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Re: [Phys-l] Albedo and GW.

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

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


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:

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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