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Greenhouse Effect [Q&A]

– Vinay

Sahiba, a middle-schooler from New Jersey, asks what is the greenhouse effect and how does it work?

We hear a lot about the greenhouse effect nowadays because of the global climate change that is happening, and the tendency is for people to think that this is something new that humans are causing and must be a bad thing. In fact, the greenhouse effect itself has been operating throughout the Earth’s existence, and it is actually essential for life to exist here. The problem arises, as with most things, when there is too much of it.

The way it works is, light from the Sun (mostly in the visual electromagnetic band) falls on the Earth and warms it up. The warmed Earth radiates in the Infra-Red and cools down. Over time, an equilibrium is established between the incoming radiation and the outgoing radiation. If there were no atmosphere, the Earth would end up with an average surface temperature of about 255 degrees Kelvin (or -18 C, or -1 F) – quite chilly, arctic conditions in the tropics, and unlivable even in the temperate latitudes.

But the Earth’s atmosphere intercepts some of the outgoing IR radiation and re-emits it, sending some out to space and some back to the Earth, thus trapping some of the energy. The Earth warms up until the heat loss again balances the incoming solar radiation. So the equilibrium temperature ends up being higher. The actual surface temperature is about 288 deg K (or +15 C, or +59 F), quite balmy in comparison.

The Earth’s atmosphere is a complex system, but one can garner most of the relevant physics from a simple model (via RealClimate).