Put your hand in the sea off Cornwall in February and the cold is the first thing you notice. The second thing is invisible: those waters are slightly less alkaline than they were when your grandparents were children. That shift has a name — ocean acidification — and it matters for coral reefs everywhere from the Great Barrier Reef to the cold-water reefs sitting in UK waters.

The chemistry, in plain terms

When carbon dioxide from burning coal, oil and gas dissolves in seawater, it does not simply sit there. It reacts with water to form carbonic acid, which then breaks apart and releases hydrogen ions. More hydrogen ions means a lower pH — the water becomes less alkaline.

The pH scale is logarithmic, so small-sounding changes are bigger than they look. Since the industrial revolution, average surface ocean pH has fallen by roughly 0.1 of a unit — which translates to something like a 30 per cent rise in hydrogen ion concentration. That is a fast change by geological standards, and the ocean has absorbed a large share of the extra carbon dioxide we have produced.

There is a second, subtler effect. The extra hydrogen ions mop up carbonate ions, the raw material that corals, molluscs and coralline algae use to build skeletons and shells. Less carbonate in the water means building a skeleton costs more energy.

Why aragonite matters most

Corals build their skeletons from a particular form of calcium carbonate called aragonite, which dissolves more readily than the calcite used by some other sea creatures. Scientists therefore watch something called the aragonite saturation state: the higher it is, the easier calcification becomes. Cold water holds more dissolved carbon dioxide and is naturally lower in saturation, which means colder seas — including the seas around Scotland and the South West — can approach difficult thresholds earlier than warm tropical water does.

What acidification does to a reef

None of this kills a coral outright in the way a heatwave does. It works more slowly, shaving away the margins.

  • Slower growth. Corals put more energy into building skeleton, leaving less for repair, feeding and reproduction.
  • Weaker structures. Skeletons become more porous and brittle, so branches snap more easily in storms.
  • Slower recovery. After a bleaching event, a reef needs to regrow. Acidification makes that regrowth harder and slower.
  • Boring and grazing. Boring sponges, worms and urchins that erode reef rock can gain the upper hand, so the reef starts to crumble faster than it can rebuild.
  • Reproduction and settlement. Larval corals can be affected at the stage when they need to find a patch of bare rock and settle — a bottleneck for reef recovery.
  • Shifting the balance. On some reefs, fleshy seaweeds cope better than hard corals, pushing a reef from coral-dominated to algae-dominated.

Lose the structure and you lose more than the corals. Reefs are natural breakwaters, nurseries for fish, and the foundation of tourism and food security in many coastal countries. Although they cover a tiny fraction of the seabed, they shelter a remarkable share of marine species.

A problem that stacks with warming

Acidification is not happening on its own. It arrives alongside marine heatwaves, which cause coral bleaching, and alongside pollution, overfishing and coastal development. Each stressor makes the others worse. A reef that is already nutrient-loaded from sewage or agricultural run-off, or stripped of the grazing fish that keep seaweed in check, has less resilience to spare when the chemistry shifts as well.

That is actually good news for management, because it means some of the levers are local and within reach — not everything depends on global emissions trajectories.

Why this matters in Britain

Britain does not have the warm, shallow, reef-building corals of the Caribbean. It does have cold-water coral reefs, built by species such as Desmophyllum pertusum — long known as Lophelia pertusa — growing slowly in deep, dark water off Scotland and the west of Ireland. The Darwin Mounds, off north-west Scotland, were among the first offshore areas in the UK to be given protection because of the reefs and mounds found there.

We also have maerl beds: unattached coralline algae that form brittle, branching structures on the seabed around Scotland, Cornwall and Wales. Maerl is a carbonate builder too, and it is slow to recover from damage. Add blue mussels, oysters and other shell-forming species, and the UK has plenty of marine life that depends on the same chemistry.

Then there are the UK Overseas Territories — the Cayman Islands, Turks and Caicos, the British Virgin Islands, Anguilla and Montserrat among them — which contain real tropical coral reefs and are directly exposed to acidification. What happens in Westminster and in the territories' own governments shapes those reefs too.

What UK policy can actually do

Ocean acidification is driven by atmospheric carbon dioxide, so domestic emissions reductions genuinely count. The UK's Climate Change Act, with its legally binding net zero target, is the backbone here. Beyond that, several practical strands matter:

  1. Keep funding long-term monitoring. You cannot manage what you do not measure. Sustained measurements of pH and carbonate chemistry in UK seas are hard to fund and easy to cut, yet they are the only way to spot trends.
  2. Make marine protected areas mean something. A boundary line on a map does not stop dredging, anchoring or nutrient pollution. Effective management of the sites we already have is worth more than designating new ones on paper.
  3. Clean up the water. Reducing sewage discharges and agricultural run-off lowers one of the stressors that combine with acidification.
  4. Protect blue carbon habitats. Seagrass, saltmarsh and maerl store carbon and support reef species. Restoring them is slow work with real returns.
  5. Push the ocean into climate diplomacy. Acidification belongs in international climate negotiations and in agreements on the high seas, where the UK has signed the High Seas Treaty and can press for ocean science and protection to be funded properly.
  6. Back the Overseas Territories. Reef monitoring, local capacity and the UK's Blue Belt Programme work all help reefs that are British by jurisdiction.

What you can do from here

Individual action will not lower ocean pH on its own, and nobody should pretend otherwise. But there are things that add up. Cut the carbon in your own life where you can, particularly flying and home heating. Choose seafood from well-managed fisheries and avoid coral souvenirs, which are taken from reefs and should never come home in a suitcase. If you dive or snorkel, keep fins, anchors and hands off the reef.

If you want to go further, volunteer with a local marine conservation group, join a shore survey or a citizen science scheme, and write to your MP about marine protected area enforcement and water quality — those two are genuinely within UK control. And if you visit a reef, pay the local conservation fee or entry charge where one exists; that money usually funds the wardens and monitoring that keep the site alive.

Reefs are slow builders, and the chemistry is shifting faster than they can adapt. That is precisely why the local, unglamorous work — clean water, protected places, measured data — is worth the effort.

Photo: paulbr75 / Pixabay