Rational & Irrational Earth Climate Energy Budgets
Alan Siddons writes: I want to comment about rational and irrational energy budget models here. First, consider the rational example show above.
In this kind of model there’s a constant flow of radiant heat to several absorbers, which mix to some extent but maintain a certain temperature DUE TO a constant flow. It’s like warming your hands over a campfire: Your warmer hands are a consequence of the fire’s heat, not a means of cooling it.
So too, if there were nothing in the way of the surface’s 51{154653b9ea5f83bbbf00f55de12e21cba2da5b4b158a426ee0e27ae0c1b44117} emission here, that 51{154653b9ea5f83bbbf00f55de12e21cba2da5b4b158a426ee0e27ae0c1b44117} would exit directly to space. As it stands, of course, all of the various absorbers/emitters and reflectors together form a 100{154653b9ea5f83bbbf00f55de12e21cba2da5b4b158a426ee0e27ae0c1b44117} return to space.
Now the irrational:
As presented (above) by the National Center for Atmospheric Research:
- In this model, something being heated by the surface represents a TEMPERATURE DROP for the surface. Observe the arithmetic at play.
- 161 hits the surface, which “loses” 17 and 80, i.e., 97, and its temperature FALLS to that extent.
- 161 minus 97 equals 64.
- And 64 W/m² equals minus 90° Celsius for a blackbody!
- So how much compensatory radiation is required to bring the surface arithmetically up to the desired 396 (16°C) with an extra 1 watt for “net absorbed”? 333. Because 64 plus 333 equals 397.
- And that’s exactly what the “downwelling” supplies.
Given what SHOULD be a constant 161 W/m², however, and a consequent steady temperature of minus 42°C, “downwelling” would only have to be 235 to bring the surface (arithmetically) up to 16°C. One may object to such flimsy physics but at least this standard is more honest.
Dr Charles Anderson adds:
You are on the right track Alan, but you can go much further with this.
There is no way that the surface of the Earth radiates as much infrared radiation as it would if there were no competing thermal loss mechanisms at the surface. A surface in vacuum satisfies the Stefan-Boltzmann Law, while a wet surface in air does not. To be more exact, portions a macroscopic surface area will obey the Stefan-Boltzmann Law, but other portions will be losing energy to bombarding air molecules or to the latent heat of evaporation of water.
The net total power emitted from that macroscopic surface area cannot equal that predicted by Stefan-Boltzmann though you will see a lower power spectrum of the expected frequency distribution with a wee bit of superposition of lower temperature frequency distributions superimposed.
The Earth’s surface does not and cannot emit the 396 W/m^2 here. Also, the atmosphere cannot absorb the 374 W/m^2 of the surface emitted radiation as shown in the 2nd Energy Budget. A black body absorber would have to be at a temperature of 140K to absorb that much of the radiation from the surface. The atmosphere is not a black body absorber. It has a window which allows about one-third of all surface radiation to escape to space. No part of the atmosphere is at as low a temperature as 140K either.
To find such a low temperature in our solar system, one has to go a distance of more than 6 Earth orbit mean radii from the sun! This particular diagram claims only 22 W/m^2 is emitted through the atmospheric window. More recent Earth energy budgets make that value about 40 W/m^2. This is one value NASA ought to be able to measure with some reliability. Three times the atmospheric window loss of 40 W/m^2 would imply about 120 W/m^2 was emitted from the surface as IR radiation.
Now adding up 161 – 0.9 – 17 – 80 -120 = -56.9 W/m^2. Back radiation cannot be great enough to supply even 56.9 W/m^2, especially once one realizes that surface radiation cannot be as great as 396 W/m^2 and the atmospheric absorbed portion of that cannot be anything like 374 W/m^2. Some other mechanism has to warm the surface.
Thanks to the fact that water vapor does absorb about 80 W/m^2 of outgoing surface emitted radiation, it absorbs a substantial portion of the incoming solar radiation in the atmosphere also, it transports a great deal of energy into the atmosphere as latent heat of evaporation, and thermals carry still more heat up into the atmosphere, there is a great deal of power to be radiated from the upper troposphere.
This in turn allows the temperature gradient in the atmosphere due to the exchange of potential energy for kinetic energy as one proceeds to lower altitudes to provide the surface with a much higher equilibrium temperature. The temperature of an ideal or perfect gas is proportional to its kinetic energy.
If you are interested, here is a set of articles I have written recently that discusses these and related issues:
mgh, Not Just Greenhouse Gases, Provides a Warm Earth
A Critical Lesson from the NASA Earth Energy Budget
The Settled Science of Catastrophic Man-Made Global Warming Violates the Laws of Physics
Water Vapor and Gravity Act Together to Warm the Earth
The fact that infra-red active gases, together with competing surface cooling mechanisms, and the transport of large amounts of energy by non-radiative means to the mid and upper troposphere causes the Earth to be warmer, does not mean that the addition of more water vapor or carbon dioxide would warm the Earth. We know that more water vapor actually cools the Earth and the same is almost sure to be true of more carbon dioxide as well.
In any case, once you realize that the absorption of surface emitted IR is only a part of the reason that the Earth’s surface is not much colder than it is and that CO2 plays no role at all in two of the main mechanisms out of the three main ones, it is very clear that the role of CO2 has been greatly exaggerated.
Knocking down the surface emission and the back radiation greatly already much reduces any role that carbon dioxide can play.