> Perhaps my previous message was not as clear as it could have been. On
> an initial doubling of CO2, the imbalance is 4W/m^2 before there are any
> other changes. As the atmosphere warms, the water vapour increases but
> the radiative imbalance *decreases* anyway, because the warmer
> atmosphere is closer to radiative equilibrium (ie outgoing LW radiation
> increases). A high sensitivity just means that the disequilibrium
> decreases slowly as the temp increases. If the disequilibrium actually
> increased with warming, the system would be unstable.
I was looking at the forcing multiplier, as if it could happen without
the concurrent temperature increase. That's very unreasonable for a
smooth temperature forcing function, and not so unreasonable with a
tipping point, where suddenly cloud cover or relative moisture
changes.
Without tipping point like changes, I suppose a lot less than 4 W/m2
is actually available averaged over 100 years to melt ice, as most
will go towards warming the upper ocean waters and as the imbalance
decreases as the temperature goes up.
On the other hand, using all the 4 W/m2 pretty much implies a tipping
point like change. The temperature of the Earth stays constant thanks
to all the melt water and still the ice is suddenly melting, when
before it was not.
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