Environmental constraints have always had important consequences for human development. However, the possibility that such constraints, including climate change, will grow significantly in this century raises concern over future advances in human development. This study uses the International Futures (IFs) integrated forecasting system to explore three scenarios: a base case scenario; an environmental challenge scenario; and an environmental disaster scenario. The purpose is to consider the impact of different aspects and levels of environmental constraint on the course of future human development. Using the Human Development Index (HDI) and its separate components as the key measures of development, the study finds that environmental constraints could indeed greatly slow progress and even, in disastrous conditions, begin to reverse it. Least developed countries are most vulnerable in relative terms, while middle-income countries can suffer the greatest absolute impact of constraints.