How does Earth's heat escape to space?
How does Earth's heat escape to space?


TL;DR: By the exact same method heat from the Sun escapes into space. Electromagnetic radiation does not require a medium. In fact, electromagnetic radiation works best in a vacuum; no medium is required. Intervening matter has an annoying tendency to absorb electromagnetic radiation. That absorption, in a nutshell, is the greenhouse gas effect.


Earth's heat escapes into space via a complex set of processes. One is convection. Eagles, hawks, and other birds use those rising thermals to get a free ride to altitude. A second mechanism is latent heat. Evaporation of water is an endothermic process, meaning it absorbs energy from the surroundings (this is why sweating keeps you cool). Condensation is an exothermic process. While falling rain cools the surface, the net transfer is upward.

Those rising columns of warm air can only go so high; they only rarely cross the tropopause (the boundary between the troposphere and stratosphere), and that is only in the case extremely unstable of conditions. Latent heat, along with birds, ride those rising columns of air. This brings up a third and the ultimate mechanism by which the Earth's heat escapes into space, which is electromagnetic radiation.

Every object, including the Earth's surface, absorbs and radiates heat electromagnetically. The atmosphere does the same. The atmosphere absorbs most of the electromagnetic radiation emitted by the Earth's surface. (I qualified that with "most" because there is a small window in the thermal infrared that goes directly into space, at least when there aren't clouds.) This absorbed radiation is quickly re-emitted as thermal radiation, but in a random direction.

The heat carried upward by thermals, by latent heat, and by surface radiation eventually does make its way into space as electromagnetic radiation.