The so-called ‘Doomsday Argument’ of Carter, Nielsen, Leslie, Gott, and Bostrom is examined, in conjunction with attempts to refute it, in relation to the real-world prospects of catastrophic climate and environmental change, and loss of biodiversity. If the late James Lovelock was right in his hypothesis that the biosphere is protected by a ‘Gaia’ self-defence mechanism, humans may well face extinction precisely because they constitute a threat to the survival of the biosphere.