Earth is setting heat records. It will be much hotter one day.

As heat waves blanket the U.S. this summer, temperature records have been spiking. Earlier this week, the global average temperature reached just over 62°F. It was the hottest average temperature ever recorded by the National Centers for Environmental Prediction since the organization began keeping records in 1979. But Earth has seen warmer days in its past and it will experience them again in the future. During so-called hothouse periods, when the atmosphere was supercharged with greenhouse gases, the planet was much warmer than it is today and the worst heat...