how do greenhouse gases trap heat in the atmosphere?
In: Physics
Greenhouse gases absorb infrared light, increase in temperature, and then re-emit that infrared light in a random direction. Some of that re-emitted infrared light will head back towards earths surface. This is an even bigger problem than at first glance though, because not all the light earth absorbs is from infrared, but a good deal of the light that it radiates out is. This means that a full spectrum of light will pass through earths atmosphere, get absorbed by the earth and radiated back out as infrared light. Some of which will then be absorbed again by greenhouse gases and be Re-emitted back towards earth.
[this futurama clip is really quite accurate ](https://youtu.be/0SYpUSjSgFg), it skips a bit out but essentially there are just so many the sunlight simply can’t pass through
The Earth gets almost all its energy from the Sun. Sunlight reaches the Earth. About a third of it actually gets reflected back out to space and doesn’t do anything and some of it gets absorbed by stuff up in the atmosphere but the *rest* of it actually reaches Earth’s surface to provide us with that sweet heat energy.
At the same time, the Earth starts releasing energy back out to space.
When there is *less* greenhouse gases, more of that energy can freely leave and not bother us. But as greenhouse gases go *up*, it traps that energy, not letting it go anywhere. Since it can’t go… it just stays on our planet to keep heating up. And since we still have sunlight heating us, too, it means that the temperature grows.
It’s called the “greenhouse effect” because it’s the same idea of a regular greenhouse – energy enters but can’t escape.
This came up in the most recent episode of [A Problem Squared](https://youtu.be/gXIh2GyIBXw).
Basically, different gasses are transparent to some light but opaque to others. This just means some light can get through but other light gets stuck.
Earth’s atmosphere is made up of a bunch of different gasses that let light through, predominantly in the visible and ultraviolet range (because evolution). Much of that light gets absorbed. The UV gets used by plants etc as food, and it gradually makes its way through the food chain. Ultimately, this eventually gets turned in to heat (as well as useful energy like movement etc.). Some of this heat gets emitted as infrared light.
The problem is, some of the gasses in the atmosphere (like CO2 and water) don’t let the infrared light through. These are the ones we call greenhouse gasses.
TLDR: greenhouse gasses let UV light in. Nature turns that in to heat in the form of IR light. The gasses don’t let the IR back out, so it gets stuck here.
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