On Apr 14, 7:35 am, jdannan <[EMAIL PROTECTED]> wrote:
> ICE wrote:
>
> > I talked to some people around here about that, they say they had
> > wondered about this but not come to any conclusion:
> > in particular, are the mecanisms causing the spread in CS (mainly
> > tropical low clouds, as apparently they say in dufresne & bony 2008
> > (J.of Clim)- for equilibrium CS) the same as the ones causing the
> > spread in "natural" variability ?
> > intuitevely, more variability means greater feedback loops - but it
> > also depends on the time-scale of these feedbacks, and...
> > allright - i'll keep on medidating on this...
>
> I was googling just now on similar issues and came across some related
> comments from a workshop:
>
> http://books.nap.edu/openbook.php?record_id=10787&page=7
>
> Gregory: There was logic in the expert judgment choices to exclude high
> sensitivity (Seq) values, since we’ve seen that the climate system
> doesn’t exhibit wide swings that would be characteristic of high
> sensitivity values. This is because the amplitude of natural variability
> depends on the same feedback processes that determine the value of Seq
> itself.
>
> also
>
> http://books.nap.edu/openbook.php?record_id=5142&page=172
>
> If we had evidence of large natural climate excursions in the past on
> the 50-year time scale, there would be two possible interpretations.
> First, if the climate sensitivity were low, then passive internal
> variability and any response to global-mean external forcing would be
> small, so we would have to invoke either significant changes in the
> thermohaline circulation or (less likely on these time scales) major
> changes in the spatial patterns of forcing (as in the Milankovitch
> effect). Alternatively, if neither of the latter two mechanisms was
> operating, we would have to infer that the climate sensitivity was
> relatively large (i.e., ΔT2× in the range suggested by GCMs, 1.5°C to
> 4.5°C), since a large sensitivity is required for both passive internal
> and global-mean externally forced changes to be substantial.
>
> James
There have been natural climate changes on a < 50 year scale.
They are called Dansgaard-Oeschger warm events. The best know
of these events in the termination of the Younger Dryas. Its
initial phase may have been as short as 3 years [Alley, 2000,
"The two mile time machine."]
Reading between the lines, you are excluding those events on
the basis that they were the result of forcings by the THC or
changes in solar flux. However, Schulz, M. [(2002), On the
1470-year pacing of Dansgaard-Oeschger warm events,
Paleoceanography, 17(2), 1014, doi:10.1029/2000PA000571] shows
that the first is not a cause and that the latter is unlikely,
since these events do not have a fixed frequency.
The idea that the Younger Dryas was caused by a switch in
the THC is now losing ground [Seager et al, 2002; Broecker,
2007] and in fact was due to the formation and disappearance
of sea ice [Gildor & Tziperman, 2003.]
Although the D-O events were not global, they affectd most
of the continental regions of the globe where the human
population reside. Thus the abrupt (high sensitivity)
climate changes of the northern hemisphere recorded in
the Greenland ice cores are more typical of what we may
experience in the future, rather than the longer steady
changes seen in the Vostock ice cores, which are only
recording the climate Antarctica.
Cheers, Alastair.
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