We are, so far as we know, the first species that has foreseen its own extinction. In the distant past, this foreknowledge took the form of myths, including visions of Armageddon. In the last couple of centuries, however, we have been able to give precise scientific shape to such forebodings. Modern astronomy and physics have proved that the solar system, galaxies and the universe itself have a finite life—although their end is so far away that people don’t usually lie in bed worrying about it. More urgently, Darwin’s discoveries have taught us that every species is mortal. Few, except very primitive ones such as thermophilic bacteria, can withstand for very long the buffets of an environment that changes perpetually. Most are only around for a fairly short period. Although he never actually spelt it out (he already had enough worries about upsetting public opinion), Darwin made it as clear as possible that human beings were unlikely to be exceptions in this respect. Darwin wouldn’t have hazarded a guess as to exactly how long homo sapiens would remain viable on the planet. Since his time, however, the prognosis for our collective life expectancy has become much clearer. It looks bleak. We know more than anyone did 150 years ago about the interactions between global vegetation, atmospheric gases, temperature and sea level. We realise with clarity the contribution we have …