Six Degrees: Our Future on a Hotter Planet. Mark Lynas

is crossed, the algae are expelled, and the ‘bleached’ corals will die unless cooler waters return quickly.

      Bleaching is undoubtedly a recent phenomenon, observed around the world's oceans only since about 1980. Scientists have drilled deep into reefs and found no evidence that such episodes have happened during past millennia. But as the oceans have warmed due to the human-enhanced greenhouse effect, bleaching episodes have hit the world's coral reefs with increasing-and devastating-regularity. The first mass bleaching event occurred on the Great Barrier Reef in 1998. Since then things have got steadily worse. In 2002 another mass bleaching event occurred-this time 60-95 per cent of all the reefs surveyed across the marine park were bleached to some extent. A small number of reefs, particularly those close to shore where the waters were hottest, suffered almost total wipeout.

      As luck would have it, I was on the Great Barrier Reef in the summer of 2002, visiting the University of Queensland's research station on Heron Island. The place was frighteningly efficient-within minutes of disembarking from the Grahamstown catamaran I had learned to tell the difference between a white-capped noddy tern and a muttonbird, and discovered that Heron Island was actually misnamed: the white birds in question are in fact eastern reef egrets. The place was stunning-‘an aviary surrounded by an aquarium’, as one of the scientists accurately put it. Buff-banded land rails scampered about like domestic chickens, in and out of the research huts. (Two female students had adopted one and named it Sheryl.) Very soon I spotted the man I had really come to see, striding purposefully around the station with his wetsuit peeled off down to the waist. Ove Hoegh-Guldberg was clearly a man happy in or out of the water. One of his favourite stories was about the time he managed to get his finger clamped in the jaws of a giant clam, and had to rip the animal off the seabed in order not to be drowned by the rising tide-only to be scolded on returning to the beach by a marine park official for damaging a protected species.

      Hoegh-Guldberg was the author of a landmark 1999 paper which first drew the world's attention to the threat posed to coral reef survival by bleaching. Having determined the thermal tolerance threshold for corals in different parts of the world, he then applied these to a model of rising sea temperatures during the twenty-first century. The results shocked even him. By the 2020s, he discovered, with less than a degree of global warming, the seas will have heated up so much that the 1998 Barrier Reef mass bleaching event would be a ‘normal’ year. Given that it takes 30 years or so for a seriously bleached reef to recover, annual bleaching events will devastate the ecosystem-transforming, as Hoegh-Guldberg wrote in the paper, ‘Great Barrier Reef communities into ones dominated by other organisms (e.g. seaweeds) rather than reef-building corals’. Other coral reef ecosystems-from the Caribbean to Thailand-would be similarly transformed. With the end of the coral reefs, one of the world's great treasure troves of biodiversity will be destroyed for ever.

      It was with this grim scenario in mind that we both went for a snorkel on the afternoon I arrived on Heron Island. Splashing through the shallows, we disturbed a huge shoal of pilchards, which turned en masse and darted off further up the shore. Halfa dozen large stingrays flapped lazily further out, where a stronger wind raised enough of a chop to make snorkelling a hazardous experience. Every so often a wave would break over the top of my snorkel, giving me a sudden gulp of salty water. Ove was unfazed, and we trod water for a while as he pointed out affected corals.

      ‘See that bright blue and red? That's actually bleached. It's ironic you get the best colours when it's bleached.’ Some of the worst-hit was the branching coral, where whole areas were blanched bone-white. In some places just the tips of the underwater antlers were bleached, whilst in others the entire structure had been hit. But only a minority were the healthy brownish colour indicating the symbiotic algae still at work.

      ‘How likely is it to come back?’ I spluttered, swallowing another wave.

      ‘So long as it stays cool from now on, most of it will probably come back,’ he replied. ‘But some of it won't, and if temperatures rise again soon much of this will probably die.’

      Hoegh-Guldberg's work has been complemented by a more recent study which gives a slightly more optimistic forecast. Work in the Caribbean and Indian Ocean by Andrew Baker and colleagues (later published in Nature) suggests that corals may be more adaptable-and therefore less vulnerable to outright extinction-than previously thought. The scientists studied coral communities that had been bleached in the 1998 event to see how far they had managed to recover, and were surprised to find that the type of symbiotic algae within the coral had changed to a more heat-tolerant version in all the places they surveyed. With a higher thermal stress threshold than before, damaged reefs may be able to survive future warmer seas without dying out completely, the scientists suggested.

      But Ove Hoegh-Guldberg doesn't agree. Even with an increase in heat tolerance with different algae, he points out, ocean temperatures are still set to get too hot for most corals to survive. He and his co-authors used the latest models and reef analysis to again project bleaching frequencies in the decades to come-and their results confirmed the earlier pessimistic analysis. Severe bleaching will occur on most of the world's reefs every 3-5 years by the 2030s, and by the 2050s will strike every two years.

      A more recent bleaching event, which struck the Caribbean in 2005, also seems to bear this out. That summer saw sea temperatures in the region reach highs never before measured during the entire 20-year satellite record. These were the same high temperatures, in fact, which made 2005's hurricane season so deadly: Hurricane Katrina hit New Orleans in 2005 after travelling over these same unusually warm areas of ocean. And they were temperatures which would be vanishingly unlikely in an atmosphere without today's loading of greenhouse gases. The effect on Caribbean corals was disastrous. According to surveys carried out by scuba divers, 90 per cent of coral bleached in the British Virgin Islands, 80 per cent in the US Virgin Islands, 85 per cent in the Netherlands Antilles, 66 per cent in Trinidad and Tobago and 52 per cent in the French West Indies. Some reefs may recover in years to come, but model predictions suggest that in this region too bleaching events of this magnitude will hit every other year by the middle of the century.

      In any case, very few of the world's reefs are in any state to take on the challenges of climate change. Direct human interference-from sewage, overfishing and agricultural run-off-has already reduced coral reefs across the globe to shadows of their former pristine selves. In total 70 per cent of the world's reefs are now either dead or dying. This is a disaster of an almost unimaginable scale for global biodiversity: second only to rainforests in terms of the vibrancy and diversity of life they nurture, coral reefs worldwide shelter and feed a third of all life in the oceans, including 4,000 types of fish.

      Heron Island's reef may be under good management, but the same cannot be said for reefs elsewhere in the Pacific. In the same trip as I visited Ove Hoegh-Guldberg, I also snorkelled along Fiji's so-called Coral Coast, at one of the few gaps I could find between the 5-star hotels and luxury resorts which now blight the entire area. Instead of vibrant-coloured reefs, teeming with parrotfish and groupers, I found piles of rubble-the shattered remains of coral-looming bleakly through a murky ocean. None of the sun-bathers crowding onto the beach seemed to mind, but for me the experience was a depressing reality check. Fiji's Coral Coast is no longer vulnerable to climate change, I was forced to conclude, because it is already dead.

      Another hot spot of biodiversity-and yet another World Heritage Site threatened by global warming-is the Cape Floristic Region of South Africa. Covering a huge coastal arc inland from Cape Town, it is home to the greatest concentration of higher plant species in the world outside the tropical rainforests. Its inauspicious rocky soils and arid Mediterranean climate support 9,000 different plants, more than 6,000 of which are found nowhere else on the planet. The most iconic plants in the region are the proteas. The king protea, with its massive sunlike flower head, entirely deserves its designated title as South Africa's national flower. The region is far from pristine, however-lions and rhinoceroses once roamed these hillsides, where now vineyards and rooibos tea plantations encroach on the last wild areas.

      According to a team of researchers based at South Africa's National Biodiversity Institute, just small changes in climate could have a devastating impact on the remaining strongholds of the proteas and other endemic species. Using the UK Hadley Centre's model for climate changes in the region by 2020, the scientific