Over the past half-million years, the world’s climate has seen four ice ages and four warm periods separating them, with extensive glaciers engulfing large swaths of North America, Europe, and Asia and then retreating, thousands of species displaced, and the shape of coastlines rearranged as sea levels rose and fell. Yet throughout these hundreds of thousands of years, the atmospheric concentration of carbon dioxide (CO2), which plays a key role in regulating the climate, has never risen above 300 parts per million.1