But how the sunlight enters the atmosphere when the greenhouse gases dont allow the sunlight pass through it towards earth?
In: Chemistry
It’s already been explained it well. But as a [visual example](https://youtu.be/Fx49t4sv7f0), consider a pane of glass. You can see the light come in, but if you were to see infrared, glass becomes an opaque mirror.
The visible light comes through, is absorbed partially -or in whole if it looks black- by various objects of different colors, and the visual frequencies reflected mostly escape.
What gets absorbed heats up the objects, which will then emit infrared. That “invisible” energy is reflected back -trapped- by the glass. GHGs do the same thing -they scatter the IR back towards the ground keeping more energy contained between the ground and atmosphere.
Think of CO2 as a one way mirror. It only blocks solar energy at very specific wavelengths. Solar energy arrives on the Earth at many wavelengths, from the invisible ultraviolet, to the visible spectrum, to the invisible infrared. CO2 only blocks the lowest energy infrared energy from coming in. It doesn’t block higher levels of solar energy.
When the wavelengths of solar energy higher than what is blocked by CO2 come into the atmosphere, they do a couple of things: they get reflected back into space by light colored clouds or ice caps, or they get absorbed by the dark earth and water. Reflected energy wavelengths bounce off into space. Absorbed energy wavelengths heat the Earth. When the Earth gets warmed, it radiates heat, like a hot pan on a stovetop. This happens at the lower energy infrared wavelengths.
This is where the one way mirror effect occurs. The energy the Earth tries to radiate, the lower energy infrared, is the specific wavelength that CO2 traps! So, most of the Sun’s energy that enters the atmosphere and is absorbed by the Earth as higher wavelength energy has no problem coming in, but it cannot get out because of the CO2 that blocks energy at that lower wavelength.
Now, the CO2 doesn’t trap all of the heat energy, but it makes it harder for Earth to radiate that heat energy to space, and the more CO2, the harder it is for the Earth to get rid of heat. Venus has a dense atmosphere that is primarily CO2, and it has a very hard time radiating heat away to space because there is so much CO2. Mars has thin atmosphere that is also primarily CO2, but it a hard time retaining heat because there is so little CO2 in the atmosphere. Earth has very little CO2 compared to Venus but a lot compared to Mars, and the amount of CO2 in Earth’s the atmosphere has almost doubled in the last 200 years. That means a lot more heat is being trapped on Earth now than in the past, and if we add more CO2 to the atmosphere, even more heat will be trapped by its one way mirror effect.
What “people” say is exactly the wrong way round.
The sunlight that gets reflected (e.g. by ice) is not the problem.
But lots of sunlight is absorbed by the surface – thats how the sun warms the Earth in the first place. Any warm object always produces infrared radiation (in a process called black-body radiation) which is the only way an object can cool down in a vacuum. And the problem is, the greenhouse gases hinder this process.
It would actually be great if we could somehow reflect more sunlight directly back into space, that would be a way of fighting global warming. Unfortunately, the opposite is happening: A warmer Earth means that there is less arctic sea ice, which then reflects less sunlight, which warms the Earth even more.
GHG reflect infrared, which is heat. That’s why it’s colder on a cloudless night because heat can get out, while a cloudy night will reflect a good portion of it back to the surface. Yes water vapour is a GHG.
Although there is definitely heat coming from the sun, not all of it gets reflected back out and it also transforms on the surface. One example is sunlight hitting asphalt on a road. Asphalt absorbs most of the visible light but heats up in the process, starting to emit its own infrared radiation. This radiation, if not allowed to get out through the atmosphere, will eventually build up and allow the overall temperature to increase.
Greenhouse gases don’t block all sunlight, the just block infrared radiation.
Sunlight goes through the atmosphere and hits the Earth, where it is absorbed and gets converted into heat. Earth then radiates this heat with infrared radiation. Without greenhouse gases, this radiation would radiate to space. Greenhouse gases however absorb this radiation, returning the heat back to Earth.
The light from the sun comes in a wide spectrum with different intensities. A lot of that is in the range of visible light which can pass through our atmosphere relatively easily. Once on the planet it often converts to a larger amount of infra-red light. When that infra-red light tries to escape, some of it is trapped by the greenhouse gasses. It only (seems to) works one way because what is trying to come in is different from what is trying to go out.
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