When the Earth was formed, it was a big ball of hot magma, that cooled off to form the Earth's crust. The core is still molten, and when it cools off, we die.

When the Earth was formed, it was a big ball of hot magma, that cooled off to form the Earth's crust. The core is still molten, and when it cools off, we die. This is not likely to happen for billions of years, so we'll probably figure out some way to handle it by then, but you can rest assured that unless we get a Trantor-like population* heat leaving the planet is a bigger concern than heat released by its inhabitants. * Going off of Asimov's description of the population density, not the numbers he mentioned, which were orders of magnitude lower than how he described it.