Posted on Dr Tamsin Edwards site 17 hours ago and approved by her. No reply yet:
I have spoken to many climate scientists who seem to think that at equilibrium, the troposphere would be isothermal, in accordance with the Maxwell-Boltzman distribution. They argue that there would be no lapse rate without radiation and convection. Do the models make this assumption too?
If they do, it seems like a grave error to me. The true situation isn’t like Maxwells ‘isolated column of gas’ for several reasons. Although it is bounded by Earth and space, its volume is not constrained. NASA has confirmed observationally that since the Sun went quiet in 2004, the thermosphere has shrunk by 30% and the average altitude of the cloud deck has dropped by 30m.
Gravity acting on the mass of the atmosphere produces a pressure gradient by pulling the gas against the solid Earth. Because air is compressible, that pressure gradient acts to produce a density gradient. Because the molecules nearer the surface are pushed closer together, there are more of them in a given volume. That makes it more likely that water vapour and co2 molecules in the volume will intercept and absorb photons of incoming solar energy and photons of outgoing long wave radiation.
Because the path length is short, these energised molecules soon share their extra energy with surrounding molecules of nitrogen and oxygen which make up the bulk of the atmosphere. The ensemble has a heat capacity. Therefore, the denser near surface air will get hotter than the less dense air at altitude.
Naturally, in accordance with the gas laws, the warmer air nearer the surface will have been expanded by the higher temperature and its density thus lowered again, but this won’t fully offset the effect due to gravity acting on mass to raise pressure and density to form a gradient. (We should calculate the value).
So even before we consider convection and radiation, there will be a ‘pre-existing’ lapse rate due to these simple thermodynamic-gravitational considerations.
Is it included in the models?