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 southeast. 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. 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. The word "aridity" in the fifth paragraph probably means