441 utenti della rete avevano questa curiosità: Spiegami: 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?
Ed ecco le risposte:
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.
Thanks for the responses everyone. I had not considered that the light gets transformed after it reaches earth.
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.
The other commenters nailed the explanation but I’ll add that we can do a thing that’s like the greenhouse effect in reverse: passive daytime radiative cooling.
There’s a small window of IR (8000-13000 nm) that isn’t absorbed by the atmosphere, so if you engineer a material that absorbs/emits strongly in that range but not others, you can use all of space as a heat sink.
Stanford spun it out into a company called SkyCool in 2016.