CO₂ emissions are a modern problem. Before 1850, they came from the burning of traditional biomass fuels, amounting to a few million tons per annum (tpa). This reflected the Earth’s relatively small human population, which remained under 1 billion people to the end of the 19^th century. But in the early 20^th century, everything changed. Abundant coal-based power drove industrialisation and economic growth forward in Europe and North America, and the invention of petroleum refining in the 1920s gave society access to practically limitless energy.