The year is 2050, and the few remaining coral reefs are thriving. The tourists who flock to admire their vibrant seascapes and their clouds of multicoloured fish need no boat or dive gear, however.

The world’s only surviving coral ecosystems are contained in enormous aquaria where conditions can be carefully regulated. Out in the ocean itself, a collision of unconstrained human impacts have reduced reefs to grey rubble.

This is the nightmare scenario envisioned by marine scientists such as Alex Rogers, of the Institute of Zoology, London, who warns that unless drastic changes are made, the only way that the rainforests of the sea will survive is through radical intervention such as transplanting them to a Noah’s Ark of enormous seawater tanks.

“We’ve got at most ten years to do something really significant to turn things around,” said Dr Rogers, who was appearing at the Times Science Festival in Cheltenham. “We have to start contemplating such huge engineering projects to preserve reefs. Of course, they would only save a fraction of the species, but it would at least be something.”

Despite the oceans’s immensity — 71 per cent of the Earth’s surface with an average depth of almost 4km (2½m) — there are indications that it is approaching its tipping point. For reefs, warming waters and acidification are closing in like a pair of jaws that threaten to make them the first global ecosystem to disappear. Fast reductions in carbon dioxide emissions are crucial for the future of reefs because of a time lag between release into the atmosphere and absorption by the ocean.

Even if a total halt on emissions were accomplished immediately, seawater would continue to absorb carbon dioxide for years to come. Overcoming this carbon dioxide “hump” is going to be a critical challenge for coral reefs, Dr Rogers said.

While reefs are likely to be the first to go, they may not be the only marine ecosystems to crash. Whole seas have been known to lose their biological structure as a result of compounding pressures.

“As a result of fishing, pollution and invasive species, the Black Sea’s ecosystem collapsed,” Dr Rogers said. “Large predatory fish were gone by the Seventies. Finishing off the smaller ones took another 20 years. The gelatinous zooplankton (comb jellyfish, in this instance) came during that time.”

Those jellyfish feasted on fish eggs, and soon there was little else but jellyfish and plankton to be found in the Black Sea.

Extrapolating from the 400,000 square mile Black Sea to the global ocean is fraught with uncertainty, but the wide-scale impacts that have already been felt cause scientists concern, not just for life but for the ecosystem services that the ocean provides, such as regulating the oxygen, nitrogen and phophorus cycles. Deprived of these systems, the planet would be uninhabitable for humans.

“What we’ve done over the last two centuries is to eliminate a very large amount of the filter-feeding animals that used to live on the seabed,” said Professor Callum Roberts, of the University of York.

“If you lose 95 per cent of filter-feeding organisms on the seabed you lose 95 per cent of filter-feeding capacity of those organisms.

“Nobody says that wild oysters are at risk of extinction. But their functional role in the marine environment has essentially disappeared because of the reduction in population sizes. That’s one of the things we need to focus on in the future. If we want to have clean water, high levels of carbon uptake from the atmosphere, then we need ecosystems intact.”

While the price of business as usual carries a harsh penalty, scientists such as Dr Roberts point out that the rewards for allowing the seas to regenerate could be enormous.

“What the seas produce today is just a fraction of what they are capable of,” he said. With proper management, fishermen could catch many times as much as they do today for only a fraction of the effort. A recovery would have profound implications not just for food security but for tourism, angling and recreation.

Foremost among the tools that could return the seas to health are networks of Marine Protected Areas, which promise benefits for both biodiversity and productivity. The challenge faced is in changing the entrenched views of senior policy-makers.

“The interest in ecosystem-based management has risen out of the failings of species-based management of marine life. The scene is being set for a big change in how the marine environment is being managed,” Dr Roberts said.

“A lot is happening compared to 18 years ago. It’s not happening fast enough, but it’s happening faster than many people at a political level are comfortable with. So that’s positive.”

If an aquatic Noah’s Ark becomes the last chance for coral reefs, their troubles will not be over: lack of genetic mixing makes captive corals vulnerable to disease, for example. Perhaps more alarming still, they will once again rely on humans for the stability of their environment.

“It’s not that difficult to get corals to grow in captivity,” said Ash Sharpe, formerly of London Zoo’s aquarium and now a coral farmer. “The problem is maintaining genetic diversity. That and human error — if one person makes a mistake with the maintenance regimes, everything’s gone.”

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