On Jan 13, 10:41 am, DrNickBone <[email protected]> wrote:
> Reasonable GCMs do sometimes show runaway warming
They show a fully cloud covered earth, similar to the outward state of
Venus, but they do not show a boiling away of the Earth's oceans.
> - Arguments that it could never happen (because of clouds etc.) are
> a bit "handwaving" in comparison
I cited a paper by Pierrehumbert which showed that a boiling away of
the oceans was not possible because the as the oceans boiled the
vapour pressure would increase the atmosphereic pressure so raising
the boiling point of the oceans.
> - Runaway warming could happen if the Earth's temperature gets above
> 300K
It is generally believed that if the global temperature rises by more
than 2K then that could be dangerous. But some island states are
pressing for a figure of 1.5K to be accepted, as even that could be
dangerous to them.
> - That level of warming (about 13 degrees above pre-industrial)
> is a stretch,
That has been seen in a very few models, but they could be right.
> but not impossible if we push CO2 up to 4 x pre-industrial levels,
> and then add lots of feedbacks (like methane release)
It seems likely that if we push CO2 high enough then we can get any
warming. The problem is that we won't stop pushing until it is obvious
it is too high, and by then it will be too late.
> - There is a historical argument that CO2 has been very high before,
> without triggering runaway, but apparently we can't be sure
> about that either.
We can be sure that CO2 has never been high enough to produce a Venus
effect, because if so we would not be here. Although, life does seem
to have survived Snowball Earth.
We do know that abrupt warmings have happened in the past, such as the
end of the Younger Dryas, and the PETM event.
What I am saying is that an abrupt climate change is a runaway event.
But it need not result in the Earth being in a similar state to
Venus. It is the term "runaway" which is confusing, and I have been
told not to use it, but I do because to me it means a positive
feedback of more than 1.
> Not very reassuring I'm afraid.
No it's not, even where I disagree with you.
Cheers, Alastair.
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