On 6/19/07, Tom Adams <[EMAIL PROTECTED]> wrote:
> Perhaps its a good time for me to ask about something I have been
> wondering about.
>
> Does this concept of "climate sensitivity" (currently estimated at 3 C
> per doubling of CO2) have a tight definition?
Yes. It essentially is about the equilibrium response of the
atmosphere-ocean-sea-ice system, given a carbon input. Ice sheet
dynamics is explicitly left out. I am not sure how surface albedo and
evapotranspiration are coupled in these models; in the real world
these can be important locally but are unlikely to be a first order
global effect.
Most importantly, carbon cycle coupling is presumed to be part of the
forcing, though; it is taken as given in evaluating "the" sensitivity.
There are efforts underway to change this, to drive the models from
emissions and land usage, and couple land and ocean carbon feedbacks
explcitly. I
'm personally very unconvinced that the huge effort toward gearing up
to very fine scale coupled carbon-cycle/climate models (called ESM or
an Earth System Models) is timely either scientifically or as a source
of practical advice to the policy sector. Note, though, that I am so
much an outlier on this topic that I don't even think it is worth
fighting about just now. Once the outputs of these models start to
appear there will be more basis for discussing how reliable and useful
they are. There is too much institutional momentum right now to bother
making the case against building them.
> I assume it is meant to apply no just to the doubling of CO2 but to
> the doubling of CO2 equivalence of all greenhouse gases?
I think that's right.
> Seems to me (the non-expert) that it takes into account some feedbacks
> but not all? I think it includes water vapor, correct?
> But is it
> meant to include every feedback?
No. The "climate system" is the atmosphere, ocean and sea ice. Land
surface process changes, again, are a grey area.
> Albedo, etc. ?
Again, I am not sure how albedo coupling from land surfaces is
treated. A good question, but probably not a first order question on
the global scale.
> Seems that it could not include the feedback where heating releases
> more greenhouse gases from the permafrost and other places since the
> level of CO2 (or equivalent) is apparently already built into the
> definition. On the other hand, if you are looking at data on the
> history of the climate (history of temperature and greenhouse gas
> levels), then that data would seem to include all the feedbacks. Is
> there (or should there be) another definition of climate sensitivity
> that includes all the feedbacks?
This is a very nicely framed question. There is a more or less
sensible answer but it's subtle.
More to follow. Meanwhile, does anyone else care to take this up?
mt
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