Tuesday, January 26, 2010

An inquiry by a reader

Recently there has been discussion of exaggerations and possibly misleading documents. In November RealClimate posted the Copenhagen document for discussion. I raised the following questions, which were posted but got no reply.

Sea level hyperbole Copenhagen report/IPCC
Executive Summary

"Sea-level prediction revised: By 2100, global sea-level is likely to rise at least twice as much as projected by Working Group 1 of the IPCC AR4, for unmitigated emissions it may well exceed 1 meter. The upper limit has been estimated as - 2 meters sea-level rise by 2100. Sea-level will continue to rise for centuries after global temperature have been stabilized and several meters of sea level rise must be expected over the next few centuries."

There is nothing I can find in the body of the report that shows a possible 2 meter rise by 2100 (see the table on p. 40). Yet the conclusion states "an upper limit of 2 meters." Why? There are three studies cited on the table. Quoting from one of them:_"Šthis relationship results in a projected sea-level rise in 2100 of 0.5 to 1.4 meters above the 1990 level."_Rahmstorf 2007 http://www.sciencemag.org/cgi/content/abstract/1135456

My understanding is that the current rate of increase is about 30 cm per century.
IPCC 2001 projected a range of 11 - 77 cm increase.
New evidence projects a range of 50 - 140 cm.
(For Americans like myself, that means that the rate of sea level rise is expected to increase from the current 1.2" per decade to somewhere between 2" - 5.5" per decade.)
Is there some evidence that makes the higher range more probable? Where does the "upper limit of 2 meters" come from? Why did the authors put this high number, and indeed one not even presented in the body of the work, in the Summary? Is it simply not sourced (i.e., no reference to the supporting research)? Or is it a case of overstating the scenario?
I would be curious about your views on the likely sea level rise, and about the strength of the underlying evidence.

I freely admit that as a layman, I could be simply misunderstanding the documentation, or misreading the evidence. So I submit this to your expertise!

Anonymous reader


P Gosselin said...

That's yet another very good question. Some time ago, I posted at this blog that the 2-meter SLR figure has already been widely rejected by many scientists.
Perhaps the figure comes from the WWF. I would not hold my breath waiting for a reply. It's obvious that the 2-meter figure is purely a scare tactic. And this is precisely what the public is now perceiving, and thus will only serve to undermine the IPCC.

eduardo said...

You may find useful to look into this older post and read the comments

Unknown said...

I hope that the question how "2 m" was given would be answered by the authors of "Copenhagen Diagnosis".

My guess is as follows: it was based on the number 140 cm in Rahmstorf et al. 2007 study, and since the authors knew that the number was not definite as for projection, they used a more round number bracketing it.

IPCC AR4 showed the range of model projections (whose upper bound is 59 cm in 100 years, see Table SPM 3 of AR4 WG1), but they also noted that the models did not fully incorporated processes of ice dynamics which are likely to accelerate disintegration of ice sheets. So the logical projection based on IPCC AR4 is "somewhat larger than 18 cm to 59 cm". But how large is "somewhat" remained open.

I think that scientists who have knowledge of ice sheet dynamics can give a range of possible speed of ice sheet disintegration. But the range may be broad (I don't know, but maybe 20 cm to 5 m in terms of sea level rise in 100 years), and perhaps they cannot give probability of each value within the possible range except very subjective guesses. Even then, I think that such educated guesses are helpful if we are informed as such.

Following what Stephen Schneider said in his book "Science as a Contact Sport", I pose a rhetorical question: Do you let the politician make the guesswork? Perhaps scientists are not entitled to make the guess for the society either, but they have some participatory role.

As a non-expert of ice sheets, but an expert in a broader sense of climatology, I subjectively guess that "the upper limit 2 m" is reasonable.

Please do not dismiss "2 m" as non-science or propaganda even if it turned out to be just a subjective guess of a scientist. On the other hand, it is legitimate to separate guesswork from more rigorous scientific work.

Nils said...

The upper limit of 2m has been stated by Pfeffer et al. 2008:
"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."
Vermeer and Rahmstorf 2009 also came close to 2m:
"For future global temperature scenarios of the Intergovernmental Panel on Climate Change's Fourth Assessment Report, the relationship projects a sea-level rise ranging from 75 to 190 cm for the period 1990–2100."
The last one was published online on 7 December, briefly after the Copenhagen Diagnosis. That might explain why it wasn't mentioned (though it could have been, I guess). A third source for the upper 2m bound is Grinsted et al. 2009:
"Pfeffer et al. (2008) estimates the plausible range of the dynamical contributions and gives a best guess for the total sea level rise at 0.8 m and 2 m as an upper limit. This range is compatible with our projections. Thus ice sheet dynamical effects are the most likely source of discrepancy between our projections and those of the IPCC AR4."

Don Shor said...

"In my opinion, if emissions follow a business-as-usual scenario, sea level rise of at least two meters is likely this century."
Dr. James Hansen
testimony to the House Select Committee on Energy Independence & Global Warming, June 23, 2008


@ReinerGrundmann said...

Re. Hansen, he made the very first statement about sea level rise back in 1981. The New York Times had a story based on Hansen and colleagues' report, saying there would be global warming of 'almost unprecedented magnitude' in the next century. 'It might even be sufficient to melt and dislodge the ice cover of West Antarctica, they say, eventually leading to a worldwide rise of 15 to 20 feet in the sea level. In that case, they say, it would "flood 25 percent of Louisiana and Florida, 10 percent of New Jersey and many other lowlands throughout the world" within a century or less.' (NYT 22 August 1981).
The WAIS is not included in the current scenarios but the early alarm is quite interesting. But it took another seven years before global warming arrived on the political agenda.

Ed said...

The question was were the 2 metes estimate came from for the IPCC AR4 published in 2007. I believe the cut off date for papers was May 2006. So it is not clear how subsequent papers published in 2008 and 2009: Pfeffer et al. 2008, Vermeer and Rahmstorf 2009, and
Grinsted et al. 2009 could have provided the 2 meter estimate in 2006 in time for IPCC AR4.

I do not think that answers the question as to where IPCC AR4 obtained its 2m estimate.

Marco said...

The 2 meter estimate is NOT in the IPCC AR4!

The 2 meter is in the Copenhagen Diagnosis report.

Unknown said...

Re #6 (Reiner Grundmann):

Hansen is certainly one of the pioneers in climate modeling, and he is still an active scientist. But in my opinion his recent popular writing has gone too alarmistic. Unlike Stephen Schneider, Hansen does not usually show uncertainty of his outlook.

In 1981, the relationship between steady-state (or "equilibrium") and time-varying ("transient") responses of the climate system to such forcing as atmospheric CO2 concentration was not well understood. A seminal paper by Schneider and Thompson appeared in the Journal of Geophysical Resaerch that year. There were no results of realistic transient climate simulation yet.

So the story of New York Times was based on steady-state response, that is, the difference between climates of two hypothetical world where the CO2 concentration is fixed at a high value and the standard value, respectively, for indefinitely long time. In the steady state with sufficiently high CO2 concentration, the whole West Antarctic ice sheet will not exist. The sea level rise of "15 to 20 feet" corresponds to that situation. Note the word "eventually" in the NYT story.

In the real world, the responses of ice sheets and the deep ocean take time, so the actual behaviour of the climate system ("transient response") lags the imaginary curve connecting instantaneous response equal to the steady-state one with simultaneous CO2 concentration.

The question, how long is the time scale of the lag, is difficult and not yet fully answered. IPCC AR4 did not reach consensus about it, as I mentioned in the previous comment (#3).

@ReinerGrundmann said...

Thanks Kooiti, very helpful background information.

In today's Guardian, there is a warning about sea level rise.


Two scenarios are compared, a "probable scenario" of 2m rise, and an "extreme sceanrio" of 84 meters. Yes, this is no typo. Go and have a look. Maybe someone with the right science credential writes a letter to the editor?