I Misunderstood the Greenhouse Effect. Here's How It Works.

We have an infographic to go with the video that you can download here: www.dropbox.com/s/mhlu3b8f53pjz9t/Infographic%20Gr…

When I first went to college I wanted to educate myself on climate change. I took a course on environmental science hoping to get a better understanding. Unfortunately I didn't realize the course was a sociology course, so we didn't actually learn anything about climate change or the environment. Instead it was about people's perception of the topics. An environmental economics course I took a couple years later was actually very good and useful, but still never really got a good understanding of the principles behind climate change. It is amazing how for such an important topic most of the conversation about it seems to not actually revolve around what it is.

I usually understand things easily when Sabine explains. Not this time, though. I will have to watch it once more.

the pressure of the atmosphere doesn't decrease with height due to the inverse square law of gravity being weaker. The difference in gravitational acceleration is negligable from the surface to 100km high which is where space begins. The pressure decreases because is given by the weight of the column of air above and as you move towards space that columns is less and less massive.

Thank you for the excellent explanation. I would like to offer what I believe is one small correction. The reduction of static pressure in the atmosphere at increasing height is due to the fact that as altitude increases, the air is supporting the weight of less air mass above. Even without the inverse r-squared variation of gravity, the pressure in the air must decrease with increasing altitude.

I'm a PhD Physicist who teaches general education climate science (when I don't have to teach the physics curriculum). This is an outstanding and clear presentation of how the greenhouse effect actually works (which I didn't fully appreciate for the first too many years I taught the class). The way you propose to modify the energy flow diagrams is spot on. Definitely some of your best work. Brava, and thank you for doing this. Heck, gonna show it in my class...

I really liked this episode, however, I think the explanation at 6:45 - 7:15 of why the roundness of earth and the inverse square law for gravitiy were relevant and why the pressure decreased with the height above the earth is totally incorrect. The pressure doesn't decrease due to the decreasing gravitational pull. In fact, the latter almost stays constant in that area. What changes, is the remaining amount of air above you that has a weight and thus exerts pressure on you. The same principle applies in water. You observe a higher water pressure at the ground of a swimming pool than at its surface. Again, that is not due to a higher gravitational pull, but due to a higher amount of water above you.

Stratospheric Cooling. That is the net proof that I didn't know before.

Having been involved in radio technology for most my life, I understood the "wiggling" a different way. I think of a molecule as a kind of antenna tuned to a specific frequency and associated harmonics. So... Basically a molecule like H2O or CO2 is like an antenna that is tuned to certain frequencies that, when excited, resonates (vibrates, wiggles)... or you can think of it like a tuning fork. Just as a tuning fork emits a specific sound regardless of what causes it to vibrate, a CO2 molecule resonates at specific frequencies of the EM spectrum. So... that's my understanding.

Sadly it's even more complicated than that. The greenhouse effect causes less than 50% of the warming effect predicted from increasing CO2, with the remainder being caused by climate feedback effects: There are a huge number of climate feedbacks, but a simple example of a "positive feedback" is that white snow reflects sunlight, but once it's melted by a warming environment, then more sunlight will be absorbed by the ground, and so the temperature will increase further (so causing even more snow to be melted, etc). An example of a "negative feedback" is that as temperature increases, there is more evaporation from the ocean, which causes more clouds to form in the lower atmosphere, reflecting more sunlight into outer space, so reducing temperature. Unfortunately these feedback effects are often not understood very well (as they are often hard to measure), hence the large variation in predictions made by different climate models (and so why the IPCC prefers to average over a large number of models). In the distant past there was probably a period known as the Snowball Earth where most(*) of the surface was covered in ice (reflecting sunlight into space) from a massive ice age, and without volcanism producing CO2 the Earth might still have been like that today. (* I have simplified to avoid writing too much.)

Air pressure doesn’t decrease with altitude because the gravitational force decreases, in a uniform gravitational field the air pressure would also decrease, and the gravitational force in LEO is pretty similar to that on the surface. It decreases because the mass of air above that point is lower.

Thank you Sabine for this excellent video! Back when I studied physics, I also took some courses on astronomy and learned that a quantity called optical depth or optical thickness is very useful when discussing stellar atmospheres. There was a rule of thumb that the radiation we see comes from an optical depth of about 1, which provided relatively easy explanations for a surprising amount of the features of stellar spectra. This rule of thumb is also useful in earth's atmosphereprovides quantitative estimates for the altitudes at which radiation is emitted. One detail about the glass houses in which we grow food - to the best of my knowledge, the main reason why they get hot is that the air inside is trapped. In experiments where the glass was replaced with infrared-transparent windows, the temperatures inside the "greenhouse" rose to almost the same levels.

1:35 that is actually not how a green house works. The main effect is not keeping the IR radiation in, but keeping the heated air inside. A greenhouse would work with IR transparent glasses as long as it reduced/prevents convection.

I didn't realize that this subject is so complicated, i almost took a break and went back to watching quantum mechanic videos to clear my mind a little, thank you for the enlightening explanation.

Wow, that just blew my mind! I've a phD in physics and still had exactly the same misunderstanding. I think, it's not just the arrows in the diagram, but most sources of information trying to make the complex topic understandable. Kind of similar to the various atomic models out there in schools and the web, which are scientifically all oversimplified, thus wrong when it comes to explaining chemistry (Schrödinger and Dirac are nodding).

That was an outstanding explanation! Thank you for not trying to simplify everything to the point at which your explanations become incorrect. I have been trying to understand how to correctly explain the warming effect of certain gases for many years and I have NEVER heard anyone explain the “altitude” issue like you did. Also, I really appreciate the explanation of stratospheric cooling and why that prediction supports the human-caused climate change story. There is quite a bit of good science content on YouTube these days, but your channel is among a very small number of really great ones!

I had to really focus but I was very impressed. Thank you for taking the time to pass on🎉 the information Bob L.

Everything's always so much more complicated than it seems 😭

Sabine! A good science communicator is one who’s not afraid to say “this is more complicated than you think”. Thank you again for the great content you put out!

I like how you do the different intellect levels. It makes an ordinary person feel smart if they understand the PhD level.