Wednesday, September 10, 2008

Another Day, Another Furphy

Dr Jennifer Marohasy, whose work at the IPA involves examining and critiquing the scientific claims of others, produced an amusing piece on glaciers yesterday. She claimed that:
A study of sea level rise from ice melt in Greenland and western Antarctica has just been published in Science and concludes that a rise of 0.8 metres is possible by 2100, but even 2 metres “physically untenable”.
If, like Dr Marohasy, you strive to be inquiring in your blogging, you might click through that link and read the abstract of the study report 'Kinematic Constraints on Glacier Contributions to 21st-Century Sea-Level Rise'. Here's what it actually has to say:
We consider glaciological conditions required for large sea-level rise to occur by 2100 and conclude that increases in excess of 2 meters are physically untenable. We find that a total sea-level rise of about 2 meters by 2100 could occur under physically possible glaciological conditions but only if all variables are quickly accelerated to extremely high limits. More plausible but still accelerated conditions lead to total sea-level rise by 2100 of about 0.8 meter. These roughly constrained scenarios provide a "most likely" starting point for refinements in sea-level forecasts that include ice flow dynamics.
It looks very much like Dr Marohasy has misread the abstract - a sea level rise greater than two metres is rise by 2100 is physically untenable, a rise of two metres is tenable but unlikely, a rise of 0.8 metres is plausible. Despite a comment pointing out this error, the post remains uncorrected.

The abstract page for the Science article has links to the full report (pay per view and to a podcast of September 5. If you're inquisitive enough to download the transcript of the podcast you can read what Tad Pfeffer, the report's lead author, had to say about the study:
Interviewer - Robert Frederick
What do you mean by dynamics?

Interviewee – Tad Pfeffer
Dynamics is the part of discharge of ice, from glaciers into the ocean, which is accomplished not by melting the ice and letting it run off as water, but by pushing ice as icebergs, right out into the ocean. So it’s accomplished by the flow of the glacier, and what we call the dynamics, rather than melting of glacier ice on the surface. When the IPCC’s Fourth Assessment was published, they left out dynamics, and they were very up front about it – they said, “This is an important process, but we don’t understand it well enough to make the kind of consensus statement about dynamics that is, you know, up to the standard of the IPCC.” So they said, “Since we can’t really deal with this in anything resembling a confident fashion, we’re going to leave it out.” And so, as a consequence, the sea-level estimates, they were known to be low because that process wasn’t taken into account, and since that time there have been a number of different groups and individuals who’ve tried to get a handle on what that dynamic contribution might be – how rapidly outlet glaciers might be able to move ice into the ocean, by having these outlet glaciers speed up.

Interviewer - Robert Frederick
How high do those estimates get in the published literature?

Interviewee – Tad Pfeffer
In the published literature, you can find estimates as high as 4 to 6 meters of sea level rise at rates occurring at something like 2 to 5 centimeters per year. Now that's very high sea level rise. It's based on analogies with past conditions, particularly at the last interglacial, when sea levels were at least 4 meters higher, and possibly a great deal higher. And those were conditions that were very analogous to what we're coming into today. And, in fact, in the next century, we may be in climate conditions that are more or less identical to the last interglacial, 125,000 years ago. But the timing of that sea level rise back then is very uncertain. And that was what our analysis is really about, is the question of, 'OK, sea level rise may be big, but how fast is it going to occur?

...we publish a range, we present a range of total sea-level rise, ranging from about eighty centimeters up to two meters. And there’s a great deal of uncertainty in there, again because we still don’t know the processes, but there are some very simple concepts that you can use to place brackets or bounds, if you like, on what outlet glaciers might do, how rapidly outlet glaciers might deliver ice to the ocean in the future. And, the reasonable values fall within about 80 centimeters to 2 meters – that’s not to say that it’s physically impossible that we could have a total sea level rise of greater than 2 meters in the next century. But to do so would require processes to come into play and things to happen in glacier systems that we’ve never seen before – it’s not impossible, but it’s not something that I would recommend that we adopt as a working hypothesis. If we’re going to try to make predictions based on our best understanding of glacier physics, it should be based on the range of values and processes that we know best.
If, after reading that, you remained in any doubt about the findings of Pfeffer and his colleagues, you might get inquiring enough to e-mail him (his e-mail address is on the abstract page too); that way you might avoid the embarrassment of mis-stating Pfeffer et al's findings in a public internet forum and having the error picked up and reproduced by Andrew Bolt. That sort of thing is no good for anyone's credibility.

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