numbers of people in poorer countries during the last three decades of the twentieth century. Recent technological successes, however, have come at grave environmental costs. Climate change, ocean acidification, and the huge dead zones at the mouths of many of world’s great rivers, are a function of the scale of contemporary agriculture and the many other demands 8 billion people make on the planet. Scientists now speak of humanity exceeding or threatening to exceed 9 planetary boundaries for safe use of the biosphere. Humanity’s unprecedented ecological impacts threaten to degrade the ecosystem services that people and the rest of life depend on—potentially decreasing Earth’s human carrying capacity. The signs that we have crossed this threshold are increasing. The fact that degrading Earth’s essential services is obviously possible, and happening in some cases, suggests that 8 billion people may be above Earth’s human carrying capacity. But human carrying capacity is always a function of a certain number of people living a certain way. This was encapsulated by Paul Ehrlich and James Holdren’s (1972) IPAT equation: environmental impact (I) = population (P) x affluence (A) x the technologies used to accommodate human demands (T). IPAT has found spectacular confirmation in recent decades within climate science, where the Kaya identity for explaining changes in CO2 emissions is essentially IPAT with two technology factors broken out for ease of use. This suggests to technological optimists that new technological discoveries (or the deployment of existing ones) could continue to increase Earth’s human carrying capacity, as it has in the past. Yet technology has unexpected side effects, as we have seen with stratospheric ozone depletion, excessive nitrogen deposition in the world’s rivers and bays, and global climate change. This suggests that 8 billion people may be sustainable for a few generations, but not over the long term, and the term ‘carrying capacity’ implies a population that is sustainable indefinitely. It is possible, too, that efforts to anticipate and manage the impacts of powerful new technologies, or to divide up the efforts needed to keep global ecological impacts within sustainable bounds among more than 200 nations all pursuing their own self-interest, may prove too complicated to achieve over the long haul. Two things can be confidently asserted regarding Earth’s carrying capacity, based on the Great Acceleration of energy and materials use, waste generation, and ecological degradation post-WW II. First, expansions in human carrying capacity have come at