[asa] Counteri-ntuitive Results of AR4 SPM on Sea Level Rise

A while back Janice crowed about how the sea level rises were lower in AR4
than TAR. The official report shows the same but not for the reasons Janice
thinks. The highest number is lower because things are worse. Say what??!!??
Yes, you heard me right. It has to do with the discussion about models
that Dr. Hamilton and I had. The ice dynamics models were found to be
wanting because they underpredicted ice melt and were dropped from AR4. It's
those pesky positive feedbacks that cause chaos which runs roughshod through
our precious computer models. In short, the effects are worse than currently
predicted -- we just don't know how much worse. From the real climate blog:

> When it comes to specific discussions, the two that are going to be
> mostly in the news are the projections of sea level rise and hurricanes.
> These issues contain a number of "known unknowns" - things that we know we
> don't know. For sea level rise the unknown is how large an effect dynamic
> shifts in the ice sheets will be. These dynamic changes have already been
> observed, but are outside the range of what the ice sheet models can deal
> with (see this previous discussion<http://www.realclimate.org/index.php/archives/2006/06/ice-sheets-and-sea-level-rise-model-failure-is-the-key-issue/>).
> That means that their contribution to sea level rise is rather uncertain,
> but with the uncertainty all on the side of making things worse (see this
> recent paper for an assessment (Rahmstorf , Science 2007)<http://www.sciencemag.org/cgi/content/short/315/5810/368>).
> The language in the SPM acknowledges that stating
>
> "Dynamical processes related to ice flow not included in current models
> but suggested by recent observations could increase the vulnerability of the
> ice sheets to warming, increasing future sea level rise. Understanding of
> these processes is limited and there is no consensus on their magnitude."
>
> Note that some media have been comparing apples with pears here: they
> claimed IPCC has reduced its upper sea level limit from 88 to 59 cm, but the
> former number from the TAR did include this ice dynamics uncertainty, while
> the latter from the AR4 does not, precisely because this issue is now
> considered more uncertain and possibly more serious than before.
>

This is shown to be the case by the fact that the measured sea level rise is
underestimated by previous IPCC models. According to Rahmstorf, 59 cm would
be the low side of the predictions and the high side would be 140 cm
instead, higher than the previous high predicted in TAR. Rahmstorf has the
advantage of being semi-empirical, taking into account recently measured sea
level rise, and thus doesn't depend as much on poorly understood ice sheet
dynamics.

Actually, I don't mind this kind of conservatism in the report. It shows
that the IPCC is not following the precautionary principle that David O. is
rightly concerned about. Note to David Opderbeck: you previously noted that
Rahmstorf was sober -- he was one I quoted -- this is by comparison more
sober. If governments and indeed individuals don't heed the sober results of
this report, they show themselves to be irresponsible.

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Received on Fri Feb 2 19:03:45 2007