Earth's average surface temperature, a basic fact of its climate, is about 15 °C. If Earth had no greenhouse gases, it would be about −18 °C, frozen over (NASA). The difference comes from the greenhouse effect, and it starts with two kinds of light.
- Sunlight, mostly visible light, passes through the air and warms the ground and ocean.
- The warmed surface gives off energy as infrared light, which we can't see.
- Greenhouse gases, such as water vapor, carbon dioxide, methane and nitrous oxide, absorb much of that infrared and give it off again in all directions, including back down.
- So the surface receives energy from both the Sun and the warm air above it, and it warms until it gives off as much energy as it gets.
Key idea
The greenhouse effect: sunlight warms the surface, the surface gives off infrared, and greenhouse gases absorb some of that infrared and send part of it back down, keeping the surface warmer. More greenhouse gas means a warmer surface.
Why it works
Nitrogen and oxygen, 99% of the air, let infrared pass right through. Molecules with three or more atoms, like CO₂ and H₂O, can bend and stretch in ways that absorb infrared. That is why a gas that is only 0.04% of the air matters so much. Adding more CO₂ makes the air absorb more of the outgoing infrared, so Earth has to warm up to send the same amount of energy back to space. How much warmer Earth gets this century depends on how much more is added.
Worked example: How much warmer?
With greenhouse gases, Earth's average surface temperature is about 15 °C; without them it would be about −18 °C. How much warming do they provide?
- Difference: $15 - (-18) = 15 + 18 = 33$.
- About 33 °C of warming.
Answer: About 33 °C
Worked example: Which light?
Do greenhouse gases mainly block incoming sunlight or outgoing infrared?
- Sunlight (mostly visible) passes through the air almost freely.
- Greenhouse gases absorb the infrared that the warmed surface gives off.
Answer: Outgoing infrared
Common mistake
Confusing the greenhouse effect with the ozone hole, or thinking greenhouse gases trap incoming sunlight. The ozone layer absorbs ultraviolet light high in the stratosphere; the ozone hole is a separate problem. Greenhouse gases absorb the infrared that the warmed Earth gives off. Check: the greenhouse effect works even at night, when no sunlight is coming in at all.