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
The light that comes in is in the visible range. The light that goes out is in the infrared range. The greenhouse effect blocks infrared light.
When light energy hits something, some energy is absorbed and some is reflected. Since the reflected light lost some energy, its wavelength gets a bit longer. That’s why the hotter a stove gets, it goes from infrared light that you can’t see to red light and if it got hotter it would turn orange then yellow.
A few things to note:
1. You know how you can’t see CO2? Like it’s completely invisible to you? This is because it’s very bad at absorbing visible light. It’s basically invisible to it for our purposes.
2. You know how when you wear a black shirt in the sun vs a white shirt, you get a LOT hotter. This is because you are absorbing a LOT more visible light which then warms you up.
3. When objects get warm, they emit heat either by warming the air/other stuff around them. They also emit light, but a much higher percentage of this light is emitted on the low end of the spectrum. It’s infrared light.
4. It turns out that infrared light can see CO2 quite readily and a lot of it is absorbed on the way back out of the planet.
Considering that the planet is always getting warmer from sun exposure, the only place for that heat to go is away in the form of light because you can’t conduct heat into a vacuum. So, anything that reduces the amount of light that radiates away from Earth will result in the planet retaining more heat.
The sunlight comes in and warms everything as things absorb the light. This is sunlight in every colour of the rainbow.
But warm things put out light in mostly infra red. Just that one colour.
Glass of a greenhouse let’s in the whole rainbow but blocks infrared getting out. CO2 does the same.thing.
The answers thus far are incomplete, this is the minimal physics to get the full picture:
* An object reaches a steady temperature when it is absorbing as much energy as it emits
* You need to know that any object that is above absolute zero emits radiation. Roughly speaking we can call this blackbody radiation and it scales with Temp^4 — hotter objects emit more energy.
* For the Earth’s temperature, the blackbody radiation is significantly infrared (IR) radiation, which is why we talk about IR for the greenhouse effect (fun fact: because animals are about earth temperature, thats why we use IR cameras to see them at night)
* For Earth, this means steady temperature when the energy absorbed from the sun matches the Earth’s (roughly) blackbody emissions.
* If the Earth were naked rock with no atmosphere you can [do the maths](https://earthscience.stackexchange.com/questions/825/what-would-be-the-temperature-of-earth-if-there-was-no-atmosphere) and get a **temperature of about 275K (2C or 35f)**.
* Essentially **physics says a sphere in the Earth’s orbit should be 275K**
* What the atmosphere does is block IR photons as they try to go to space from the rocky surface. The photon gets absorbed by an atmosphere molecule and the energy of the photon therefore “stays” with Earth. This is called being “optically thick” — most photons are stopped.
* The only way for an IR photon to get to space is if it is emitted not from the ground, but from a molecule high up in the atmosphere. The photon then has a shorter distance to travel to space and the atmosphere in the way in much less dense. There is some height where the atmosphere becomes “optically thin” and the average photon gets to space.
* **The height at which the atmosphere becomes optically thin is the part of the Earth in thermal equilibrium with the sun, and is therefore at about 275K**
* Due to gas physics, the **air below this point is continually hotter the lower you go, and therefore the surface of the Earth is kept much warmer than 275K**
* (Advanced topic: yes [temperature vs height in the atmosphere](https://www.eoas.ubc.ca/courses/atsc113/flying/met_concepts/03-met_concepts/03a-std_atmos/images-03a/std-atmos-temperature-color.png) is complex, but it gets optically thin in the mid-Troposhere, so we can just take the simple case)
* **Extra greenhouse gases make higher the height at which the atmosphere becomes optically thin, and therefore there is a deeper layer of air between it and the surface, and therefore the planet’s surface gets hotter**
TL,DR: The Earth is covered in a blanket, at the temperature of the top of the blanket is fixed. More greenhouse gases make the blanket thicker.
Light comes in with ultraviolet rays. Light is radiated into heat and goes out with infrared rays. Greenhouse gases trap infrared rays
Edit:I’m not a scientist, but:
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Visible light goes through glass very easily, but heat goes through glass more slowly. That’s how actual greenhouses work.
Visible light goes through carbon dioxide very easily, but heat goes through carbon dioxide more slowly. That’s how the greenhouse effect works. Visible light hits the ground/ocean, warms it up, but the heat generated doesn’t escape because too much CO2 in the atmosphere.
Some of the light changes into multiple light waves when it hits stuff on the earth. Those lower energy light waves are the ones that get absorbed by the greenhouse gases.
I watched a great Carl Sagan video on this recently. Basically the light you see – visible light – comes from the sun and warms the Earth. The Earth absorbs this heat and radiates out infrared light. Now if we measure the ratio of visible in vs infrared out, the world would be around 20-30 degrees Celsius colder than it is now, all the water would freeze eventually.
But Carbon dioxide absorbs light at a specific wavelength. This is the infrared light because it’s at the right wavelength and that’s what warms the Earth up!
Sunrays heat the earth, which then radiates heat out, and due to the greenhouse gasses can’t get out. It’s very symilar to how actual greenhouses work (hence the name) or most forms of insulation, by trapping heat.
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