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wili

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RC: 5th IPCC report's SLR estimates still too low
« on: October 17, 2013, 03:34:36 AM »
Sea level in the 5th IPCC report

by Stephan Rahmstorf

The take away really seems to be in the last two sections:

Quote
Coastal protection professionals require a plausible upper limit for planning purposes, since coastal infrastructure needs to survive also in the worst case situation. A dike that is only “likely” to be good enough is not the kind of safety level that coastal engineers want to provide; they want to be pretty damn certain that a dike will not break. Rightly so.

The range up to 98 cm is the IPCC’s “likely” range, i.e. the risk of exceeding 98 cm is considered to be 17%, and IPCC adds in the SPM that “several tenths of a meter of sea level rise during the 21st century” could be added to this if a collapse of marine-based sectors of the Antarctic ice sheet is initiated. It is thus clear that a meter is not the upper limit.

It is one of the fundamental philosophical problems with IPCC (causing much debate already in conjunction with the 4th report) that it refuses to provide an upper limit for sea-level rise, unlike other assessments (e.g. the sea-level rise scenarios of NOAA (which we discussed here) or the guidelines of the US Army Corps of Engineers). This would be an important part of assessing the risk of climate change, which is the IPCC’s role (**). Anders Levermann (one of the lead authors of the IPCC sea level chapter) describes it thus:

    In the latest assessment report of the IPCC we did not provide such an upper limit, but we allow the creative reader to construct it. The likely range of sea level rise in 2100 for the highest climate change scenario is 52 to 98 centimeters (20 to 38 inches.). However, the report notes that should sectors of the marine-based ice sheets of Antarctic collapse, sea level could rise by an additional several tenths of a meter during the 21st century. Thus, looking at the upper value of the likely range, you end up with an estimate for the upper limit between 1.2 meters and, say, 1.5 meters. That is the upper limit of global mean sea-level that coastal protection might need for the coming century.

Outlook

For the past six years since publication of the AR4, the UN global climate negotiations were conducted on the basis that even without serious mitigation policies global sea-level would rise only between 18 and 59 cm, with perhaps 10 or 20 cm more due to ice dynamics. Now they are being told that the best estimate for unmitigated emissions is 74 cm, and even with the most stringent mitigation efforts, sea level rise could exceed 60 cm by the end of century. It is basically too late to implement measures that would very likely prevent half a meter rise in sea level. Early mitigation is the key to avoiding higher sea level rise, given the slow response time of sea level (Schaeffer et al. 2012). This is where the “conservative” estimates of IPCC, seen by some as a virtue, have lulled policy makers into a false sense of security, with the price having to be paid later by those living in vulnerable coastal areas.

Is the IPCC AR5 now the final word on process-based sea-level modelling? I don’t think so. I see several reasons that suggest that process models are still not fully mature, and that in future they might continue to evolve towards higher sea-level projections.

1. Although with some good will one can say the process models are now consistent with the past observed sea-level rise (the error margins overlap), the process models remain somewhat at the low end in comparison to observational data.

2. Efforts to model sea-level changes in Earth history tend to show an underestimation of past sea-level changes. E.g., the sea-level high stand in the Pliocene is not captured by current ice sheet models. Evidence shows that even the East Antarctic Ice Sheet – which is very stable in models – lost significant amounts of ice in the Pliocene.

3. Some of the most recent ice sheet modelling efforts that I have seen discussed at conferences – the kind of results that came too late for inclusion in the IPCC report – point to the possibility of larger sea-level rise in future. We should keep an eye out for the upcoming scientific papers on this.

4. Greenland might melt faster than current models capture, due to the “dark snow” effect. Jason Box, a glaciologist who studies this issue, has said:

    There was controversy after AR4 that sea level rise estimates were too low. Now, we have the same problem for AR5 [that they are still too low].

Thus, I would not be surprised if the process-based models will have closed in further on the semi-empirical models by the time the next IPCC report gets published. But whether this is true or not: in any case sea-level rise is going to be a very serious problem for the future, made worse by every ton of CO2 that we emit. And it is not going to stop in the year 2100 either. By 2300, for unmitigated emissions IPCC projects between 1 and more than 3 meters of rise.

(My emphases.)

Thoughts?
"A force de chercher de bonnes raisons, on en trouve; on les dit; et après on y tient, non pas tant parce qu'elles sont bonnes que pour ne pas se démentir." Choderlos de Laclos "You struggle to come up with some valid reasons, then cling to them, not because they're good, but just to not back down."

ritter

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Re: RC: 5th IPCC report's SLR estimates still too low
« Reply #1 on: October 17, 2013, 06:03:03 PM »
Thoughts?

Go big or go home. Seriously. If we're going to implement adaptation strategies to protect our infrastructure from sea level rise, we're going to have to start thinking in terms of generations, not years. Planing to the year 2100 is the same short-sighted bs that has gotten us to the place we are now. And even then, the worst-case models won't be used because, why pay today when you can pay tomorrow? Frankly, I don't think we have the time or resources to implement the measures that would be required to save places like Venice and Florida.

Lennart van der Linde

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Re: RC: 5th IPCC report's SLR estimates still too low
« Reply #2 on: October 18, 2013, 05:05:23 PM »
Based on current knowledge and uncertainties I think we should plan for a worst-case global SLR of about 1.5m by 2100 and about 10m by 2300, while taking into account a risk that it could be even more. Meanwhile we should decarbonize as fast as we can to keep the risks of such rapid SLR as small as possible. If during the coming decade(s) the risk of more than 1.5m of SLR by 2100 seems larger than expected now, we'll have to start planning for maybe 2m or even more by 2100.

wili

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Re: RC: 5th IPCC report's SLR estimates still too low
« Reply #3 on: October 18, 2013, 05:46:51 PM »
The question would seem to be--Which areas are we going to try to preserve with sea walls and other measures, and which areas have to be abandoned, either slowly or immediately. I think the second option will start to/ is already starting to happen as insurance rates along the coasts go up or insurance is simply not available.

I wonder also if at some point emergency workers will just say that they aren't going to risk their lives to try to save people or property for folks who, against all logic and science, continue to insist on living near sea level. That may sound harsh, but at some point it does not seem to me to be fair to make others suffer for ones own stupidity or pig-headedness.

One strategy, that was used before hurricane Katrina iirc, might be to require everyone who insists on living near sea level to have the parts of their body tattooed with identifying numbers so they can be put together for ID and burial purposes after dismemberment. That tends to get the message across fairly powerfully and viscerally.
"A force de chercher de bonnes raisons, on en trouve; on les dit; et après on y tient, non pas tant parce qu'elles sont bonnes que pour ne pas se démentir." Choderlos de Laclos "You struggle to come up with some valid reasons, then cling to them, not because they're good, but just to not back down."