What phenomenon causes Earth's surface to be warmer than it would be without an atmosphere?

The phenomenon that makes Earth’s surface warmer than it would be without an atmosphere is the greenhouse effect.

Sunlight, mostly shortwave radiation, passes through the atmosphere and warms land and oceans. The warmed surface then emits infrared radiation. Greenhouse gases—including water vapor, carbon dioxide, methane, and nitrous oxide—absorb some of that outgoing energy and re-emit it, slowing the loss of heat to space.

The natural greenhouse effect is essential: without it, Earth’s average surface temperature would be about −18 °C rather than roughly 15 °C. The atmosphere therefore does not simply “trap sunlight”; it mainly affects outgoing infrared radiation. The enhanced greenhouse effect refers to the extra warming caused by rising greenhouse-gas concentrations from human activities.

This is often confused with the ozone layer or albedo. Ozone shields Earth from much ultraviolet radiation, while albedo describes how much incoming sunlight surfaces reflect. Both influence climate, but neither is the name of the heat-retaining process described here.