Why is the greenhouse effect only one way?

Question

So what I’m reading is that these gas absorb the light from the sun and keeps it trapped on the earth.

What I don’t get is how is it letting the light and heat in from the sun in, but not the light and heat reflected from the Earth out? If it’s a barrier, shouldn’t it block both ways? If it’s not a barrier, how is it trapping the heat?

In: 950

Answer 1

Ok, so you have the sun and it is shining on the earth. The earth starts to heat up because of all that energy from the sun coming into the earth.

As the earth heats up, it does emit some of that energy back out into space. The light it emits is lower energy than what comes in from the sun (it’s called outgoing longwave radiation). As the earth gets warmer, it emits more and more of this energy.

So, it reaches a point where the energy coming in from the sun is balanced with the energy going out from the earth. This is a very good thing, to have that nice balance. That is what keeps the earth nice and warm (mostly) for humans to live.

What the green house gases do (and it is basically carbon dioxide from burning fuel) is push that balance out of whack a bit. These are really good at absorbing the sun’s energy, so the earth needs to get back into balance by getting a bit warmer. It warms up, then it can reach this new balance – but at a higher temperature.

The greenhouse gas is absorbing more energy from the sun, so the earth gets warmer to the level where it can emit that same level of energy, and get back into balance.

So it only ‘goes one way’, because it is a good absorber of energy, so it makes the earth hotter. However, you could remove all the greenhouse gases and the effect would be that the earth would cool down a bit. But, if you are adding more absorbers, it gathers more energy from the sun, and the earth gets warmer.

(also, fun fact, carbon dioxide stays in the atmosphere for a long time, on the order of a century. So when we add carbon dioxide to the atmosphere, it just stays there. That’s kinda where the real problem lies. The level of CO2 just keeps going up and up and up.)