Greenhouse gases A post (below) on the Bishop Hill blog relating to climate change asserts that no warming effect can be attributed to $\mathrm{CO_2}$. I don't know whether the author is really a physicist but it sounds impressive (Planck spectra and black-body radiation etc). Can someone explain in layman's language whether the assertions are valid.
Quote from http://www.bishop-hill.net/blog/2012/4/5/greenhouse-reversal.html
There is no greenhouse, so it can't be reversed.

Many so-called skeptics are not really basing their arguments on the true
physics of the atmosphere. By failing to do so, they are demonstrating
that they also have fallen for the IPCC bluff that radiation from a cooler
atmosphere (including so-called backradiation, but also initial radiation)
can transfer thermal energy to a warmer surface.

This is not correct physics and the sooner this is made clear to the
public the better. True physics, backed up by basic phenomena such as the
fact that radiation in a microwave oven is not absorbed in the usual sense
of the word, shows why this is the case. No one has ever proved anything
to the contrary in any empirical experiment, and never will.

The only thing any such radiation from the atmosphere can do is slow down
that third or so of surface cooling which occurs by way of radiation that
does not escape to space via the atmospheric window. Radiation from the
atmosphere can have absolutely no effect on evaporative cooling, chemical
processes or sensible heat transfer. These non-radiative components plus
the radiation to space make up about 70% of all surface
cooling. Furthermore, the effect of carbon dioxide with its limited
frequencies is far less than a true blackbody, and less per molecule than
water vapor. No gas can radiate outside its Planck spectrum (i.e. more than
a true blackbody) and so there is no way that carbon dioxide (1 in 2,500
molecules) can contribute a very large amount of radiation anyway.

The other cooling processes merely accelerate and compensate for any
minuscule slowing of radiative cooling. Thus there is absolutely no
warming attributable to carbon dioxide. It is time for skeptics to get
their facts right and stop giving in to part of the hoax. Only truth will
prevail in the long run. 

EDIT:
The author of the blog post was 'Doug Cotton', who has published a related paper at http://principia-scientific.org/publications/psi_radiated_energy.pdf and has a website at http://climate-change-theory.com/
 A: Let's look a bit closer at the claims of Doug Cotton, and of  Claes Johnson, whose work Doug relies upon. It's important because this is one of the strongest claims made by those who choose to reject the notion of anthropogenic climate change.
Here's the core claim, from Doug Coton:

The assumption is made that so-called "backradiation" from a cold atmosphere is able to transfer thermal energy to a surface which is warmer than the source of the radiation. This is a physical impossibility as is proven theoretically by Prof Claes Johnson and empirically by Prof. Nasif Nahle

The claim, contradictory to over a century of thermodynamics, is that a body will only radiate heat towards bodies colder than it, and never towards bodies hotter than it. Note that this is not a claim about net heat transfer, but about any radiated heat.
Apparently, this happens through the following mechanism according to Professor Claes Johnson:

[a body] reads the temperature of the surrounding from its spectrum, and then decides to cool or warm depending on its own temperature

The Johnson/Cotton theory is that almost everyone other than them misinterprets the Stefan-Boltzmann Law, and that there is no such thing as photons of infra-red radiation.
And the problem with all of this is that this is an extraordinary claim that lacks any evidence at all. Thermodynamics, the theory of photons, and of black-body radiation, have all made astonishingly successful predictions, and are supported by many decades of empirical evidence.
A: The article you quoted frankly reads very poorly. It quotes a lot of stuff without once noting that greenhouse effects absolutely are real and critical to the earth being habitable. I don't know who this fellow is, but if he posted here directly I'd give it an instant negative vote.
You, sir, I'm giving a thumbs up for taking the trouble to ask in a forum where you are likely to get some answers. More people should do that when they hear odd science claims!
Now, with that said, it's absolutely true that both carbon dioxide and methane are bit players in the overall greenhouse effect.
The main greenhouse is water vapor, by about two orders of magnitude. My recollection without looking it up is that 97 to 98 percent of the greenhouse effect is caused by water vapor. This is why it gets so cold in the desert at night, for example.
The Nobel-prize winning models for global warming do not invoke direct warming from carbon dioxide. Instead, they postulate and model using computer programs the idea that the very small additive impacts of carbon dioxide, methane, and other minor greenhouse gases throw off the balance of the major player, water vapor. I do not know how they do that part of the model. It has to be complicated, since water vapor levels vary with near-fractal complexity from day to day and from region to region.
A: I believe the professor Johnson, for one simple reason, If you heat a piece of metal and subject it to air temperature of the room it will cool off fast at first, then as the air and metal temperatures come close to the same, before slowly coming the the normal ambient of the room, this is all basic thermodynamics, 
  And in my limited knowledge, there are a lot of things that I still am learning about, however there is on thing that is conveniently omitted in the GH Gas circles, is water vapor, and the variable amount of sunlight we receive each day. 
  we on earth see the same sun each day, but have no idea of how much radiation it sends our way, and that the sun spots actually increase the amount of radiated heat we get, these gusts of solar radiation (winds)have been correlated with ice core samples that show that during the little ice age a few hundred yrs back, correspond to a period of little or no sunspots during that period, and we have just recently come through a period high spot activity,  but when all appeared calm seem's that 2 spots have reappeared, Personally I think it's absurd that whatever amount of CO 2 we add to the atmosphere, there is no way that we can overpower the sun and water vapor.        
