Tuesday, May 14, 2013

Data Wars: Climate

I recently got to argue with a bunch of old friends and former colleagues about climate. It seems that they had formed a little group of climate warriors who would meet for breakfast and the bashing of the conventional climate wisdom. One point of physics they were confused on was the surface temperature of Venus. the guru of their group, a retired meteorologist and meteorological researcher, was convince that the thickness of the atmosphere and the gas law alone could explain the temperature at the surface. Of course they also didn't know that Venus, despite being only 2/3 of our distance from the Sun, absorbed less solar radiation than Earth.

Heat is transported in the atmosphere through three mechanisms: conduction, convection, and radiation. Most of the heat energy we get from the sun is absorbed at the surface, and since we know that the planet has been at roughly the same temperature for billions of years, essentially all of that absorbed energy is returned to space. Space is too empty to carry away heat by conduction or convection, so that all that energy the planet absorbs is reradiated into space. The absorption of solar energy, as I mentioned, takes place at the surface, but most of the reradiated energy is emitted from a region near the top of the troposphere. That fact, and the processes that mediate it, is responsible for greenhouse warming. If the atmosphere were transparent to infrared radiation the re-radiation would take place from the surface, and the planet would be a lot colder than it is.

The lower part of the Earth’s atmosphere, the troposphere, has a temperature structure that is dominated by convective processes. That fact is due to it being relatively opaque to infrared radiation – when radiation transport is efficient it dominates convection since light moves a lot faster than molecules. The dominance of convection produces the well known decreasing temperature with altitude, with a rate of decrease that tends to approach but only locally exceeds the adiabatic lapse rate – that is the lapse rate achieved by transporting a parcel of atmosphere adiabatically to higher altitude.

It’s this low atmosphere dominance of convection over radiation that creates the troposphere, and when the density of absorbing gases gets small enough, radiation dominates and convection halts. Note that putting more absorbing gases into the atmosphere tends to increase the depth of the troposphere, since it increases the height at which radiation dominates.

Once we appreciate this point, we can see why Venus has such a thick troposphere. Of course it is the thick atmosphere, about 90 times as massive as that of Earth that makes it possible, but it is the opacity due to that thick atmosphere which is responsible for the thickness of the convecting zone. An equally massive atmosphere of much smaller opacity would lead to a lower tropopause and surface temperature.