> I get the general idea of a tipping point, in short and in the extreme,
> once ice sheets start breaking down, we may get to a new equilibrium,
> where even pre-industrial carbon dioxide levels won't be enough to get
> us back to the previous equilibrium.
>
> Hansen writes that it is commonly assumed that ice sheet disintegration
> will take millennia, and he thinks, though that is outside of his area
> of expertise (he actually says that in the paper I linked in my reply
> to Coby), it's probably only a few centuries.
>
> To get from that to a decade by pointing to lags in energy
> infrastructure is where I either misunderstand him, or think his
> argument is very weak.
Hansen begins the article you quote with the following paragraph:
"In a recent article (Hansen, 2004) I included a photograph taken by Roger
Braithwaite with a rushing stream pouring into a hole in the Greenland ice
sheet. The photo relates to my contention that disintegration of ice sheets
is a wet, potentially rapid, process, and consequent sea level rise sets a
low limit on the global warming that can be tolerated without risking
dangerous anthropogenic interference with climate."
I suspect that you do not realise the significance of a "wet process" and
why it is potentially rapid. Wet in this case means water, not ice or
steam. But water forms water vapour, a cold steam, and water vapour is the
main greenhouse gas. The problem with water is that the water vapour it
produces increases exponentially with temperature (because of the
Clausius-Clapeyron relationship.) Thus once the surface of an ice sheet
starts to melt and become wet, then there is potenitally a runaway effect as
the surface water warms and the greenhouse effect from the water vapour
increases causing more melting.
This means that once an ice sheet starts to melt there is no way to stop it
melting completely. So the tipping point is not when the ice sheet
disappears, but when it starts melting. That is the point of no return.
Hansen obviously thinks we have ten years in which to prevent the ice sheets
from beginning to melt. I, and Manlowski think otherwise.
Cheers, Alastair.
>
> >
>
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