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LRC1962

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Great youtube University background course
« on: December 12, 2014, 01:26:27 AM »
For all those that would like to have the abc's of Cryosphere and AGW, I stumbled on this youtube series.
It is an acual university course, and in 26 45 minute classes gives you a very good grounding for a good many things that are talked about in this forum. Such as formulae that are used and a very good grounding over all to understanding what is happening and why.
Do plan to work through some of it and see how far I can go before it gets too much.
"All truth passes through three stages: First, it is ridiculed; Second,  it is violently opposed; and Third, it is accepted as self-evident."
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Neven

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Re: Great youtube University background course
« Reply #1 on: December 13, 2014, 10:56:35 AM »
Thanks, LRC1962, that looks very interesting. Now to find the time to watch this.  :)
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LRC1962

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Re: Great youtube University background course
« Reply #2 on: December 23, 2014, 12:42:42 PM »
Two part lecture on 1)Social responsibility for GW 2)Possibilities of ASLR and their impact.

"All truth passes through three stages: First, it is ridiculed; Second,  it is violently opposed; and Third, it is accepted as self-evident."
       - Arthur Schopenhauer

Lennart van der Linde

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Re: Great youtube University background course
« Reply #3 on: December 24, 2014, 08:33:11 AM »
Thanks, LRC, those are nice lectures by Jim White, warning for 20m of SLR. The only question seems to be: how fast and abrupt will this happen?

Laurent

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Re: Great youtube University background course
« Reply #4 on: December 24, 2014, 09:54:07 AM »
Somewhere beetween 500 to 1000 years with a bell type curve, I guess...we are climbing that bell right now !

wili

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Re: Great youtube University background course
« Reply #5 on: December 24, 2014, 02:08:36 PM »
At its fastest at the end of the last ice age, sea levels rose at something like a meter every twenty years, iirc. I'm not sure we can completely rule out that rate happening again, maybe even this century, but I am glad to defer to ASLR and others more knowledgeable on the subject.
"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."

Lennart van der Linde

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Re: Great youtube University background course
« Reply #6 on: December 24, 2014, 02:50:48 PM »
See figure 3 in Rohling et al for their probability estimates based on the geological record:
http://www.nature.com/srep/2013/131212/srep03461/full/srep03461.html

They seem to estimate about a 1% chance of 20m SLR by 2500, with an average potential SLR of 4-5 meters per century. But that's only based on the geological record, which never had a climate forcing as strong as the human forcing is now, and which is still growing.

So even Rohling et al may still be too cautious in order to not be accused of alarmism. Jim White argues scientists shouldn't be afraid of such accusations and just communicate the risks as clearly and honestly as possible. Even if that can be somewhat intimidating as times, I suppose.

Lennart van der Linde

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Re: Great youtube University background course
« Reply #7 on: December 24, 2014, 02:59:44 PM »
Rohling et al 2013 estimate a 2.5% chance of 1.8m of SLR by 2100. Jevrejeva et al 2014 estimate a 5% chance of 1.8m by 2100:
http://iopscience.iop.org/1748-9326/9/10/104008/article

Why they don't consider the tail beyond this 1.8m in their figure 3, is not so clear, and maybe also a form of conservatism?

Lennart van der Linde

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Re: Great youtube University background course
« Reply #8 on: December 24, 2014, 03:03:38 PM »
Kopp et al 2014 do consider smaller chances of extreme SLR:
http://onlinelibrary.wiley.com/doi/10.1002/2014EF000239/full

However, in their table 1 they estimate only about a 0.5% chance of circa 1.8m by 2100. So how risk averse should we be?

Lennart van der Linde

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Re: Great youtube University background course
« Reply #9 on: December 24, 2014, 03:28:46 PM »
Also see the expert assessment by Horton et al 2014 (among 90 of the most publishing SLR-experts):
https://marine.rutgers.edu/pubs/private/HortonQSR_2013.pdf

Their table 1 estimates a 17% chance of 1.2m and a 5% chance of 1.5m by 2100 as worst-cases. This is more than the 1m and 1.2m Kopp et al 2014 estimate for a 17% and 5% chance respectively, which is about the same as the IPCC-estimates.

For 2300 Horton et al estimate a 17% and 5% chance of 3m and 4m respectively as worst-cases. This seems less than Kopp et al who estimate a 17% and 5% chance of 2.8m and 3.7m by 2200, not 2300, as worst-cases.

Extrapolating Kopp et al to 2300 would imply maybe a 5% chance of 7-8m as a worst-case. Compare this to Rohling et al 2013 who estimate a 2.5% of about/almost 9m by 2300.

So what should we take as relevant worst-case probabilities: 17%, 5%, 2.5%, 1%, or even less? And whose estimates should we use? And how far should we want to look into the future: one, two, three, or even more centuries?

Lennart van der Linde

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Re: Great youtube University background course
« Reply #10 on: December 24, 2014, 04:12:30 PM »
So for 2100 Jevrejeva et al 2014 have the most pessimistic probability estimate with a 5% chance of 1.8m as their worst-case. Rohling et al estimate a 2.5% chance of 1.8m by 2100 and a 2.5% chance of 5m by 2200.

So maybe we can extrapolate Jevrejeva et al 2014 as saying that they estimate a 5% chance of 5m by 2200 as a worst-case? And maybe a 5% chance of 9m by 2300? And almost 14m by 2400? And 19m by 2500? And 24m by 2600?

That would be about 19m between 2200 and 2600, comparable to Meltwater Pulse 1A over 14.000 years ago:
http://www.sciencemag.org/content/299/5613/1709

And would SLR start to slow down by that time, because most marine basins would be gone by then? So maybe 11m more meters from 2600-3000, so about 35m in total by the year 3000? And 55m by the year 4000? And 70m another millennium later, with the planet ice-free by then?

Or could it go even faster? Of will it be slower? Will we do what we need to do to prevent this most extreme meltdown? Can we still prevent 20m of SLR in the long run? Or 10m? Or is it already too late for that?

It seems at least 10m of SLR in this millennium is pretty much unavoidable by now. If so, our choice is only how fast or slow, and how far beyond 10m it will go.

Laurent

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Re: Great youtube University background course
« Reply #11 on: December 24, 2014, 05:10:47 PM »
Personnaly, I am thinking it is not a question of many thousands of years of SLR, the earth will catch up with what we have already put in the atmosphere within this next thousand years. That mean more than 30 meters within that thousand years 100% certain, certainly mutch more if we insist to dump green house gazes in the atmosphere. I already said it, if we want to be safer we have to absorb all the CO2 emitted since 1950 and certainly much more since natural CO2 and methane are starting to be massively released, 300 ppm CO2eq is the "safe" limit.
Of course since I am not a scientist, I am waiting for knowledgable point of view !

wili

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Re: Great youtube University background course
« Reply #12 on: December 24, 2014, 05:56:08 PM »
Quote
We announced that we had collected enough observations to conclude that the retreat of ice in the Amundsen sea sector of West Antarctica was unstoppable, with major consequences – it will mean that sea levels will rise one metre worldwide.

What's more, its disappearance will likely trigger the collapse of the rest of the West Antarctic ice sheet, which comes with a sea level rise of between three and five metres. Such an event will displace millions of people worldwide.

Two centuries – if that is what it takes – may seem like a long time, but there is no red button to stop this process.

That sounds to me as if they are expecting up to 5 meters slr by 2200 or sooner just from this one source. Add GIS, glaciers and heat expansion and I would think you would at least double that figure. But again, others here are much more on top of this stuff than I am.

http://www.theguardian.com/commentisfree/2014/may/17/climate-change-antarctica-glaciers-melting-global-warming-nasa
"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."

Lennart van der Linde

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Re: Great youtube University background course
« Reply #13 on: December 24, 2014, 06:54:57 PM »
I think Rignot refers only to the Amundsen sector of WAIS with his two centuries remark. Also see:
http://www.washingtonpost.com/national/health-science/research-casts-alarming-light-on-decline-of-west-antarctic-ice-sheets/2014/12/04/19efd3e4-7bbe-11e4-84d4-7c896b90abdc_story.html

There it says:
Quote
Eric Rignot of UC-Irvine, suggested that in his view, within 100 to 200 years, one-third of West Antarctica could be gone. Rignot noted that the scientific community “still balks at this” — particularly the 100-year projection — but said he thinks observational studies are showing that ice sheets can melt at a faster pace than model-based projections take into account.

So maybe about 1m from WAIS in 100 years, which could bring total SLR by 2100 close to or maybe even over 2m. This could then possibly be on course to 20m by 2500 and 35m by 3000, who knows?

I doubt it's a certainty that, as Laurent says:
Quote
the earth will catch up with what we have already put in the atmosphere within this next thousand years. That mean more than 30 meters within that thousand years 100% certain, certainly mutch more if we insist to dump green house gazes in the atmosphere.

As far as I understand it takes the ocean at least 900 years to fully catch up with a strong climate perturbation. So it takes a while before the full forcing can work on the ice. And then it takes a while for the ice to melt. How long noone really knows, but it seems it would take longer than a thousand years, maybe a few thousand?

So even with RCP8.5 it would seem hard to melt more than 30-35 meters in this millennium, unless positive feedbacks would be so strong that climate sensitivity (ECS) turns out to be more than 6 degrees C per doubling. Which could be possible, for all I know, but seems not a certainty yet.

Lennart van der Linde

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Re: Great youtube University background course
« Reply #14 on: December 24, 2014, 06:58:44 PM »
wili,
Actually, if one third of WAIS could be gone in one century, then (almost) all of it could probably be gone in two centuries, so maybe 3m from WAIS alone by 2200 and 5-6m in total?

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Re: Great youtube University background course
« Reply #15 on: December 24, 2014, 07:08:26 PM »
Also from the WP-article linked above:
Quote
the ice sheet of West Antarctica would, if it collapsed entirely, contribute about 3.3 meters, or nearly 11 feet, to global sea-level rise, Alley said

So one third from WAIS is about 1.1 meters. And 100 years from now would be around 2114, so by 2100 it would be somewhat less than this full one third, maybe 80 cm or so instead of 1.1m? But then between 2100 and 2200 it could well be another 3-5 meters, it seems.

And all this is supposing a pretty gradual speed-up, not a potentially faster abrupt surge followed by a relative slow-down. So as ASLR has stressed many times, the risks are more serious than all these estimated probabilities can tell.

Lennart van der Linde

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Re: Great youtube University background course
« Reply #16 on: December 24, 2014, 07:49:42 PM »
Of course it also depends on the potential ice loss from GIS and EAIS, as mentioned here in the Greenland-Jakobshavn thread:
http://forum.arctic-sea-ice.net/index.php/topic,154.msg41913.html#msg41913

Pfeffer et al 2008 pointed to possible kinematic constraints. And Hansen & Sato have pointed to a potential negative iceberg cooling feedback. Such mechanisms could imply a sort of maximum speed limit for SLR. Also see figure 2 in Rohling et al 2013:
http://www.nature.com/srep/2013/131212/srep03461/full/srep03461.html

They estimate SLR faster than about 4 meter/century as improbable, but not impossible. More than 10 meter/century they seem to exclude as physically implausible.

As for the response time of the ice they think a few centuries is likely, but it could also be more than a millennium.

Then again, this is all based on the natural past, not on the antropogenic present and future, so faster rates than in the past seem quite likely to me.

Lennart van der Linde

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Re: Great youtube University background course
« Reply #17 on: December 24, 2014, 08:43:07 PM »
See about a potential speed limit for SLR also this comment about Rohling et al 2013:
https://forum.arctic-sea-ice.net/index.php/topic,130.msg17964.html#msg17964

I checked this comment with Rohling himself, who completely agreed with it.

Particularly relevant seems this quote from Rohling et al:
Quote
The disequilibrium between rapidly increased anthropogenic climate forcing and slow ice-volume responses amounts to many metres of SLR at current atmospheric CO2 levels, and may not change significantly with increasing CO2 to 700 ppmv (Figure 1)24. Temperature adjustments to emission scenarios within that range follow a sigmoidal pattern with time, accelerating and then decelerating, with adjustment timescales between 150 years for low-emission scenarios and 400 years for high-emission scenarios15. We infer that the present disequilibrium is already sufficient to cause build-up toward major ice-sheet responses, and that further warming will occur over similar timescales as the developing ice-sheet responses, so that increased forcing would cause (rapid) shifts toward extremes of the parameter ranges considered here (high ultimate SLR rates and rapid adjustment timescales). The median global radiative forcing projections for high-emission RCPs is ~12 W m−2 (ref. 15). For deglaciations, this was 8–10 W m−2 (refs. 17,28). We infer that the long-term consequences of high-emission RCPs may be suitably gauged from the SLR adjustment rates and timescales of deglaciations, if we assume that such consequences would develop via naturally precedented processes (e.g., because the response becomes rate-limited).

Forcings and temperatures can rise faster than ice can melt, so there must be a limit to the melting rate somewhere. The question is what that limit is, and that's not easy to answer, since nature seems to have never done this experiment before.

The natural maximum rate of SLR over several centuries during past interglacials seems to have been about 1 meter/century, maybe some more for shorter intervals. So if the current climate forcing is or will be at least 10 times as strong as back then, even with strong mitigation, then maybe 10 meters/century would be a possibility in the coming centuries, unless this would be above the maximum rate that's physically possible.

But can we exclude that for example 4-5 meters per century is possible? There is less ice now than during the last deglaciation when this rate was apparently reached during several centuries (Meltwater Pulse 1A, about 14.000 years ago). But then again, it's warmer now, and there's still enough vulnerable, marine based ice available. So with a forcing 10 times or more stronger than in the past, who knows what is possible or not?

wili

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Re: Great youtube University background course
« Reply #18 on: December 24, 2014, 09:00:52 PM »
Thanks for the discussion and links, LvdL. There are certainly many uncertainties, and uncertainties are not necessarily our friends.
"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."

Laurent

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Re: Great youtube University background course
« Reply #19 on: December 25, 2014, 05:27:33 PM »
Do not forget that what we have experienced for 1 million year is very different from what the earth used to. Each 120.000 years we did oscillated between cold and "warm" (suitable for us) climate but the earth used to oscillate between very cold (snow ball earth to warm (much warmer than recently). The reason why we could not go more than 300 ppp (1°C) is because of the arctic, unfortunately we will explode sooner or later that limit, so the earth will tend to stabilize toward a higher level of CO2, temp and SLR. Of course tha will take more than a thousand year...

LRC1962

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Re: Great youtube University background course
« Reply #20 on: December 29, 2014, 07:03:48 PM »
I believe we are all in unknown territory. All past charts that go in sync are based on CO2 leading the pact, but at a spead that SLR and temps can keep pace. The problem we are having now is that CO2 is rising so fast and so far ahead of SLR and temps that we are in uncharted territory. We have a pretty good idea what happens with the regular rhythms of the charts, but can we equate what happened in the past to what will happen now? A lesson that I still believe is relevant is that 10 yrs ago we all thought that a major melt off of the Arctic ice cap could not happen until after 2100. Now most are saying a total Sept ice free within 20 yrs.
Please correct me if my line of reasoning is wrong.
Floating ice on oceans does not rise SLR when it melts.
Major portions of the current grounding lines of the WAIS are at or close to the point of where the terrain starts dropping off to well below sea level.
Water always searches for the lowest level it can find.
It is the grounding line that prevents the ice of the WAIS from affecting the sea level.
Once that final ground line high point is breached would not the WAIS start influencing SLR without much more melt even occurring by in effect turning it into a shelf ice? Part of my reasoning is that water would begin forcing its way between the ice and bedrock from the ocean. Granted, the weight of the ice would be great enough that still would push itself down very much like when the ice selves where grounded farther out into the ocean. A major difference I see (although maybe not as much as I presume) you no longer have the horizontal pressures from the back pushing it out onto the grounding line.
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Lennart van der Linde

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Re: Great youtube University background course
« Reply #21 on: December 29, 2014, 09:58:24 PM »
LRC,
I think you're right: we're in unknown territory, and we don't know how the ice will behave in the coming decades and centuries.

Your point about grounding line retreat and glaciers starting to float seems a real concern. Also it will probably speed up calving, as for example Bassis & Jacobs 2013 argues:
http://www.nature.com/ngeo/journal/v6/n10/full/ngeo1887.html

See this blogpost by Greg Laden for some explanation:
http://scienceblogs.com/gregladen/2013/07/23/important-new-science-on-melting-glaciers/

This is not included in IPCC-reports yet, and they can't say how much it could accelerate SLR, so it could mean IPCC is substantially underestimating future SLR.