Re: [Phys-l] Albedo and GW.

[Bernard Cleyet <bernardcleyet@redshift.com>]

Panzers, however, I had forgotten about water and was thinking of CO2

http://ceos.cnes.fr:8100/cdrom-98/ceos1/science/dg/dg1.htm

Shows the importance of water -- CO2 is not as obvious, and not very 
relevant as JD showed.

bc

John Denker wrote:

> Bernard Cleyet wrote:
>  
>
> > "The melting of the Arctic and Antarctic ice
> > caps reduces the fraction of solar energy reflected back into space,
> > and so increases the temperature further."
> >
> > Is my assumption correct that there is an asymmetry ...
> >    
>
> BC raises an excellent point.
>
> In the absence of a strong wavelength-dependence in the albedo, the
> "ice-albedo feedback" argument would be diametrically wrong, as you
> can see from the following line of reasoning.  If we temporarily and
> hypothetically assume a wavelength-independent albedo, then:
>
>  1) Any patch of surface has its own radiative-equilibrium temperature, Tr.
>
>  2) A typical patch of surface in the arctic has an average temperature
>   *above* Tr, because of non-radiative heat "leaks" coming in from warmer
>   parts of the earth.
>
>  3) Therefore, each such patch is (on average) emitting more radiation than
>   it receives.
>
>  4) Melting lowers the albedo. This raises the efficiency of process (3), which
>   in turn causes global cooling and negative feedback (not runwaway global warming).
>
> But there *is* a strong wavelength-dependence in the albedo.  Ice, as you know, looks
> white;  i.e. it has a high albedo in the visible wavelengths.   Melting lowers the
> albedo in the visible, but doesn't greatly lower the albedo in the thermal IR, because
> the latter was near zero to begin with.
>
> Reference:  figure 1 in:
>  http://fiesta.bren.ucsb.edu/~dozier/Class/esm236/Reading/Konig.pdf
>
> So all those guys who talk about ice-albedo feedback are not crazy ... although
> they have done a terrible job of explaining the physics of the situation.
>
>
>  
>
> > Without an atmosphere they would be equal?
> >    
>
> Nope.  Dry air is nearly transparent, in the IR as well as visible.  You can
> verify this for yourself by walking around in the high desert at night:  skin
> exposed to the sky feels cold, even if the air is not particularly cold.
>
> The air in the arctic is verrry dry.
>
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