01 October 2015

Why David Evans' greenhouse effect definition can't work

On the Jo Nova website is a recent attempt to portray a greenhouse effect:

Source: New Science 6: How the Greenhouse Effect Works and “four pipes” to space

I think the reduced earth-emissivity theory is the closest thing to a functioning definition of the greenhouse effect there is. (A similar portrayal on the SkS site here).

The idea is that earth's emissivity is reduced by addition of more atmospheric greenhouse gases, because the subsequent emission takes place from the higher cooler layers of the atmosphere.

At first glance, this mechanism appears to be valid. But upon closer examination falls apart.

The only way an object in a vacuum can gain temperature without gaining energy is for its emissivity to decrease.  This is in the Stefan-Boltzmann law of a grey body emission, which notice has no greenhouse term:

j = ɛσT4

The problem with the reduced-emissivity model is that it treats the 3D atmosphere of the earth like a 2D surface.

If you take a sphere with a flat surface and compare it to one enveloped by a thin layer of gas at different temperatures at different altitudes, you won't be comparing apples to apples.

If the surface of emission is from several different levels, such as the ground, the mid-troposphere, the upper troposphere, etc. the SB-law does not apply, since the different layers are not in a vacuum but are connected by air that moves heat between the levels in ways other than EMR, such as convection.

Shifting the emission at CO2-absorption wavelengths from a slightly lower cooler layer of the troposphere to a warmer higher one can only be a temporary state of affairs. 

At first it seems earth's emission is reduced by such a displacement, with the "missing EMR" being absorbed by the greenhouse gas layer.

But such a reduction can not ultimately be sustained because the layer absorbing the heat must necessarily warm. And in turn such warming would then cancel the original emissivity-reducing effect.

We really should be seeing that atmospheric hot spot if there was any credence to the foregoing theory at all. But the hot spot isn't there:

Empirical evidence for lack of magical 33C greenhouse effect. Sourced from the Jo Nova website.

What we find in reality is that no matter how much absorption allegedly occurs, the vertical temperature gradient (VTG, also known as the adiabatic lapse rate) never ever warms – the gradient never lessens.

There is no absorption overall in practice because every photon that is absorbed is matched by one that is emitted. Hence no warming, and no greenhouse effect.

This missing hot spot means that absorption does not exceed emission; absorption and emission are always equal no matter how much of any greenhouse gas is added to the atmosphere (including water vapour).

(Water vapour does keep humid areas warmer than drier areas at night. But that is because the adiabatic lapse rate is changed by water vapour not a greenhouse effect.)

Nor has the emission of OLR to space as measured by satellites reduced over the last few decades during the time of greatest CO2 increase:

Other alleged CO2-warming fingerprints also are missing.

Warming has ceased for almost 19 years:

And cooling in the stratosphere, another alleged fingerprint, ceased 21 years ago:

Stratospheric fingerprint fail: cooling stopped 21 years ago

The EMR flux internal to any object can not warm it.  Hence discussions of geometries, the curvature of the earth, "pipe" analogies etc., don't come into play.

In terms of emission, only the emissivity on the outside of earth makes any difference whatsoever to its temperature.

In summary, the mistake is to assume the vertical temperature gradient is never-ending and eternal, and somehow thermodynamically independent of the heat it supposedly traps.

If an upper, cooler layer of air really was trapping heat, then that layer would warm until the layer emitted as much as before, and the initial emissivity-reducing effect was cancelled. 

The VTG is the confounding factor that, to many, appears to allow a greenhouse effect to exist. Yet the utter reluctance of the VTG to warm due to such greenhouse absorption, shows there is no overall absorption happening in the atmosphere despite the upper layers being cooler.

Whatever factor it is that maintains the tropospheric VTG – and I'm not convinced we yet fully understand that factor – that mechanism is more important than, and overwhelms any, alleged greenhouse effect.

No comments:

Post a Comment