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
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.
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