Dienstag, 14. Januar 2014

The layman's approach: The properties of greemhouse gases

The layman's approach: The properties of greenhouse gases

In my course about climate change I got some explanation about the so-called greenhous gases. But this is not sufficient to see the complete picture and the role they play within the climate system. Being a layman, I want to find out how these work and try to get an easy to understand but yet exact enough picture for everybody. As an example I use the CO2, but the other GG are working after the same principle.

What are the properties of CO2?

  • CO2 can absorb a vast spectrum of infrared (heat)rays. The CO2 molecule gets hotter and radiates again infrared radiation, partly toward the earth's surface, partly towards space.
  • The time between absorbing heat for IR radiation and emitting it again lies in the fraction of a nanosecond.
  • CO2 can collect heat from the earth surface or from other surrounding non-greenhouse-gas-molecules through physical contact. Even this heat is radiated in both directions.
  • CO2 content of the air is about 400ppm (parts pe million, 1 ppm = 0.0001 %). In very direction of an air filled space, every 16th molecule is CO2. So it is unlikely that infrared radiation doen't meet such molecules. In fact, even after only few cm, a big amount of the  IR radiation has already met CO2 molecules.
In short, CO2 can absorb and emit IR/heat rays. Or: I can be heated up and will cool down by itself. The proinciple is like that: Two matters are emmiting heat/IR rays, one is hotter than the other. As the hotter one radiates more than the cooler one, the difference between both heat transfers is the net amount, which will heat up the cooler one.

How is CO2 in the atmosphere working?

near the earth surface: CO2 is hit by a IR ray, is heated up and emits half of the extry energy toward the surface and half of in direction of the space. But there are lots of other CO2 Molecules, wihch also re-direct the half amount from this fraction towards space and half of it towards the surface. The next one does the same, etc. So the fractions redirected are getting smaller and smaller.The more the IR rays are comming to the outer layers of the amosphere, due to the thinner air let CO2 molecules ar hit and most of it will go out into the space.  CO2 can be considered as brake for the IR radiation, which slows down the cooling process of heat transfer through IR radiation.

on the outer edge of the atmosphere towards the space: CO2 is absorbing heat rays comming from the earth via other CO2 molcules and extra heat from physical contact to other non-CO2-molecules. As the temperature of the space is -273 centigrad, it radiates as long energy towards space, until it is exhausted. But still some few IR is comming from the earth. Here on the outer edge CO2 is helpng to send any energy away from the earth, which means that it cools the atmosphere. Yes. some few IR rays are still sent back to earth, but they are few compared to the net sum of those who are directed towards space.

What are the properties of the NON-greenhous gases O2 and Nitrogen? 
  • They can collect heat through physical contact to the earth's surface and to other air molecules.
  • They can't neither absorb IR rays nor emit/radiate heat away. They are insulating.
How would an atmosphere work without Greenhous gases?
  • IR radiation could pass through towards space, causing more heat loss.
  • But O2 and Nitrogen are also heated up by contact to the surface, but this heat can't escape from the atmosphere, because both cannot emit heat through IR rays.
  • So the whole atmosphere would have the same temperature as the surface.the only cooling could happen through contact to colder parts of the earth, heating them up, and causing them to radiate the excess heat towards space.
In consequence, an atmoshphere without Greenhouse gases could be hotter than with, as it misses the cooling through IR emitting gases. Is it really like this? Consider the fact, that now at the outer edge of the atmosphere are temperatures of -100 centigrades or lower. Without cooling, those areas would have the same temperature as the surface.

Reflections: How cold/hot it would be without Greenhouse gases?

 It depends on the amout of heat which is emitted from the earth's surface vs. the the amount of heat which is collected from the atmosphere through contact. We understand, that wind / convection is steadily bringing new air molecules to the surface to be heated up.

The heat is only radiated away, when the surface has been heated up. In deserts the day temperature will go up to 70 centgrades or more. At the same time the air has chance to get heated up by surface contact. And it is trapped as long there, as it hits a colder area of the earth, say, the poles. Using the poles to cool down the hot desert air, would cause them to melt. 

Possibly it's ´not so bad to have some CO2 in the air. And possibly the rate of it is not so crucial. More heat mean more convection / exchange with the outer layers of the atmosphere, allowing there the CO2 to do some cooling work.

Anway, a good scenario to start a discussion.

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