On Jan 14, 9:33 am, "[email protected]" <[email protected]>
wrote:
> > 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.
>
> Very strictly speaking there is no boiling. Boiling is the formation
> of bubbles of vapour in the water, because the pressure inside the
> bubble is bigger than the pressure of the atmosphere. For a kettle
> exposed to the open atmosphere, that external pressure stays constant
> at 1 atmosphere. The water vapour released has no measurable impact on
> the external pressure.
>
> When the whole world warms to 100C, no bubbles with water vapour will
> form in the ocean; rather the atmospheric pressure will go up to
> around 2 atmospheres.
>
> But that does not mean the water on Earth could not in its entirety
> turn into vapour (though strictly speaking again, the 400 bar or so
> implied from evaporating 4 km of ocean get us beyond the critical
> point for water of 214 bar and 372C beyond which there is no phase
> boundary between the liquid and gas phases of water)
Yes you are correct. The oceans would not boil if the only heat was
from solar radiation. They would evaporate. But the issue is not
would they evaporate. It is would they evaporate completely as it is
surmised has happened on Venus. What I am saying is that this is not
possible because with such a high vapour density, the world would be
covered in cloud and the resulting albedo would reduce the solar
radiation that was causing the evaporation.
This also means that if Venus had the same amount of water as Earth
then it must have lost its water clouds to the solar wind.
> > 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.
>
> Just talk about tipping points that result in Earth moving from one
> stable equilibrium to another. Then people won't think you are talking
> about Earth turning in Venus.
The problem is that there are two types of tipping point. The first
is when the overall feedbacks become positive, and the system is no
longer stable, and the second is when the positive feedback exceeds
100% and the system runs away. Thinking about positive feedbacks is
pretty muddled especially in the climate modeling community. For
instance Gavin Schmidt and William Connolley have tried to get each
other to correct Wikipedia. I don't dare do it, although I have
tried.
Cheers, Alastair.
--
You received this message because you are subscribed to the Google Groups
Global Change ("globalchange") newsgroup. Global Change is a public, moderated
venue for discussion of science, technology, economics and policy dimensions of
global environmental change.
Posts will be admitted to the list if and only if any moderator finds the
submission to be constructive and/or interesting, on topic, and not
gratuitously rude.
To post to this group, send email to [email protected]
To unsubscribe from this group, send email to
[email protected]
For more options, visit this group at
http://groups.google.com/group/globalchange
