If Greenhouse Gases are such a Small Part of the Atmosphere, How Do They Change Its Temperature?

June 17th, 2010

Some of the questions I receive from the public tend to show up repeatedly. One of those more common questions I receive arrived once again yesterday, from a airplane pilot, who asked “If greenhouse gases are such a small proportion of the atmosphere,” (only 39 out of every 100,000 molecules are CO2), “how can they heat or cool all the rest of the air?”

The answer comes from the “kinetic theory of gases”. In effect, each CO2 molecule is a tiny heater (or air conditioner) depending on whether it is absorbing more infrared photons than it is emitting, or vice versa.

When the radiatively active molecules in the atmosphere — mainly water vapor, CO2, and methane — are heated by infrared radiation, even though they are a very small fraction of the total, they are moving very fast and do not have to travel very far before they collide with other molecules of air…that’s when they transfer part of their thermal energy to another molecule. That transfer is in the form of momentum from the molecule’s mass and its speed.

That molecule then bumps into others, those bump into still more, and on and on ad infinitum.

To give some idea of how fast all this happens, consider:

1) there are 26,900,000,000,000,000,000,000,000 molecules in 1 cubic meter of air at sea level.

2) at room temperature, each molecule is traveling at a very high speed, averaging 1,000 mph for heavier molecules like nitrogen, over 3,000 mph for the lightest molecule, hydrogen, etc.

3) the average distance a molecule travels before hitting another molecule (called the “mean free path”) is only 0.000067 of a millimeter

So, there are so many molecules traveling so fast, and so close to one another, that the radiatively active molecules almost instantly transfer any extra thermal energy (their velocity is proportional to the square root of their temperature) to other molecules. Or, if they happen to be cooling the air, the absorb extra momentum from the other air molecules.

From the above numbers we can compute that a single nitrogen molecule (air is mostly nitrogen) undergoes over 7 billion collisions every second.

All of this happens on extremely small scales, with gazillions of the radiatively active molecules scattered through a very small volume of air.

It is rather amazing that these relatively few “greenhouse” gases are largely responsible for the temperature structure of the atmosphere. Without them, the atmosphere would have no way of losing the heat energy that it gains from the Earth’s surface in response to solar heating.

Such an atmosphere would eventually become the same temperature throughout its depth, called an “isothermal” atmosphere. All vertical air motions would stop in such an atmosphere, which means there would be no weather either.

Now, I will have to endure the rash of e-mails I always get from those who do not believe that greenhouse gases do all of this. But that issue will have to be the subject of a later FAQ.