Near the surface in an atmosphere with little or no greenhouse gases, Earth’s surface would constantly emit infrared (IR) radiation upward, both during the day and at night (L↑). On average, incoming energy from the sun (K↓) would equal outgoing energy from the surface, but the surface would not receive IR radiation from its lower atmosphere (no atmospheric greenhouse effect). Earth’s surface air temperature would be low, and most water would be in the form of ice. With greenhouse gases, Earth’s surface not only receives K↓, but also additional IR energy from the atmosphere (L↓). Annual average incoming energy still equals outgoing energy for the whole system, but the added L↓ from the greenhouse gases raises Earth’s average surface temperature to a more habitable level.