New talkshop visitor ‘David’ has dropped a fruitful link on Wayne Jackson’s recent thread which, after a bit of sleuthing via AstroBio.net, leads to a new paper from the Trieste Astrobiology Group led by Giovanni Vladilo. This will be of great interest to our friends Nikolov and Zeller, because it vindicates their contention that atmospheric pressure is the principle determinant of planetary surface temperatures. However, there is a twist. As well as affecting the near surface heat capacity, evaporation rates and meridional energy transport, atmospheric pressure also affects the atmospheric optical depth of atmospheres, and this explains the role of ‘greenhouse gases’ and their radiative properties in contributing to the overall distribution and magnitude of energy at planetary surfaces. Although not dscussed in the paper, I think it will also be the case that regardless of extra emissions of a greenhouse gas such as carbon dioxide, since the pressure is the primary variable, the optical depth will remain constant, as NASA Physicist Ferenc Miskolzci found. If so, the Man Made Greenhouse Panic is over.
The key passage from the paper is this one:
4.2.1. Surface Pressure and Planet Temperature Variations of surface pressure affect the temperature in two ways. First, for a given atmospheric composition, the infrared optical depth of the atmosphere will increase with pressure. As a result, a rise of [pressure] will always lead to a rise of the [radiative] greenhouse effect and temperature. Second, the horizontal heat transport increases with pressure. In our model, this is reflected by the linear increase with [pressure] of the diffusion coefficient D (Equation (A5), Appendix A.2). At variance with the first effect, it is not straightforward to predict how the temperature will react to a variation of the horizontal transport. In the case of Earth, our EBM calculations predict a rise of the mean temperature with increasing D. This is due to the fact that the increased diffusion from the equator to the poles tends to reduce the polar ice covers and, as a consequence, to reduce the albedo and raise the temperature.
The main thrust of the paper is the effect of pressure has in widening or narrowing the ‘habitable zone’ for a planet in relation to distance from its star.
At the Trieste Astobiology Group’s website, further scenarios testing the model are also animated:
More animations showing the seasonal evolution of the surface temperature are provided here. This is the Earth at p=0.2 bar: one can see that, due to the low surface pressure, the mean global annual temperature is low (Tm=-12 C) and the seasonal excursions of the zonal temperatures are quite intense. This is the Earth at p=5 bar: in this case, due to the high pressure, the mean global annual temperature is high (Tm=+52 C) and the seasonal excursions of the zonal temperatures are quite moderate. A more exotic case is shown here, we you can see an Earth-like planet with axis obliquity at 90 degrees and p=0.3 bar: in this case an equatorial belt of ices is formed as a consequence of the high inclination of the planet rotation axis.
This pdf of the group’s 2012 AGU presentation makes the effect of pressure on temperature even clearer. Look at the temperature difference for exoplanet GI581d between 7 and 8 bar of pressure! H/T