It is extremely difficult to divine what is happening below the water from what is visible at the surface. Ghost crab volcanoes rise from the sand beside a lagoon of duck-egg blue. The air is clear and still cool as the first sun touches the palm crowns. Noisy mynahs argue over the contents of a plastic bag stranded on a beach that is smoothed to perfection by the falling tide. Below the glassy surface, stingrays pass like shadows with movements so subtle they seem to glide without propulsion. A little offshore, sudden splashes and arcing cascades of silvered fish give away the primordial interplay of hunters and those hunted.

This coral reef in the Maldives lies at the heart of the Indian Ocean, where the fight for life goes on daily, as it has for countless millennia. But a new struggle is under way against an invisible force that has not been felt for millions of years, upending the relationship that built these reefs, a partnership first formed during the age of the dinosaurs between coral and a microbial seaweed. These seaweeds, called zooxanthellae, somehow evolved to live within the coral animal’s tissues. As plants, they gifted corals with abundant food produced by photosynthesis, enabling the corals to grow more rapidly than their predecessors, whose only source of food was passing plankton. In return, living deep within coral tissues cupped inside hard limestone skeletons, zooxanthellae gained protection and used exhaled carbon dioxide to make food. Photosynthetic calories fortified the corals as protein supplements do bodybuilders. Thus endowed, corals could lay down limestone fast enough to outpace the forces of destruction: rasping grazers, burrowing worms, snails and sponges, chemical dissolution and storm waves.

Over tens of thousands of years, corals built structures that criss-cross thousands of kilometres of ocean and descend hundreds of metres into the abyss. It is hard to reconcile the immense geological antiquity of the coral-seaweed partnership with its present susceptibility to disruption. Corals love warmth, basking in tropical seas that rarely dip below swimming-pool temperatures, but equally, they wilt under unusual heat. Although we rarely think of them this way, the oceans are afflicted by heatwaves too, and, just as on land, the frequency and duration of these are on the rise due to global warming. When oceanic temperatures increase just a degree or two above the typical maximum for a region, and stay there for a month or more, the relationship between coral animal and internal zooxanthellae turns nasty, swinging from benefit to cost. In this fevered state, for reasons that are still unclear, the corals must rid themselves of the zooxanthellae quickly or they will die. Because the seaweeds produce most of the colour in coral, when they are expelled the corals turn a deathly bone white in a phenomenon known as mass bleaching. A bleached coral is a starving coral, and since corals store little fat, unless temperatures fall quickly and they can regain more zooxanthellae, they perish.

The Maldives is a nation wholly dependent on coral. It consists of twenty-six atolls built upwards by coral growth over thousands of years as sea levels rose after the last ice age. The reefs are topped by 1,100 islands created from dead coral fragments and sand swept up by Indian Ocean waves and bound together by shrubs and palm trees. From space, the country appears as a double string of bright emerald rings that runs north to south across a thousand kilometres of dark sea. Zoom in closer and, inside the giant circles of reef that make up the main atolls, dozens of ring reefs rise from the bottoms of pale lagoons. This is one of the major coral provinces of the world, comparable to Australia’s Great Barrier Reef in size and complexity. The Maldives’ profusion of reefs, developed over a period that we know from the geological record, has been one of the most prolific eras of reef construction in planetary history. But in 1998, tragedy struck.

In December 1997, far away from the Maldives, an El Niño – a large-scale disturbance of oceanic and atmospheric circulation – began to spread a pool of warm water across the eastern Pacific. First it scorched the Galapagos Islands, then in early 1998 it spread like a hot tongue along the equator, wrapping around Australia and Indonesia before slipping into the Indian Ocean. By May the hot pool had crossed to the coast of East Africa and entered the southern Red Sea. As the heat engulfed reefs along its journey, the corals bleached before dying en masse. Indian Ocean reefs were devastated, losing between 70 and 95 per cent of their coral. In the Maldives, living coral plunged from covering between half and two-thirds of the reef to less than a twentieth.

With hindsight, we realise that this global mass mortality of coral was an early example of the immense planetary upheaval that is visible everywhere today. The world is changing in ways beyond experience. Places where benign climates have nurtured civilisations into life now see wildfire, drought or tempest in their midst; seas that have lapped the same shores since the end of the last ice age are rising again as glaciers are burned off mountainsides or surge into the ocean, unmoored from their rocky seabed anchors by probing warm water. There is a febrile atmosphere about these times, as the earth’s thermostat drifts upwards. Each broken record – each new disaster – is like a gasping whitecap on a steepening wave, the harbinger of a coming crash.

Climate sceptics (and there are still a few weaving fairy tales to explain away the implacable mountain of evidence) love to remind us that the planet has often changed throughout its history. During the early Eocene epoch, 50 million years ago, the climate was over 10°C hotter than now, there was no ice at the poles and the sea above the Arctic Circle was an improbable 20°C. The sceptics’ argument implies that today’s changing climate is somehow less serious because changes have occurred previously, but there are two crucial differences between past and present. The first is that changes today are happening faster than previously. Four of the past five mass extinction events were marked by a tremendous release of greenhouse gases leading to global warming and ocean acidification (from carbon dioxide dissolved into the sea). The mother of mass extinctions was the end-Permian collapse, brought on by vast reservoirs of lava bubbling up through today’s Siberia, vaporising coal measures and carbonate rocks into carbon emissions at a rate of a fifth to a third of a gigaton per year. Today’s emissions blow into the atmosphere thirty to forty times more rapidly.

The second vital difference between past and present is us. Past changes had natural causes, like volcanoes or orbital wiggles, but present warming is unequivocally our fault: you can’t release aeons of solar energy stored in fossil fuels without disruption. But the heat produced by these fuels is only a tiny part of the warming; it is the greenhouse effect, by which released carbon dioxide and gases like methane trap incoming solar energy, that makes the greatest difference. In the time that it takes you to read this sentence, the world will have trapped additional heat that is equivalent to the explosion of thirty Hiroshima-sized atomic bombs. Read that again, it wasn’t a misprint. By the time you have, the total will be sixty atomic bombs.

I first began to take my master’s students to the Maldives to experience its marvellous coral reefs in 2014. Sixteen years had passed since the El Niño warming had shrivelled its corals and their renewal was nearly complete. Just as a tree which falls in the forest can offer opportunities to others, space freed up on a reef is soon colonised by seaweeds and a riot of invertebrate life, including new corals. Coral growth is slow, typically ranging from about a centimetre a year for colonies with rounded stony shapes to twenty centimetres for the most vigorous arborescent species. Although visible scars remained, the wounds had more or less healed. Corals tumbled into the depths down slopes half obscured by moving clouds of fish. Table corals spread seaward from the reef front like stepped ledges, around which schools of snapper fish flowed in braided streams, their bodies lined with gold, dark eyes circled in glowing daffodil yellow. Such resilience was uplifting. I began to believe again that coral reefs might outlive today’s relentless human destruction.

Then in 2015, history repeated itself. A giant El Niño disturbance erupted in the eastern Pacific and spread towards Australia. By March 2016, the northern Great Barrier Reef had lost 60 per cent of its coral. By May, corals throughout the Maldives had bleached and, by July, 50 to 70 per cent were dead. One particularly exquisite reef near the island my students and I use as a base, a showpiece gem in a richly decorated crown, lost 99 per cent of its coral.

Three years on from this catastrophe, I am back with another class. We slip underwater with our scuba tanks and glide gently down to the reef. It is beginning to look healthier, less damaged. Fish that depend on coral for food or shelter suffered greatly when the corals died, but are coming back. Tiny Chromis, newly arrived from their drifting planktonic larval stage, fill the water like emeralds, their scales glinting and sparkling above their coral shelters. Black-and-white humbug fish, neat and rounded like the boiled sweets after which they are named, retreat among the branches of a coral head at our approach, then blossom outward again after we pass. Newly settled corals speckle the bottom, like young plants pushing up from recently thawed soil.

But despite the exuberant colours, the play of light and wondrous forms, I see the absences: animals that should be there but are not. These ghosts haunt me as I pass by towering columns that should be half-living coral but instead are rock spattered by sponges and tufted with hydroids or weed. Within their clefts, giant groupers and spangled emperor fish should be hiding. The occasional encounter with a hulking fish, round-bellied and scarred by age, only serves to remind that there should be more. Most of my students are fresh to the scene and revel in the immediacy of the experience. To them, the blanks are invisible.

I also see things that should not be there. The sponge that covers the bottom like fungus is a pretty turquoise patch to my students. To them, the matted algae that stain the seabed rust brown shiver in the current like waterweed in a summer river. For me, they are a cloak that chokes off recovery, snuffing out tiny corals before their lives have really begun.

The difference in our perceptions illustrates a phenomenon called ‘shifting baseline syndrome’. Shifting environmental baselines are intergenerational changes in how we perceive our world. Each generation sets its mental benchmark of normality by how the world looked when first encountered, often in youth, and sees change relative to this. Younger generations accept as normal a world that seems tainted and degraded to older people.

Shifting baselines weave through many aspects of life, usually unnoticed, since that is their power. In 1871, John Ruskin, the English critic and artist, railed in ‘The White-Thorn Blossom’ against what he saw as the desecration of a small piece of heaven in the Derbyshire hills by a railway viaduct:

There was a rocky valley between Buxton and Bakewell, once upon a time, divine as the Vale of Tempe; you might have seen the gods there morning and evening – Apollo and all the sweet Muses of the Light – walking in fair procession on the lawns of it, and to and fro among the pinnacles of its crags. You cared neither for gods nor grass, but for cash (which you did not know the way to get). You thought you could get it by what the Times calls ‘Railroad Enterprise’. You enterprised a railroad through the valley, you blasted its rocks away, heaped thousands of tons of shale into its lovely stream. The valley is gone, and the gods with it; and now, every fool in Buxton can be at Bakewell in half-an-hour, and every fool in Bakewell at Buxton; which you think a lucrative process of exchange, you Fools everywhere!

The twist in this tale is not Ruskin’s rage, which many of us feel daily at the destruction by commerce of natural wonders and beauty, but what happened a century later. By the 1960s, the railway had become uneconomical and the line was closed. Instead of taking swift advantage to remove the blight, the Headstone Viaduct was Grade II listed and preserved for its grandeur. Looking at photographs of this softly wooded valley, spanned by five elegant arches of moss-covered stone, I find it hard to agree with Ruskin: my baseline was set in a world where viaducts were admired, not castigated.

Shifting baselines have a darker side. The recent resurgence of right-wing ideology has been blamed on many things, with one view being that immigration and globalisation have placed immense strain on feelings of cultural and national identity, leading to a backlash. Another is that growing inequality breeds resentment and despair that usher the dispossessed into the arms of populist leaders. Such leaders are again spouting the fascist and authoritarian tropes used by Stalin, Mussolini and Hitler. Their words were taboo and almost unspoken by generations of people who lived through the horrors of two world wars or grew up in their aftermath, but younger people today are falling for the rhetoric in part because their personal experiences of peace and stability tell them little, or nothing, of its danger.

These are extraordinary times. The Renaissance heralded in a resurgence of art, literature and technology, expressions of the period’s renewed curiosity about how the world worked and our place in the universe. Not even the greatest minds of the day, however, predicted that within a few centuries we would transition from being at the mercy of nature to reshaping it. That headlong growth in understanding and influence – that fevered rebirth of culture between the Middle Ages and modern times – is itself a wonder of nature, but it far outstrips growth in our common sense. We know what we are doing to the world, we know how to stop the harm, but again and again we fall short when it comes to action. Like a teenager, our capacity for calamity has grown faster than our self-restraint, a mismatch that leaves us dangerously exposed.

The question of the age is whether we can adapt from fulfilling our own selfish interests to fulfilling the self-interest of humanity before it is too late. If we let the growing flood of human impact run its course, nature’s beauty and abundance will be pruned so savagely that there will be only remnants left. They will hint at past majesty, as a marble torso or shattered face pulled from the earth lets us glimpse the nobility of ancient Greece. Those of us who document nature’s struggle, as the boot of human progress presses ever harder upon the earth, look on with feelings of helplessness as the world did when the Taliban destroyed the Bamiyan Buddhas, or when ISIS sacked Palmyra. Each forest cleared is another assault; every coral bleaching, a silent tragedy.

Scientists are trained to record and report dispassionately, distancing themselves from the phenomena observed. But sometimes it is hard not to get caught up by emotion. I once dived off an island in the Arabian Gulf and found the reef pulverised to rubble by anchors thrown from fishing vessels. The anchor of a neighbouring boat had smashed a hole amid the delicate fawn branches of an Acropora thicket, lying within the hollow like an iron claw. A fragment-strewn path passed through the coral to the anchor, cut by dragging chain. The loss felt all the more heartbreaking because the destruction was not yet complete. Here and there turrets of living coral emblazoned with the liveries of a hundred species of invertebrates hinted at
past glories.

If humanity were to feel the cumulative burden of loss and damage over millennia, it would already be unbearable, but shifting baselines have a protective influence.