Many readers of this blog would like progress towards the solution of the great questions in climate science. Other readers have stopped by still pondering the basics.
Some of those pondering the basics might have read many of the exciting claims on the internet that the “greenhouse” effect can’t exist because it would violate the 2nd law of thermodynamics.
It’s not a claim that you find in any books on atmospheric physics by the way, it’s strictly an “internet phenomenon”.
Just a few basics in case this is the first post you have read from this site.
The inappropriately named “greenhouse” effect can be summed up in a few sentences:
Longwave radiation from the earth’s surface is absorbed by many trace gases, including water vapor and CO2. The absorption causes these gases to heat up and energy is radiated back out – both up and down. The upward radiation is effectively “no change”. The downward radiation adds to the energy received from the sun and heats up the surface of the earth more than if this downward radiation did not occur.
If there was no absorption of radiation by “greenhouse” gases the surface of the earth would be a lot colder. Here is a very simplified graphic to draw people’s attention to the fact that “something big” is going on:
(TOA = top of atmosphere). If there was no absorption and re-radiation back down the two numbers would be the same. (Note that the downward radiation is not shown to crystallize the issue)
These numbers are global annual averages under a clear sky. Under a cloudy sky the numbers are different but similar – and still the radiation from the surface of the earth is a lot greater than that leaving through the top of atmosphere. For more on this take a look at CO2 – An Insignificant Trace Gas? – Part Six – Visualization and the followup CO2 Can’t Have That Effect Because.. as well as the start of the series on CO2.
Many people have said that the numbers are obviously wrong, I’m mixing up solar radiation and longwave radiation, it can’t happen, the temperature varies a lot from equator to poles so that’s why the radiation numbers are wrong..
As one person said on another blog, possibly commenting on one of these earlier posts:
I even saw one where the guy had 100 watts/m2 going into the atmosphere MORE than was coming out FOREVER.
Sharp-eyed readers might notice that I haven’t drawn in the downward radiation. Energy is balanced in the atmosphere because of the downward radiation from the atmosphere (not drawn). This is the “greenhouse” effect.
Onto the Imaginary Second Law of Thermodynamics
How can a colder atmosphere add heat to a warmer surface?
Can a candle warm the sun?
There are many popular restatements of the imaginary 2nd law. These two should be a representative sample. And so follows the Q.E.D. claim that the “greenhouse” effect plainly contradicts the second law of thermodynamics.
What is the second law?
The Real Second Law of Thermodynamics
My boring thermodynamics books and I have long since had a parting of the ways, so I looked it up on Wikipedia. Not a 100% reliable source, but the (real) second law is just as I remember it so I looked no further.
It’s possible that the imaginary second law has taken a strong hold because anyone who does look it up finds statements like dS/dt>=0, where S is entropy. Wow. Clever people. What’s entropy? How does this relate to candles? Candles can’t warm the sun, so I guess the second law has just proved the “greenhouse” effect wrong..
According to Wikipedia, Clausius expressed the second law (validly) like this:
Heat generally cannot flow spontaneously from a material at lower temperature to a material at higher temperature
Again, that seems right and it doesn’t have any entropy involved in the description. I never did like entropy. It never seemed real.
Perhaps this formulation has been the inspiration for the imaginary second law. As a not very precise definition many people might read this and think no energy at all can flow from a cold body to a hot body.
In fact, no net energy can flow from a cold body to a hot body.
In the case of the real “greenhouse” effect and the real 2nd law of thermodynamics, net energy is flowing from the earth to the atmosphere. But this doesn’t mean no energy can flow from the colder atmosphere to the warmer ground.
It simply means more energy flows from the warmer surface to the colder atmosphere than in the reverse direction.
Another likely reason the imaginary second law has become popular is most people are much more familiar with conduction of heat than radiation. Conduction of heat only appears to flow one way.
A Thought Experiment
We’ll do a thought experiment to demonstrate why the imaginary second law of thermodynamics is wrong. It’s simpler, safer, cheaper AND more reliable than assembling equipment. After all, we are going to look at radiation and if we do an experiment we would need to ensure that no convection or conduction was taking place.
And the thought experiment will, I hope, be more powerful. Plus it will have the added benefit for those already convinced by the beguiling imaginary second law that they can say “you haven’t proven anything, it’s all in your head” and so the popular imaginary law can live on.
In our thought experiment we will consider the sun. It’s hot. It doesn’t conduct or convect any heat outside its immediate surface because space is a vacuum and heat can only travel by radiation through a vacuum.
So energy is radiated out from the sun equally in all directions. At a 1000km distance from the sun, we get our measuring instrument out and find that energy radiated is 10,000W/m2 (because I can’t be bothered to work out the actual number).
Now we fly in a cold large rock and park it at 1000km from the sun. Energy from the sun is absorbed on this cold rock and it heats up to some equilibrium value where it is also radiating out what it is receiving.
The temperature of this large once-cold rock is now a toasty 648K (375’C). All is well with both the real and imaginary formulations of the 2nd law, so far.
Now, from a galaxy far far away, we fly in a new star. Before we started moving it we checked the radiation 1000km away from the star and found that it was 11,000W/m2. We are careful in our relocation of this star that nothing changes in its inner generation of radiation. The new star is parked 1000km away from the once-cold rock and 1000km away from the sun.
It’s a love triangle. Due to the new star’s welcome appearance, the rock heats up further. It now receives 21,000W/m2. Its new equilibrium temperature is 780K. All is still well with both the real and imaginary formulations of the 2nd law.
Trouble in Paradise
But now a problem.. the new star is radiating out in all directions. Believers in the imaginary second law have no problem with the idea that the sun receives energy from the new star. After all the sun was a little colder.
But what about the sun? It is also radiating out in all directions. Or it was before the new star arrived.
Now that the new star is parked 1000km from the sun, squarely in the path of some portion of the sun’s radiation we have to ask ourselves what actually happens?
Believers in the real second law of thermodynamics are quite happy. No cognitive dissonance there. The energy from the sun which is incident on the new star’s surface actually increases the new star’s surface temperature compared with what it was before.
11,000W/m2 are flowing from the new star to the sun, and 10,000W/m2 are flowing from the sun to the new star. Some kind of new equilibrium might be reached, but for real second law believers there is no angst. The net flow of energy is from the hotter to the colder.
Believers in the imaginary second law, what happens?
One obvious suggestion is that the sun’s paltry 10,000W/m2 which was flowing through that exact spot now divert around the new star as if it had some kind of force field. Perhaps all the energy lines completely redistribute so that (depending on the diameter of this new star) about 10,015W/m2 flow in all directions except through the location of the new star.
Another obvious suggestion is that the sun “realizes” the new star is there and energy is flowing from the new star to it so just stops radiating in that exact direction. I put “realizes” in quotes of course because we all know the sun is not sentient. It’s just terminology. Some process that drives the imaginary second law will no doubt make this happen.
And the most likely suggestion of all is that this radiation from the sun, when it strikes the surface of the new star, just bounces off. Or is absorbed but doesn’t actually heat up the surface of the new star (unlike the inner radiation of this new star which does warm the surface from the inside).
I can’t help thinking that all my explanations for the imaginary second law have their own problems. And so I welcome explanations from promoters of the theory for the physical processes that take place near the surface of the new star.
Perhaps the problem is in the thought experiment itself. After all, you can’t just fly a star in from another galaxy and park it close to the sun. Barking mad!
And so the Imaginary Second Law of Thermodynamics lives on!
Update – the imaginary law also covered (possibly created by) On the Miseducation of the Uninformed by Gerlich and Tscheuschner (2009)
Update – a worked example with the maths, Radiation Basics and the Imaginary Second Law of Thermodynamics
Update – and more explanation with reference to one advocates explanation of this imaginary law, Intelligent Materials and the Imaginary Second Law of Thermodynamics