The signature environmental and social quandaries of our time—climate change and income inequality—are mutually amplifying. In the latest example, a new study finds that the gap in per capita income between the richest and poorest countries is likely 25 percent greater than it would have been without climate change. How come? Warmer temperatures have slowed economic growth in tropical regions, where most poor countries are located, but not in cooler, industrialized regions. This slower growth stems from declines in labor productivity, crop production, and human health. The differential impacts between rich and poor underscore a fundamental global injustice: those least responsible for climate change suffer most. The case grows ever stronger for rectifying this international inequity through international law, economic compensation, and binding treaties for a just and sustainable future.