In the past half-century, though, these beautiful, biodiverse structures have been put under pressure by human activity. About a quarter of all coral cover has died. The reefs that are in worst shape are those off the most crowded beaches. “People don’t leave enough time for their sun cream to soak in, so it gets in the water,” says one deckhand with Eo Wai’anae Tours, which organises boat trips off Oahu. More damage is caused by fertiliser-rich run-off from farms, leading to algal blooms which block light the corals need. Fishing near reefs cuts the number of herbivorous fish, allowing vegetation to grow out of control. Some fishing methods are particularly harmful: for example, blast fishermen in Colombia, Tanzania and elsewhere use dynamite to stun and kill fish without regard to the harm done to nearby reefs…In the South China Seaisland-building and fishing for giant clams are crushing some reefs beyond the possibility of recovery (seearticle)….

Tourism generated by the Great Barrier Reef is worth about $4.6 billion annually to nearby Queensland alone. Australian bigwigs bent over backwards last year to keep the UN from listing the reef, a World Heritage Site, as “in danger”. Estimates suggest that the economic value of Martinique and Saint Lucia’s corals comes to $50,000 per square km each year, thanks largely to tourism. But overdevelopment threatens the reefs the visitors come to gawp at. Sediment from construction clouds waters, burying corals and blocking the light they need. Hotels close to the shore may be convenient for tourists, but the process of building them can kill the reefs that snorkellers like to swim over…The three countries with the largest numbers of people who fish on reefs are all in the coral-triangle region: Indonesia, Papua New Guinea and the Philippines. In Indonesia and in the Philippines, up to 1m people’s livelihoods depend on reefs.

Averting a tragedy of the commons means agreeing which activities should be restricted and enforcing the rules. For coral reefs—and other biodiverse marine environments—the usual approach is to give ecologically sensitive areas special status under local or regional laws. In such “marine protected areas” (MPAs), activities that are deemed harmful, such as fishing, drilling and mining, can then be restricted or banned, with penalties for rule-breakers.

The Aichi targets, agreed in 2010 under the UN Convention on Biological Diversity, seek to reduce “anthropogenic pressures” on coral reefs to “maintain their integrity and function”. The aim is to have at least 17% of inland water and 10% of coastal and marine areas under conservation by 2020. Most countries have signed up. But the targets are far from being met. Less than 3% of the ocean’s surface is within an MPA.

The most urgent action is needed close to shore. The nearer humans are to reefs, the worse their effect on the fragile ecosystems. A global register of fishing vessels, long under discussion, would also help identify wrongdoers. And beefing up the UN law of the sea could inspire further action. Decades old, it has little to say about biodiversity.

But simply declaring an area protected does not make it so. In 2009 George Bush junior, then president of America, established three national marine monuments in the Pacific, including nearly 518,000 square km of coral islands and surrounding areas. Their remoteness makes it hard to stop vessels entering illegally; Hawaii’s coastguard is already stretched.

Satellites are sometimes used to police MPAs, but they pass over infrequently. In the future, sailing robots could play a larger role. America’s National Oceanic and Atmospheric Administration (NOAA) has been working with a private firm, Saildrone, on hardy models equipped with carbon-fibre fins. They cost less than $500,000 each and can roam remote ocean regions for months, making them far cheaper than manned boats.

Such drones could photograph rogue fishing vessels, obtaining hard-to-gather evidence for any criminal proceedings. And they could carry out other useful work at the same time, such as monitoring ocean temperature and acidity or tracking tagged members of endangered species. Saildrone plans to provide its robots as a service, so that universities and other cash-strapped organisations do not have to buy one outright…

Even if the right policies are adopted to keep corals healthy in the immediate future, longer-term threats loom. Neither oceanic warming nor acidification can be kept out by an MPA. And both may be happening too fast for corals to adapt, especially as recent global climate deals will not slow them much. Back slaps and handshakes accompanied the inclusion of an aim to limit global warming to just 1.5°C above pre-industrial levels in the Paris Agreement last year. But only an incorrigible optimist would bet on that aim being achieved.

So researchers are turning their attention to ways to help corals cope. Their global diversity, scientists hope, may hold the key. The same coral will grow differently under different conditions: corals of the western Pacific near Indonesia, for example, can withstand higher temperatures than the same species in the eastern Pacific near Hawaii….The characteristics that help some reefs survive unusual conditions could allow others to endure climate change. But tough corals from one place cannot simply be transplanted to another. So a team at the Hawaii Institute of Marine Biology is in the early stages of engineering reef ecosystems, with $4m from the Paul G. Allen Foundation, a charity set up by Bill Gates’s former business partner.

Organisms respond to environmental changes through both genetic processes (adaptation) and non-genetic ones (acclimatisation). With corals, the nature of their symbiotic relationships can also alter. So selectively breeding and conditioning them, and investigating whether certain types of algae confer resistance to heat or acidity, could create hardier varieties faster than they would develop naturally.

These could then be used to repopulate ravaged reefs—once more is known about how and where to transplant them. “We’re assisting evolution,” explains Ruth Gates, who leads the research.

Marine conservation: Rejuvenating reefs, Economist, Feb. 13, 2016, at 57