... this topic has been bugging me all weekend.
The source of my ire is statements like this:
"Certain gases in the atmosphere are transparent to the incoming short-wave solar radiation but trap (absorb) the outgoing long-wave terrestrial radiation. This increases the kinetic energy of the gas molecules causing the temperature of the atmosphere, and subsequently the Earth’s surface, to rise.
Most absorption of infrared radiation takes place in the lower atmosphere, the troposphere. This warming phenomenon (33C) is known as the natural greenhouse effect..."
Nobody disputes that the average surface temperature at the surface is 33C higher than it 'should' be. But it is quite simply untrue, and few people are daft enough to claim, that the whole atmosphere is 33C warmer than it 'should' be.
I find it cathartic if I write things down on this blog. So here we go again (hopefully for the last time)...
As always, first write down what you know and what most people agree on (or can easily Google for themselves, which I why I haven't bothered putting in many links).
1. Earth's gravity pulls everything, including air, down. It must do, or else it all would just float off (as do some very light atoms).
2. Atmospheric pressure at sea level is whatever it is, defined as 760.00 mmHg; 1013.25 mbar; or 101.325 kPa. Those are just units.
3. Every gas molecule (most of the atmosphere is molecules, let's ignore the few atoms) is pulled down by gravity and pushed down by the air above it; it is also pushed up by the air (or ground) beneath it. At any height, the forces must balance overall.
4. Let's not worry about the subtle differences between conduction, convection and radiation. Gas molecules are so close together as makes no difference. In practice, the biggest movements are winds, which usually go horizontally, not up and down. But for the sake of this explanation, let's assume a calm, still night.
5. The main source of heat for the Earth's surface and atmosphere is the Sun. It heats by day and things cool down at night. In the absence of an atmosphere, the average temperature on Earth would be the same as on our Moon i.e. 255K. The atmosphere increases that by 33C/K to 288K.
6. Air pressure and temperature fall steadily as you go upwards. This is hardly surprising. Temperature is just a measure of "how often atoms or molecules bump into each other". The more there are, the more bumps and the higher the temperature; the fewer there are, the fewer the bumps and the lower the temperature.
7. The molecules at the surface can't rise to dissipate heat (as some people argue), they are only warm because of where they are. The pressure/heat some think should cause them to rise is equal and opposite to the pressure from the air above pushing them down and gravity pulling them down. You might as well ask, why don't the colder molecules fall down. They are being held up by air pressure from below. And if some swap places, so what? The equilibrium is unchanged. As I said, in this explanation, it is a calm, still night.
8. If you are clever enough, you can calculate the rate at which temperature and pressure fall with height. Those, and the relationship between them are derived from other constants and equations.
9. In practice, assuming average humidity, temperature falls/increases by 6.5K for every km you go up/down in the lowest 11 km of the atmosphere (aka the moist adiabatic lapse rate). And pressure falls/increases by about 12% for each km you go up/down. These rates get smaller and smaller as you go up towards and past 11 km, but that need not concern us down here on the surface and where the actual weather happens.
10. Note - the height of 11 km and the lapse rates I'm using are averages. Over the Poles, the atmosphere is thinner and the lapse rates are higher; over the Equator, the atmosphere is thicker and the lapse rates are lower.
11. Let's bring that 255K expected temperature (from 5. above) back in to the proceedings. There's a certain amount of air in the atmosphere. If you start at sea level, 288K and deduct 6.5K for every km you go up, and you go up until half of the air is beneath you and the air pressure is half what it is at sea level i.e. up 5.5 km, the temperature is 288K minus (5.5km x 6.5K/km) which is pretty close to 255K. In other words, the overall average temperature of the atmosphere is +/- 255K
12. Unsurprisingly, above that height, temperature and pressure keep falling, all the way down to 217K (-57C) and close to zero pressure, where it flattens off (and need not concern us). The bottom half is warmer than you are supposed to expect (255K) ... and the top half is colder than what you are supposed to expect (255K). All that heat that should be there is simply in the lower half.
13. So far this has all been non-contentious, consensus stuff I hope. Here's the contentious bit - mainstream global warming theory explains why it's warmer than expected at the surface. But does not explain why it's colder than expected higher up. If the surface were 33C warmer than expected and the top of Mount Everest were at least a tiny bit warmer than expected, or at least, no cooler than expected, fair enough. But it's not. It's a damn' sight colder than expected.
14. Crude analogy - imagine a long spring with regularly spaced coils. You turn it vertically, rest one end the Earth's surface and pull up the other end until just before the point where the bottom end is lifted from the surface. The coils which started off regularly spaced will now be closer together at the bottom and further apart at the top end.
15. That's the same as the distribution of molecules in the atmosphere. The apparent temperature difference is merely a result of them being squashed together at the surface and pulled apart at the top, see 6. above. Which is why it's so cold at the top of Mount Everest, even on a sunny, still day.
16. So we can rephrase the original statement thusly:
"The presence of the atmosphere and the effect that gravity has on its density cause the Earth’s surface temperature to be +/- 33C warmer than it would be in the absence of an atmosphere. It is conjectured by some that gases such as CO2 or CH4 might increase this effect, but the amount of warming they cause is difficult to establish."
Debate Magazine
