The latest report of Working Group I of the Intergovernmental Panel on Climate Change (IPCC) combines new and better observations with improved models to provide a clearer and more definite picture of humanity’s influence on the climate system, and the paths we can choose for the future. The atmosphere continues to warm, with the last three decades each being warmer than the one before, and all three being warmer than any other decade since reliable observations began in the 1850s. Warming is seen almost everywhere on the Earth’s surface, and many observed changes are unprecedented on scales of decades to millennia. We also now have...