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Why would anybody be surprised ?

Washington Post records that –
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1 comment to Why would anybody be surprised ?

  • The fundamental assumption of the greenhouse effect is that back radiation has warmed the surface from 255K to 288K. But this assumption is itself based on a false assumption.

    Roy Spencer (in his post about Greenhouse misunderstandings) claims in his point (6) that the atmosphere would have been isothermal at 255K in the absence of any GHG.

    An isothermal atmosphere in a gravitational field would violate the Second Law of Thermodynamics, which reads: “An isolated system, if not already in its state of thermodynamic equilibrium, spontaneously evolves towards it. Thermodynamic equilibrium has the greatest entropy amongst the states accessible to the system”

    In isothermal conditions there would be more potential energy (PE) in eash molecule at the top, and, because kinetic energy (KE) is homogeneous, molecules could “fall” downwards and do work in the process. hence it was not an equilibrium state, let alone one of maximum entropy, as is required by the Second Law of Thermodynamics.

    The Second Law of Thermodynamics has to be obeyed. So (PE+KE) has to be homogeneous, because otherwise work could be done, and so the system would not be at an equilibrium with greatest entropy, as the Second Law requires. In the process of reaching such equilibrium it is inevitable that molecules at the bottom have more kinetic energy, and there are more of them in any given volume, and so that does give a measure of higher pressure, yes. But the whole column could still cool down, maintaining the same gradient and pressure.

    So pressure does not maintain temperature. The relationship in the ideal gas law only applies in adiabatic conditions, but the atmosphere can radiate heat away. If you “turned off” the Sun, Venus atmosphere and surface would eventually cool down.

    We need to consider how the thermal energy actually gets into the Venus surface, especially at the poles. The facts are ..

    (1) the poles receive less than 1W/m^2 of direct insolation.

    (2) the atmosphere 1Km above the poles is at least 9 degrees cooler, and not absorbing much insolation either. It could have at most 1W/m^2 coming back out of the surface, which (at 0.5 absorptivity) would raise it to a mere 7K.

    (3) Rather than being 7K, the lowest Km of the Venus atmosphere is around 720K, just a few degrees less hot than the surface.

    If all convection (resulting from absorbed incident insolation at various altitudes) only went down the thermal gradient (ie towards space) how would enough energy get into the surface, especially if it were even just 1 degree hotter than the base of the atmosphere?

    My answer is that the sloping playing field (the thermal profile) becomes a level playing field due to gravity, so all energy absorbed in the atmosphere (mostly incident insolation) spreads out in all directions, creating convection both up and down, and also diffusion and convection right around the globe producing equal temperatures at equal altitudes, but higher temperatures at lower altitudes. Then intra-atmospheric radiation reduces the magnitude of the net gradient by about 10% to 15% on Venus, (as best I can work out) but by about a third on Earth. Some of the extra reduction on Earth. though, is probably due to release of latent heat.

    Here’s a thought experiment. Construct a perfectly insulated sealed cylinder filled with pure nitrogen gas. Suppose there are two insulating dividers which you can now slide into place one third and two thirds up the cylinder, thus making three equal zones. Warm the middle zone with a heating element, which you then turn off. Allow equilibrium to establish with the warmer nitrogen in the central zone. Then remove the dividers. Those molecules which move to the top zone will lose some KE as they gain extra PE, whereas those which fall to the lowest zone will gain KE as they lose PE. Hence, when the new equilibrium is established, the highest zone measures a lower temperature than the middle zone, and the lowest zone measures a higher temperature than the middle zone. Hence the highest zone measures a lower temperature than the lowest zone. QED.

    So there is no need for any greenhouse effect to raise the surface temperature, simply because gravity cannot help but do so, because the atmosphere must obey the Second Law of Thermodynamics.

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