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

Excellent point, Rich. Good post. Yes, I've heard other reports along these lines. Sea level rises are significantly underestimated in IPCC because they don't take the ice melting into account. I hadn't heard why and your post clarifies that point. The expansion of water due to warming is the part that is understood much better now and that's the main factor IPCC included. To show how big that discrepancy is, note that Jim Hansen obtained an estimated sealevel rise of 25 +/- 11 meters for a temperature rise of 3 degrees C. That's a big range and a lot higher than Rahmstorf. No wonder they want more independent assessments to get a better handle on it. The message is that the IPCC report is quite conservative and is likely to be revised upward in future years as these positive feedbacks are better understood.

Randy

  ----- Original Message -----
  From: Rich Blinne
  To: asa
  Sent: Friday, February 02, 2007 7:03 PM
  Subject: [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). 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)). 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 21:23:17 2007