As a heat wave roasted the western United States this week, temperatures in California’s Death Valley soared to a blistering 130 degrees Fahrenheit, marking the hottest temperature measured anywhere on Earth since 1931 and the third hottest day ever recorded on our planet, period.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 waves were correspondingly nightmarish. And while human carbon emissions haven’t pushed Earth into a new hothouse state yet, climate change is making heat waves more frequent and severe, meaning Death Valley’s extreme temperatures are unlikely to stand for long. Earth won’t be as scorching and uninhabitable as Venus anytime soon—temperatures there are hot enough to melt lead—but heat that challenges the limits of human tolerance will occur more often as the century wears on, scientists say.And in the very, very distant future, Earth might actually become like Venus.The scorching pastIt might not feel like it if you live in California or Japan right now, but Earth is currently in what geologists consider an icehouse climate: a period cold enough to support an ice-age cycle, in which large continental ice sheets wax and wane near the poles. (Right now the one in the northern hemisphere has retreated to Greenland.) To get a glimpse of what a much warmer world would look like, we need to go back at least 50 million years to the early Eocene.“That was sort of the last really warm climate the Earth experienced,” says Jessica Tierney, a paleoclimatologist at the University of Arizona.