Climate change has been projected to affect species distribution(1) and future trends of local populations(2,3), but projections of global population trends are rare. We analyse global population trends of the emperor penguin (Aptenodytes forsteri), an iconic Antarctic top predator, under the influence of sea ice conditions projected by coupled climate models assessed in the Intergovernmental Panel on Climate Change (IPCC) effort(4). We project the dynamics of all 45 known emperor penguin colonies(5) by forcing a sea-ice-dependent demographic model(6,7) with local, colony-specific, sea ice conditions projected through to the end of the twenty-first century. Dynamics differ among colonies, but by 2100 all populations are projected to be declining. At least two-thirds are projected to have declined by > 50% from their current size. The global population is projected to have declined by at least 19%. Because criteria to classify species by their extinction risk are based on the global population dynamics(8), global analyses are critical for conservation(9). We discuss uncertainties arising in such global projections and the problems of defining conservation criteria for species endangered by future climate change.