification – and he says we should be very worried about what is going on: “There are presidents for acidification in the paleo-record, but you have to go back a long way.”

He points out how nearly all of the six major mass extinctions recorded in the fossil record were linked with elevated carbon dioxide and more acidic oceans, but he points out how that “All took place more slowly that what is happening now. We’re increasing the carbon dioxide extremely fast, and its quite dangerous.”

The main biological effect comes down to the ability of organisms, from corals to oysters and from sea-butterflies to shell-forming phytoplankton, to make the exoskeletons they need to complete their life cycles.

When carbon dioxide dissolves in seawater it combines with water to form carbonic acid (H₂CO₃). Carbonic acid is composed of weak chemical bonds, so it eventually breaks down, releasing hydrogen ions (H+) into the oceans, together with carbonate (CO₃=) and the far more abundant bicarbonate (HCO₃-).

Marine creatures from corals to snails and plankton make their shells from dissolved calcium and bicarbonate ions – a process which actually emits CO₂ back into the seawater.

MacDonald points out that biology is more difficult to predict than chemistry, but says: “The worry is that we’ll affect the lower part of the food chain as we alter the level of calcium carbonate available for the small ocean critters that make shells, such as phytoplantkton and so on, and that this will transmit up the food chain and affect fisheries significantly.”

There is also some risk that the acidification will ultimately affect the ability of the oceans to absorb carbon dioxide.

One third of our CO2 emissions have ended up in the oceans

Since the beginning of the industrial revolution about a third of the carbon dioxide we’ve emitted has been absorbed the oceans and much of that has been taken into the deep ocean, having been incorporated in the biomass of marine organisms, says MacDonald:

“There’s something called the biological pump and some of the shells that get created sink down and that’s how we make limestone and chalk. If the upper ocean gets impoverished of these species then the whole ecosystem will change.”

In fact, a major change has recently been detected in the North Atlantic, where coccolithophores – carbonate shell-forming phytoplankton that are abundantly represented in chalk – have increased tenfold in abundance over the last 45 years. But as Science News notes:

“This finding was diametrically opposed to what scientists had expected since coccolithophores make their plates out of calcium carbonate, which is becoming more difficult as the ocean becomes more acidic and pH is reduced.”

And while this shows that coccolithopores are unexpectedly thriving under higher CO₂, it’s not such good news as it’s actually reducing the oceans’ ability to soak up CO₂ from the atmosphere. It’s also bad news for diatoms – silica-shelled phytoplankton which are an important food for fish and efficient conveyors of ocean carbon into the depths – as their abundance has diminished.

Amid so many unknowns, known and unknown, best play it safe

What this illustrates above all is the unpredictable and poorly studied workings of ocean ecology on which major climatic outcomes depend. Even simple questions like ‘What eats coccolithophores?’ – the ones whose abundance in the North Atlantict just increased tenfold – remain unanswered.

So there is grave concern here at COP21 as to the adequacy of the commitments on the table to achieve a maximum temperature increase of two degrees. Alarm is also being expressed by experts as to the implications for the seas.

“We really have to put the brakes on carbon dioxide and very fast. These effects are already becoming very large and there are huge uncertainties as to how this will affect among other things food production. To bring this under control we really need to be aggressive”, says MacDonald.

Some creatures will likely survive the changes now under way, including some of the Earth’s oldest multicellular organisms. They have survived multiple mass extinctions, but whether their rise in an ocean in decline will be good for us and the rest of life is a different matter.

MacDonald puts it like this: “An ocean full of jellyfish is not going to be an ocean that’s very friendly to humans.”

Tony Juniper is a campaigner, writer, sustainability advisor and leading British environmentalist who is attending COP21 on behalf of The Ecologist. For more than 25 years he has worked for change toward a more sustainable society at local, national and international levels. His website is at tonyjuniper.com.

Additional reporting by The Ecologist.