AT THE top of Green Mountain, the central peak of Ascension Island, there is a small pond, dotted with lilies, shadowed to one side by the fronds of a pandan tree. It is the only open body of fresh water on the island—and for a thousand kilometres in any direction. Around Dew Pond grows a grove of towering bamboo, beyond which the trade winds blow incessantly from the south-east. Within the grove the air is still and damp.

Along the trailing ridge of the summit are fig trees, Cape yews and a garland of remarkably vigorous ginger. Below, on the mountain's lee side, trees and shrubs from all parts of the world spread down the hillside to a landscape of casuarina trees—ironwood, or she-oak—and thorny chaparral around its base. Even on the bleaker windward slope, grasses and sedges are dotted with Bermuda cedar and guava bushes. Above, the bamboo scratching at their bellies, are the clouds the trade winds bring; some days they cover the mountain top.

Once seen as too dry to be worth inhabiting, Ascension Island is becoming greener at an increasing rate. People are responsible. In part, their contribution was unwitting: the thorny mesquite that anchors a lot of the island's scrub was introduced for a landscaping project just 50 years ago. But the forest on the peak of Green Mountain represents a deliberate attempt to change the island's climate to make it more habitable. It is the centrepiece of a small but startling ecological transformation which is part experiment and part accident, part metaphor and part inspiration.

Ascension was discovered by the Portuguese in 1501. Just to the west of the mid-ocean ridge that separates South America's tectonic plate from Africa's, it is the top of a volcano which rises steeply from abyssal plains more than four kilometres below the surface of the ocean. The volcano made it above that surface only a million or so years ago, since when the island has grown to about 100 square kilometres. Before people arrived it was home to just a flightless bird, a land crab and no more than 30 species of plant, none as big as a bush. It was so barren and isolated that during the following three centuries of assiduous empire-building neither the Portuguese nor any other nation bothered to claim it. When Captain Cook passed by in 1775, Georg Forster—later to become renowned for his accounts of exploration—wrote it off as a “ruinous heap of rocks”, drearier even than Tierra del Fuego and Easter Island. But Forster's naturalist father Johann saw something more promising:

This barren island with very little trouble might be settled and made a very useful place of refreshment ... I am persuaded that if the common furze, which thrives so well on St Helena, were planted on this island, it would no doubt equally thrive here, and were these Furzes everywhere growing, grass and other plants would no doubt immediately grow between them ... The more the surface of the earth is covered with plants the more would they not only evaporate but even attract the moisture of the air ... after grass and water were more plentiful in the isle certainly many a tree would soon grow and thus afford fuel.

Islands had a particular hold on the imaginations of explorers like Forster. It had long been widely held that the varieties of humankind reflected the action of different climates. In the late 18th century the opposite notion began to take hold among sailors, scientists and administrators: that humankind might itself act to change the climate, either for the worse or for the better, mainly through what it did or didn't do to trees. A decade after Cook and the Forsters, a French explorer, La Pérouse, visited Easter Island. Noting the island's “dreadful aridity” in the midst of an immense ocean, he blamed the ancestors of the island's inhabitants, who had cut down the trees.

Those imprudent ancestors have become symbols for mankind's short-sighted carelessness with his environment. As environmentalists began to preach the gospel of finite resources, and satellites sent home images of the Earth looking like a small island in a vast dark sea, the fate of Easter Island seemed like a fearful parable. In his jeremiad, “Collapse”, Jared Diamond described Easter Island's story as “the closest approximation that we have to an ecological disaster unfolding in complete isolation”.

Yet it would be a mistake to place too much weight on this tale. The familiar story—deforestation leading to environmental degradation; subsequent population collapse, possibly including cannibalism; eventual endemic misery—has been revised in recent years. Some suggest that the Easter Islanders' fate was not purely self-inflicted: seed-eating rats, European slavers and climate change were in part responsible. And although apocalyptic stories have a power that brighter tales lack, mankind's record is more nuanced than the Easter Island story suggests. People have created fertile ecosystems as well as destroyed them. Ascension Island is a supreme example.

A kaleidoscope of connections

Ascension's key advantage over Easter Island is that it is remote, but not entirely isolated. Once it was eventually settled, it remained connected to the rest of the world for all sorts of purposes and in a succession of different ways. Britain first took possession of it in 1815 lest it be used as a staging post to rescue Napoleon Bonaparte from exile in St Helena. Later it became a supply base for the navy's campaign against the slave trade; the steady warm air of the trade winds meant it also made a good sanatorium for sailors and freed slaves. For administrative purposes it was treated as a vessel, HMS Ascension, “sloop of war of the smaller class”. Subsequently it provided succour to ships, both naval and merchant, that found themselves in distress. The fact that the island could supply magnificent turtles—they migrate from Brazil to lay eggs on, or in, the beaches—as a delicacy to the lords of the Admiralty probably helped justify its garrison, too.

Like an alien planet rearranged for human life, Ascension Island has been “terraformed”

As the 19th century waned, steam and the Suez Canal meant that there was less and less call on Ascension for services to shipping. Then, in 1899, a telegraph cable connecting Britain to Cape Town came ashore amid the jagged rocks of Comfortless Cove. It was soon joined by cables from Sierra Leone, Buenos Aires and Rio de Janeiro. During the first world war radio receivers were strung over the lava like washing lines to provide communication with ships at sea.

In 1922 the “stone frigate” HMS Ascension was decommissioned, and the island became in law a dependency of St Helena; in reality it became a fief of the Eastern Telegraph Company, which was subsequently absorbed by Cable & Wireless. Two decades later, it found itself part of a different connection—one that ran from the aircraft manufacturers of America to the North African and Mediterranean theatres of the second world war. American troops built an airstrip on the lava plains in the south of the island where the wide-awake terns nested; 20,000 fighters and bombers flew in from Brazil, refuelled and went on to Africa. Forty years after that, Ascension provided an air bridge when Britain fought for the Falkland Islands.

These days it is a communications hub. Wires strung between two sets of tall towers transmit the BBC World Service's broadcasts to 85m listeners in Africa and beyond. Nearby, strange geometries of short-wave systems connect the British and American armed forces to ships and aircraft. Aerials that look a bit like fish-skeletons are used by the spooks at Britain's GCHQ, a strategic eavesdropping organisation. Dishes track space launches from Cape Kennedy and European space launches from French Guiana, and monitor tests of submarine-launched missiles.

Not all the information that washes across Ascension is picked up by electromagnetic means. The Met Office station measures greenhouse gases. Seismometers listen out not just for earthquakes but also for illicit nuclear explosions. Infrasound monitors do the same job for the atmosphere, picking up the inaudible but remarkably persistent sound waves that circle the world when a bomb goes off. Offshore instruments near the American base listen out for the underwater sound of such blasts.

Building a new Eden

In the mid-19th century one Joseph Hooker visited Ascension Island. He was the son of the director of Kew Gardens, a job he later took on himself. Hooker advised the Admiralty to plant trees over the top of the mountain, encourage brambles, aloes and briar rose in the ravines and establish acacia, casuarina and eucalyptus on the lower slopes. Shipments of plants from the Cape and from Kew started soon thereafter, drawing on the entire botanic inventory of empire. In four months of 1860, John Bell, the island's horticulturalist, is reported to have supervised the planting of some 27,000 trees and shrubs. Surveying the results 140 years later, a British ecologist, David Wilkinson, turned to science fiction for the appropriate metaphor. Like an alien planet rearranged for human life, he wrote, Ascension Island had been “terraformed”.

The chief aim was to provide more rain and soil for the farm that had been established when the garrison was founded. In this, the scheme proved a long-term failure: the farm is no more. With two flights from Britain and one flight from America every week as well as regular visits by the Royal Mail ship St Helena, it is now cheaper to import food than to grow it.

Nor is it clear that the foresting of Green Mountain has increased rainfall. Precipitation varies on Ascension; in the years that Bell was expanding the plantings it was particularly heavy. Later it fell back and the farm, as well as some of the plantings, suffered. No one has documented any long-term trend in rainfall in response to the plantings, which is not surprising. The degree to which forests encourage rainfall is a matter of considerable debate. Those mechanisms that can plausibly be called into play at larger scales—such as the way that transpiration through leaves recycles water to the air, allowing the same moisture to fall as rain repeatedly—seem unlikely to apply on Ascension. The constant trade winds ensure that air passing over the mountain is back over the ocean in less than an hour.

In practice rainfall, like farming, is not much of a problem any more. The BBC, which has ended up producing most of Ascension's electricity because its transmitters are the biggest energy users, puts some of that power to work desalinating seawater. The American base has a desalination plant, too. Ascension could get by with no rain at all, if it had to. What the trees certainly do, though, is catch moisture directly from the clouds. As air climbs the mountain it cools, encouraging water vapour picked up from the warm ocean to condense. The trees provide copious surfaces on which that condensation—“occult precipitation”, to ecologists—can take place. That is what provides the water for Dew Pond, for the moist air under the spreading yews and figs below it, and for the soil. The more trees, the more moisture, the more trees.

This explains the success of the plantings on the mountain. It may also explain some of the greening that has swept down the mountain's south-eastern flank; water from higher up may be percolating through rock and soil. But not entirely. Stemson Stroud, the island's conservation officer, first arrived from St Helena to work at the Apollo tracking centre in 1967. He and others contend that the island's subsequent greening has been far more widespread than the slopes of Green Mountain.

The thorny mesquite is undoubtedly another factor. It was introduced to the island in the 1960s, when the BBC built a new village, Two Boats, for the people working on its World Service transmitters. Intended as decorative erosion-proofing the mesquite quickly took off, helped by the fact that its seeds pass happily through the digestive tracts of the island's small population of feral donkeys.

Around Two Boats, which is near the foot of Green Mountain, the mesquite has teamed up with acacia, yellowboy (a shrub in the jacaranda family) and prickly-pear cactus to make thick scrub. It has also spread to the west and down to sea level. Mr Stroud and his colleagues spend a fair bit of time hacking it back and poisoning the stumps—and through them, they hope, the prodigiously deep roots—in order to preserve the lifeless volcanic splendour of at least some parts of the island. Goats, Mr Stroud speculates, might help them in their task. To hear a conservationist speak warmly of the notoriously omnivorous and disruptive goat is to get a sense of how potent a foe the mesquite has become.

But the greening is not just an invasion. Nor is it merely a result of increased soil moisture. Just look at Mountain Red Hill, an impressive cinder cone that lies south-west of Green Mountain. Contrary to its name, Mountain Red Hill is increasingly green, but not with mesquite, and not thanks to groundwater, unless it is a special type that flows uphill.

The green side of global warming

One possibility, far from proved, is that Ascension is benefiting from global warming. Warmer seas impart more moisture to the winds blowing across them: more mists, more clouds, more condensation. Although the temperature on Ascension has not changed appreciably in the past 30 years, sea-surface temperatures upwind of it jumped by more than a degree in the 1980s before levelling off. This warming may be a natural variation; it may well not. Rainfall measured by the Met Office has not increased over 30 years; but its rain gauge, at the southern tip of the island, is in one of the drier spots.

If there is more moisture in the air condensing as dew, you might expect to see the effects high up and to windward, on somewhere like Mountain Red Hill. If there's more rain, you might expect to see it in the lee of Green Mountain's central peak—and there is indeed a rainier strip, the locals say, stretching across the island from Two Boats to Comfortless Cove, a frequent source of teatime drizzle in the rainier months. It is along that strip that the mesquite and yellowboy grow most strikingly. If nearby ocean temperatures climb higher still, as climate projections would have them do, Ascension will probably become ever moister and greener. All those Victorian plantings mean that there are dormant seeds, both of plants that prospered and of those that didn't, all over the island, biding their time. Euan Nisbet, a Zimbabwean geologist and climate scientist, speculates that after a century or two of further warming the island may be green from top to toe.

With plants in place and seed banks built up in the soil, such a greening might continue unassisted. It may, in time, have to. The fact that Ascension has always found new uses to replace old ones does not mean the trick can be carried on indefinitely. And the electromagnetic connectedness on which much of the island's usefulness now rests allows it to get by with fewer and fewer inhabitants. Even on Ascension Island, which is about as far off shore as you can get, jobs can still go offshore; the contracting companies that run the island's many antennae are all looking to reduce their costs and their presence when possible. And since the crown allows no right of abode to anyone not working or dependent on a worker, nor the right to own private property, no jobs means no people.

If left to itself, Ascension would probably decline into dull, scrubby simplicity. Humans can help avoid that by creating the sort of balance that cannot evolve for itself on human timescales. To do so, though, is to make choices. Should the mesquite be allowed to kill the casuarina trees by drilling its roots deeper and depriving them of water, as in some places it seems to be doing? Which cinder cones should keep their bleak red beauty? What new elements should be introduced into the ecology in attempts to reinforce it? Would goats be OK? Would giraffes?

Such questions are easier on Ascension, where the ecological canvas was almost empty to begin with, than in the other novel ecosystems that humans are, mostly by accident, setting up around the planet. Yet decisions must be made. In 2002 the island set about eradicating its population of feral cats. Introduced to control the rats that had arrived with sailors, they had instead chosen to prey on the vast colonies of sea birds that roosted on the lava plains, wiping them out. Every species on the island retreated to a small islet offshore, Boatswainbird Island, except for a few individuals that held on to the most inaccessible cliffs, and the phenomenally scrappy wide-awake terns that visit the southern plains to breed. With traps, poison and guns, over 500 cats were wiped out. The birds have started to return.

Not all indigenous species are so easily accommodated. The grasses, sedges and shrubs that have been brought to the island handily out-compete the native species, many of them ferns, that were making such a poor fist of greening the island before people came. This is hard to regret, seeing the result. What's more, the new painting need not cover up all of the original, almost bare, canvas beneath. The creation of the new can, with care, make room for the conservation of the old.

Restoring the balance

Beneath the summit ridge on Green Mountain, on the lawns of a small garden, Mr Stroud nurtures indigenous plants. He discovered one fern only a couple of years ago—a species hidden for centuries. He plants the successes under a huge fig tree on the ridge. When they flourish he takes them further out into what on other islands would be the wild, but here is the artifice, returning occasionally to check up on them and take more seed. While he and his successors are here, those ferns and grasses will be safe from extinction. And a few are taking the initiative themselves. Xiphopteris ascensionis, a tiny endemic fern, had never seen a tree before the Victorian planters came. Now it lives in and on them, nestled in their moist bark, pioneering the epiphytic way of life familiar from ancient forests around the world and discovered afresh in their youngest cousin. Life, with helping hands, adapts.

The lesson that Easter Island teaches humanity is bleak. Ascension Island's story has a more hopeful message. It shows that environments not remotely natural in their origins can become lovely to inhabit. People like Mr Stroud can and will act not just to preserve the environment but to improve it, making it more, not less, than it otherwise would be.

Winding down the flank of the mountain, there is a graceful fluttering in the woods off to the side of the road. Free from the threat of cats, fairy terns have returned to the island—and forsaken their ancestral cliffs for a new life among the leaves and branches. They flash bright white and beautiful against the green.