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 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.
Latest Answers