…The Earth is not shifting from a cold period to a warm period, as it did when the ice sheets began to melt 18,000 years ago. It's shifting from a warm 'inter-glacial' to an even warmer episode, where temperatures are likely to be outside the evolutionary experience of many species.

The threshold of thermal tolerance is already being crossed for tropical coral reefs, for example, which have been struck by severe mass bleaching episodes in recent years, as a direct result of the warming seas. Bleaching reached catastrophic levels in the 1998 El Niño event, when a sixth of the entire coral reef ecosystem on the planet was destroyed. Yet disasters of this scale are likely to become commonplace within a mere two decades, and within thirty to fifty years severe bleaching could be an annual event - a test the vast majority of corals will not be able to survive.

The likely disappearance of tropical reefs is a very big deal indeed. Coral reefs are the most biodiverse marine ecosystems on the planet, containing as many as nine million different types of plants and animals, including a quarter of all known sea fish. Most of the rest of the planet's biodiversity is in tropical rainforests - which are endangered not only from direct threats of logging and farming, but from shifts in climate which are already making them vulnerable to fire. In Indonesia 80% of the original forest cover has now been destroyed, and a drought in 1997-8 led to the worst fires in history.

Computer models developed by the meteorological office suggest that a 'tipping point' exists where global warming will lead to a rapid collapse of rainforest ecosystems and their eventual conversion to savannah. This is particularly the case for the Amazonian forest, where the largest undisturbed forest areas remain. In Australia, the tropical rainforests of north-eastern Queensland, which are listed as a word heritage area due to their high biodiversity, face an "impending environmental catastrophe", according to Stephen Williams, a scientist who has spent years studying them.

Williams has modelled the effect of temperature rises on some of the species unique to the area, and discovered that even small rises lead to the loss of valuable habitat. Indeed for a warming of anything over 3.5°c, the wipeout is almost total…