It is barely possible for models to run for decades and track decadal
changes that are sufficiently small.
Until quite recently climate models were only run in a
quasi-stationary world, trying to capture a particular climate.
Recently, they have more or less replicated the observed changes of
the twentieth century, but those changes are small. It seems fair to
say that the models have some validity for changes sufficiently small.
Nobody really knows how large the changes would have to be for the
models to fail to track the changes. The more we rock the boat the
less we can trust our models of the boat. Climate is not unusual in
this regard.
The climate system sensitivity is fairly well constrained to 3 C per
CO2 doubling. James had a lot to do with firming up this constraint. I
think it's solid. The idea that it is in excess of 4.5 K is so
unlikely as to be in asteroid bombardment territory. You have enough
to worry about without focusing on that.
The main problems with this 3 C per doubling value is that it excludes
carbon feedbacks in the system, so it doesn't tell us the sensitivity
with regard to a unit of emissions. Nature added carbon in a closed
loop. We have a good idea about themagnitudes of this phenomenon
during the quaternary (the most recent 1.5 million years) but we don't
have a good grasp on the mechanism. The CO2 now being injected into
the system is being injected open loop; that is, far faster than most
postulated response mechanisms in pre-anthropic nature. We know the
very short-term response of the carbon cycle and we know the very long
term response, but the transient bumps along the way are on the time
scale of human history, and we have very little grasp of what to
expect on those scales.
If we understood the carbon cycle, we'd still have to model the
response of the oceans to large transients. Our ocean models are
arguably not up to the task either.
So it is beyond present modeling capacity to explicilty model very
large changes and/or very long time scales. People have high hopes in
this regard but I personally think the relevant scientific community
is not going about it very well. Be that as it may, you are asking
good questions, and I don't think we are ready to answer them.
mt
On Nov 24, 2007 1:53 PM, Phil Hays <[EMAIL PROTECTED]> wrote:
>
>
> James Annan wrote:
> > AdamW wrote:
> >> Phil Hays wrote:
>
> > >> Isn't there also the "Snowball Earth" state switch? Both to the
> > >> "frozen death" state, and back again after enough CO2 accumulates?
>
> > > Isn't that dependant on the ice-albedo feedback, which is excluded
> > > from climate sensitivity?
>
> > Sea ice and snow cover are both counted, which probably could achieve
> > the same end. But that's a long way from the present day situation. IIRC
> > someone on RC said that they thought a venus-like runaway was
> > impossible, but I'm not sure what basis that had.
>
> Sea ice would be required, for if the oceans don't freeze we don't
> match the geologic record of such events ~700 million years ago.
> Nearly complete sea ice coverage for many thousands of years is
> indicated by the presence of "BIF's" or Banded Iron Formations.
> "Snowball Earth" is not as far away from the current climate as we are
> likely to change the climate, however in the other direction. Taking
> a doubling of CO2 as a unit of measure, roughly two halvings of CO2
> from pre-industrial. We are moving the other way, so this isn't much
> of a concern for us, except for one point. Last I heard, current
> climate models can't reproduce these events, unlike older climate
> models. The models do produce a substantial non-linearity, but the
> cooling stops well before the ocean has completely frozen. Has this
> changed, or in other words does one or more of the modern models
> reproduce these events? What about the models you work with?
>
>
> --
> Phil Hays
>
>
> >
>
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