Figure 1.13: The balance of incoming and outgoing radiation and the greenhouse effect. To maintain an even temperature, Earth has to balance energy coming in with energy going out. Energy coming in is mostly sunshine, and energy going out is mostly radiant heat. Here the sunshine, or incoming solar radiation, is shown in yellow: some reflects right back into outer space, a little gets absorbed in the air, and about half warms the ground. The numbers represent averages—obviously there is usually more sunshine at noon than at midnight! Heat, mainly infrared radiation, is shown in orange: quite a lot bounces and flows around near the surface in various forms. Clouds, dust, smoke, water vapor, and certain other gases tend to keep it there. But what finally reaches outer space almost exactly balances the amount of sunshine absorbed. These days, scientists find that extra greenhouse gases released by humans are causing Earth to retain extra energy—outgoing infrared radiation seems to average nearly 1 watt per square meter (W/m²) less than incoming solar radiation, so average temperatures on Earth are rising.