A reasonable template might be the last time our planet was three degrees warmer than today, as it was some 10 million years ago. Then, sea levels were twenty-five metres higher than at present, with continental glaciers entirely absent from the Northern Hemisphere.
In this world, much of the Amazon basin would become a desert, and the glaciers which provide drinking water for much of China and the Indian subcontinent would all but disappear. The southern belt of the United States, countries bordering the Mediterranean – not to mention the Middle East, Australia and much of Africa – would become too hot to sustain their present populations. Alongside all of this, there would be a major rise in extreme weather events and a profound disturbance to the hydrological cycle. Here ‘once in a century’ weather events would be happening all of the time, with the previously extreme becoming routine.