There are many misconceptions about how atmospheric processes work, and one that often seems to present a mental barrier is the idea of How much work can one molecule do?
This idea – presented in many ways – has been a regular occurence in comments here and it also appears in many blogs with eloquent essays on the “real role” of CO2 in the atmosphere, usually unencumbered by any actual knowledge of the scientific discipline known as physics.
Well, we all need mental images of how invisible or microscopic stuff really works.
When we consider CO2 (or any trace gas) absorbing longwave radiation the mental picture is first of trying to find a needle in a haystack.
And second, we found it, but it’s so tiny and insignificant it can’t possibly do all this work itself?
How much can one man or woman really do?
This article is really about the second mental picture, but a quick concept for the first mental picture for new readers of this blog..
Finding a Needle in a Haystack
Think of a beam of energy around 15.5μm. Here is the graph of CO2 absorption around this wavelength. It’s a linear plot so as not to confuse people less familiar with log plots. Water vapor is also plotted on this graph but you can’t see it because the absorption ability of water vapor in this band is so much lower than CO2.
The vertical axis down the side has some meaning but just think of it for now as a relative measure of how effective CO2 is at each specific wavelength.
Here’s the log plot of both water vapor and CO2. You can see some black vertical lines – water vapor – further down in the graph. Remember as you move down each black horizontal grid line on the graph the absorption ability is dropping by a factor of 100. Move down two black grid lines and the absorption ability has dropped by a factor of 10,000.
Now, I’ll add in the absorption ability of O2 and N2 – the gases that make up most of the atmosphere – check out the difference:
Spectralcalc wouldn’t churn anything out – nothing in the database.
15.5μm photons go right through O2 and N2 as if they didn’t exist. They are transparent at this wavelength.
So, on our needle in the haystack idea, picture a field – a very very long field. The haystacks are just one after the other going on for miles. Each haystack has one needle. You crouch down and look along the line of sight of all these haystacks – of course you can only see the hay right in front of you in the first one.
Some magic happens and suddenly you can see through hay.
Picture it.. Hay is now invisible.
Will you be able to see any needles?
That’s the world of a 15.5μm photon travelling up through the atmosphere. Even though CO2 is only 380ppm, or around 0.04% of the atmosphere, CO2 is all that exists for this photon and the chances of this 15.5μm photon being absorbed by a CO2 molecule, before leaving this world for a better place, is quite high.
In fact, there is a mathematical equation which tells us exactly the proportion of radiation of any wavelength being absorbed, but we’ll stay away from maths in this post. You can see the equation in CO2 – An Insignificant Trace Gas? Part Three. And if you see any “analysis” of the effectiveness of CO2 or any trace gas which concludes it’s insignificant, but doesn’t mention this equation, you will know that it is more of a poem than science. Nothing wrong with a bit of poetry, if it’s well written..
Anyway, it’s just a mental picture I wanted to create. It’s not a perfect mental picture and it’s just an analogy – a poem, if you will. If you want real science, check out the CO2 – An Insignificant Trace Gas Series.
CO2 – The Stakhanovite of the Atmospheric World?
Back in the heady days of Stalinist Russia a mythological figure was created (like most myths, probably from some grain of truth) when Aleksei Stakhanov allegedly mined 14 times his quote of coal in one shift. And so the rest of the workforce was called upon to make his or her real contribution to the movement. To become Stakhanovites.
This appears to be the picture of the atmospheric gases.
Most molecules are just hanging around doing little, perhaps like working for the _____ (mentally insert name of least favorite and laziest organization but don’t share – we try not to offend people here, except for poor science)
So there’s a large organization with little being done, and now we bring in the Stakhanovites – these champions of the work ethic. Well, even if they do 14x or 100x the work of their colleagues, how can it really make much difference?
After all, they only make up 0.04% of the workforce.
But this is not what the real atmosphere is like..
Let’s try and explain how the atmosphere really works, and to aid that process..
A Thought Experiment
For everyone thinking, “there’s only so much one molecule can do”, let’s consider a small “parcel” of the atmosphere at 0°C.
We shine 15.5μm radiation through this parcel of the atmosphere and gradually wind up the intensity. Because it’s a thought experiment all of the molecules involved just stay around and don’t drift off downwind.
The CO2 molecules are absorbing energy – more and more. The O2 and N2 molecules are just ignoring it, they don’t know why the CO2 molecules are getting so worked up.
What is your mental picture? What’s happening with these CO2 molecules?
a) they are just getting hotter and hotter? So the O2 and N2 molecules are still at 0°C and CO2 is at first 10°C, then 100°C, then 1000°C?
b) they get to a certain temperature and just put up a “time out” signal so the photons “back off”?
c) other suggestions?
The Real Atmosphere – From Each According to His Ability, To Each According to His Need
What is the everyday life of a molecule like?
It very much depends on temperature. The absolute temperature of a molecule (in K) is proportional to the kinetic energy of the molecule. Kinetic energy is all about speed and mass. Molecules zing around very fast if they are at any typical atmospheric temperature.
Here’s a nice illustration of the idea (from http://www.chem.ufl.edu/~itl/2045/lectures/lec_d.html).
At sea level, a typical molecule will experience around 1010 (10 billion) collisions with other molecules every second. The numbers vary with temperature and molecule.
Think of another way – at sea level 8×1023 molecules hit every cm2 of surface per second.
Every time molecules collide they effectively “share” energy.
Therefore, if a CO2 molecule starts getting a huge amount of energy from photons that “hit the spot” (are the right wavelength) then it will heat up, move even faster, and before it’s had time to say “¤” it will have collided with other molecules and shared out its energy.
This section of the atmosphere heats up together. CO2 can keep absorbing energy all day long even as a tiny proportion of the molecular population. It takes in the energy and it shares the energy.
If we can calculate how much energy CO2 absorbs in a given volume of the atmosphere we know that will be the energy absorbed by that whole volume of atmosphere. And therefore we can apply other well-known principles:
- heating rates will be determined by the specific heat capacity of that whole volume of atmosphere
- re-radiation of energy will be determined by the new temperature and ability of each molecule to radiate energy at wavelengths corresponding to those temperatures
The ability of a CO2 molecule to be “effective” in the atmosphere isn’t dependent on its specific heat capacity.
Molecules have embraced “communism” – they share totally, and extremely quickly.
Update – New post on the related topic of understanding the various heat transfer components at the earth’s surface – Sensible Heat, Latent Heat and Radiation