Monday, August 15, 2011

The Tropopause

One reason it's hard to argue with conservatives is that they mostly just can't do the arithmetic. Here is a global warming denialist argument I have been hearing: Venus is so much warmer than Earth not because it has all the CO2 but because it has such a thick atmosphere - about 90 Earth atmospheres worth. This fact alone, it is claimed, means the surface temperature needs to be warmer.

Like Earth, Venus has a tropopause, that is, the temperature decreases as you go up from the surface in a more or less steady way until you get to a point where it starts increasing again - the tropopause.. The denialo-argument says that because the atmosphere is so much thicker, the temperature has to decrease a lot more on the way up to that tropopause. That part is sort of true. The claim that this doesn't have anything to do with CO2 is totally bogus.

Remember that the tropopause is a colder region sitting above one warmer region and below another. That is not a thermodynamically stable arrangement. Why does a planet have a tropause at all anyway?

The first point is that the tropause is a side effect of greenhouse gases in the atmosphere - it wouldn't exist on a planet with a purely transparent atmosphere. Two competitive processes exist for transporting heat from the surface up to space - radiation and convection. Convection can only occur if a parcel of air transported upward expands enough to be lighter than the air above it - when either its sensible or latent heat content will make it warmer than its surroundings in the elevated state.

This condition does not occur when the atmosphere is radiatively thin - radiation transports away any excess heat before it can build up enough below to support convection. The tropopause occurs where the atmospheric opacity at infrared wavelengths becomes small enough for heat to escape rapidly to space. It's that refrigerator above that powers convection from the heater below. In Earth's atmosphere, most of the opacity comes from water vapor and clouds, with a boost from CO2. For Venus it comes mostly from CO2. It's the thickness of the CO2 in the Venusan atmosphere that makes it infrared opague and consequently infernally hot. If it had 90 atmospheres of nitrogen instead of carbon dioxide, things would be very different - and much cooler.