Why is the greenhouse effect only one way?


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

It’s not the light that’s reflecting off the Earth and then going back up that’s the problem, it’s the light that gets absorbed by the Earth (making the Earth warm) and then released back out that’s the problem. When the warm ground released energy back out as a form of light, it’s a different ‘color’ of light than originally hit the Earth and made it warm in the first place. The greenhouse effect is a barrier based on the ‘color’ of the light so it lets in the Sunlight no problem, but it becomes to a barrier to the Earthlight.

Greenhouse gasses (and the glass of actual greenhouses) are transparent to light in the visible spectrum and near-infrared just below the visible part of the spectrum, but they are *not* very transparent to the infrared farther down the spectrum. The peak energy of sunlight is in the yellow/green part of the visible spectrum, although there’s also a lot of infrared at various wavelengths.

So what happens is that although a lot of light from the Sun is reflected by the atmosphere and the rest of the Earth, a lot of it gets absorbed, adding energy to whatever absorbed it. That energy will later get released back, but at a lower wavelength. A lot of it *does* pass right through the atmosphere and leave, but all that near-infrared gets turned into far-infrared, which gets absorbed and reflected back towards the ground.

You are correct to think that any far-infrared from the Sun will *also* be absorbed and reflected, but when photons are emitted it’s a very random event so about half of the energy that is absorbed by the atmosphere gets emitted downwards towards the Earth. More importantly, though, the Sun is beaming a lot more energy into the Earth in wavelengths that the atmosphere (and glass) are not transparent to, while more of the energy that the Earth (and the inside of a greenhouse) is trying to beam back out is in wavelengths that the atmosphere (and glass) are not transparent to.

So the thing about greenhouse gasses is that they are transparent to visible light but opaque to infrared radiation.

So light from the sun passes through the atmosphere. The energy is absorbed by things on the planet’s surface. Those things then release that energy through blackbody radiation, which will be in the infrared range, which we experience as heat.

That radiation ideally reaches space and beyond, cooling the planet. But greenhouse gasses reflect it back to the earth’s surface instead.

It’s exactly like a literal greenhouse, except with gas instead of glass.

Thanks for the responses everyone. I had not considered that the light gets transformed after it reaches earth.

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.)