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Climate Hustle

Global Sea Level Rise: Pothole To Speed Bump?

Posted on 7 February 2012 by Rob Painting

As indicated in a press release from the NASA Jet Propulsion Lab last year, short-term trends in global sea level rise are greatly affected by temporary exchanges of water mass between the land surface and ocean - creating 'potholes' and 'speed bumps' in the sea level record. This a consequence of changes in precipitation (rainfall & snow) resulting from the El Niño-Southern Oscillation (ENSO).

During La Niña the sea surface is cooler-than-normal and rainfall is concentrated over land, which leads to a temporary fall in global sea level. With El Niño the surface of the tropical Pacific Ocean becomes warmer-than-normal, and rainfall gets concentrated over the ocean. This, combined with the drainage of water from land, causes a temporary spike in global sea level.

ENSO is principally responsible for the large year-to-year fluctuations evident in the global sea level record, but neither of these two phenomena (El Niño/La Niña) alter the long-term sea level rise which results from the melting of land-based ice, and the thermal expansion of the oceans as they warm. They do, however, cause sufficient 'noise' to obscure the long-term sea level rise when viewed at short intervals.

In the last two years two back-to-back La Niña have temporarily lowered sea level, but La Niña appears to have weakened in recent months and accordingly we would expect an uptick in sea level rise as conditions move closer to neutral. A quick look at AVISO confirms this, see Figure 1.

Figure 1 - The reference mean sea level since January 1993 (left) is calculated after removing the annual and semi-annual signals. A 2-month filter is applied to the blue points, while a 6-month filter is used on the red curve. By applying the postglacial rebound correction (-0.3 mm/year), the rise in mean sea level has thus been estimated as 3.18 mm/year. Image from AVISO.

Rather than focusing on the potholes, as the skeptics do, one needs to consider the broader picture. That means factoring in the speed bumps too. See figure 2.

Figure 2 - University of Colorado global mean sea level data with a 12-month running average, and short-term declines. This animation does not include the latest sea level updates indicated in figure 1.

What happens to global sea level rise over the short-term will depend on which aspect of ENSO develops in the tropical Pacific this year. Whatever the case may be, global sea level rise will continue over the long-term because of the accelerating melt of land-based ice and continued warming of the oceans.  

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Comments 51 to 73 out of 73:

  1. Steve, a 5 meter rise by 2100 is a worst case potential discussed in the paper. Uncertainties and negative feedbacks that mitigate against this potential are clearly discussed by Hansen and Sato. The graph is a simple model of an ice-sheet loss doubling regime derived from paleo-records, and is not meant to represent an actual scenario for the year-by-year rate of sea level rise acceleration (Hansen says this explicitly in the 2007 paper the graph is modelled from). As mentioned above, the more likely realization of such a potential would come with short periods of massive inundations rather than a smooth rate increase.

    Hansen is not saying that he believes the 5 meter rise by 2100 is the likeliest outcome, just that it is not implausible and shouldn't be discounted. His general theme in this and other papers is that the official projections, like those in the IPCC, are too conservative presenting upper bounds. And even though we should be wary of focussing on short-term data, as you have done, it's worth pointing out that global sea level rise (and Arctic sea-ice decline) have so far been underestimated by the IPCC. Hansen may have a point; and he is aware of the limits of and arguments against his thesis, and has called for more study to test it. Whether consciously or not, you have misrepresented his argument here.

    Here's the monograph underpinning the graph that's got your attention:

    Hansen (2007): Scientific reticence and sea level rise

    Read the whole thing for comprehension and context. It's not long.
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  2. Thanks for this. I didn't realise that sea levels would vary with El Nino and La Nina. It makes sense.
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  3. (novice question)

    "Summer melt on West Antarctica has received less attention than
    on Greenland, but it is more important" says M. Mann in his reticence paper.

    Because of ocean temperatures or are the other or more reasons?
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  4. Though I've not read the paper, I'd suggest it is that it is because, of these two ice sheets, the West Antarctic Ice Sheet (almost a physically separate entity from the East Antarctic Ice sheet because of the Transantarctic Mountains) has a great deal more potential for rapid melt/collapse than the Greenland Ice Sheet. Both contain broadly comparable potential sea level rise components, but Greenland has a lot of mountains round the perimeter to buffer the ice sheet, which the WAIS does not have. The WAIS is also grounded well below sea level, and so warmer sea water can readily melt it from the base (Greenland also, but sea water access is mostly limited to the narrow fjords). If the WAIS begins to go, there's a lot less to stop it, and a consequently rapid sea level rise.

    Having said that, I think Greenland has more melting on its surface, and an observed flow acceleration already, so is not immune to significant melt!
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  5. Hi Barry, nice to see you here.

    #52 Barry

    That's right, Dr. Hansen is not saying the 5 meter rise is the likeliest outcome, he just says it's plausible. I've run the numbers on his 5 meter every ten years rate doubling scenario and have determined that sea level would be going up at a rate of nearly one millimeter per day by 2100. Further comment is not necessary, it stands as its own testimony.
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  6. Steve Case, your argument from incredulity is just that. Just because you have a hard time imagining the collapse of an ice sheet, and the resulting pulse of sea level rise, does not negate the plausibility of it happening. Meltwater Pulse 1A had 20m SLR in ~200 years, a mean of 0.26mm/day for two hundred years. Assuming a sigmoidal curve for the pulse, which is certainly reasonable, the peak sea level rise was quite possibly close to, or even larger than, 1mm/day. This is a matter of palaeoclimatic record, and we presently have two ice sheets (Greenland and West Antarctica) that have been known to be a great deal smaller in global conditions with this much CO2 in the atmosphere.
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  7. Skywatcher #55, thanks

    Mean annual rate of ice mass loss Greenland: 200-300 Gt
    Mean annual rate of ice mass loss Antarctica: 70-210 Gt
    Permafrost warming: 0.5°C to 2°
    Freshening of part of the Arctic Ocean: 2006–08 increase 8400 ± 2000 km3

    I am prepared to worry about either or both but I still don't quite see why Antarctica should be favoured eventhough the WAIS is indeed perhaps more prone to melt at its base. Is there evidence (yet)? In favour of "Artic worries" might be the the permafrost melt run-offs draining into Arctic waters.

    Am I being obtuse?
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  8. #58: West Antarctica is buttressed by two great ice shelves, the Ross and the Ronne-Filchner. As we have already seen on the Antarctic Peninsula, ice shelves can spectacularly collapse in very short spaces of time, leading to great armadas of icebergs (Larsen A & B, Wilkins - video). If the main ice shelves go, sea levels will suddenly rise much more quickly as the grounded glaciers that are buttressed by the ice shelves accelerate. An interesting paper on a past example driven by warming seas.

    Ultimately, gradual acceleration is most likely with Greenland (and is observed), while West Antarctica has the potential for more sudden acceleration, but isn't presently losing mass so fast.
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  9. skywatcher - Agreed; Greenland is a prime candidate for steady increases in ice loss/sea level rise, while Antarctica is a risk for jumps in sea level due to grounded glaciers cutting loose.

    Either way, folks (are you listening, Steve Case?) rises on the order of 1mm/day are entirely possible, given that warming is something like an order of magnitude faster than anything in the paleo record. And hence, so are the possibilities of sea rise rates.

    Personally, I'm not encouraging coastal real estate investments for anyone I know...
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  10. Skywatcher/KR

    Understood ... collaps means exactly that. Nothing gently timeconsumig about it. (I got waylaid by language, like: economic collapse)

    Thank you for clearinng the head. It isn't a story easily told to Monckton enthousiasts though (there are still a few left).
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  11. #60 KR at 16:09

    Yes, I read you post.
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  12. @ Steve Case #62

    I am always interested in "contrarian" opinions ... they sometimes offer a glimpse of hope in - what I consider - a bleak picture.

    Do you not agree with the figures or with the conclusion? Would you be kind enough to tell me why or refer me to something to read which reflects your view?
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  13. #63 Sascha @ 02:28

    As Skywatcher says, I think the rate of sea level rise increasing more than 100 times the current value is incredible, "argument from incredulity" he says. I like to go to the source of these sorts of claims and run the numbers. I'm always in fear of getting the sign wrong or getting decimal places wrong or going off half-cocked. Somewhere up thread it was pointed out that Dr. Hansen's curve isn't an asymptote but sigmoid or geometric. They were right. Eventually it has to level off. I can't go on forever. Considering that the geometric nature of that curve hasn't started yet, see my post #2 on this thread; I wonder when it will start. I would think anyone would.
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  14. Thanks for an informative post and many useful comments. For what it may be worth, the term "collapse" should probably be approached with caution. Living near the southeast US coast and being uncertain about what the term might mean with regards to WAIS, I asked Robert Binschadler to help me out. He allowed that a "collapse" of WAIS might give rise to a 2 m/century SLR.

    Here's an interview with Binschadler

    Here's what's happened more recently at Pine Island Glacier

    Not comforting...
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  15. Steve Case, the current ice contribution to total sea level is relatively small, and so you won't see a geometric curve in the total SLR, such as the 20-year figures shown above, as it is hidden by other components. Accelerations in ice mass loss (such as observed, e.g. Veliconga et al 2009) will come to dominate the SLR budget if the accelerations in mass loss of the ice sheets continue. As it is, the SLR curve for the whole 20th Century shows acceleration too, not visible on a 20-year plot. Quite how fast the acceleration continues, once ice loss comes to dominate, is the matter of question, not whether the acceleration has started.

    David Stoney, thanks for the links. Always worth listening to what Bindschadler has to say about Antarctica, the first link is particularly informative.
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  16. #66 Skywatcher @ 8:59

    According to the Church and White paper I've seen, the big jump in acceleration occurred in about 1925 for what reason I don't know.
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  17. Steve Case - The Church and White 2006 paper, if that is what you are referring to, does show a jump in acceleration around ~1930. "The slopes are 0.71 ± 0.40 and 1.84 ± 0.19 mm yr^-1 respectively, respectively, implying an acceleration of 0.017 ± 0.007 mm yr^-2 (95%)" They relate this to the more rapid rise in temperatures (with a 20 year lag) beginning in the early 1900's.

    From their conclusions: "From 1993, the rates of rise estimated from tide gauge and altimeter data (after correction for GIA effects) are about 3 mm yr^-1, faster than the quadratic (about 2.3 mm yr^-1) at this time." (the quadratic for the entire 20th century).

    There are ~10 year +/- variations in the rate of sea level rise, but it's definitely accelerating if you look at all of the data, rising faster than quadratic in rate.
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  18. ...Except that Church and White (2011) themselves estimate an accelerating sea level rise:
    The linear trend from 1900 to 2009 is 1.7 ± 0.2 mm year-1 and from 1961 to 2009 is 1.9 ± 0.4 mm year-1. However, there are significant departures from a linear trend. We estimate an acceleration in GMSL by fitting a quadratic to the time series, taking account of the time variable uncertainty estimates. From 1880 to 2009, the acceleration (twice the quadratic coefficient) is 0.009 ± 0.003 mm year-2 (one standard deviation). This estimate is slightly less than but not significantly different from the (one standard deviation) estimate of Church and White (2006) of 0.013 ± 0.003 mm year-2, but still significantly different from zero at the 95% level.
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  19. In a laboratory setting (constant temperature, no wind) how does ice melt? Any acceleration?

    (Please tell me to go away if I'm too ignorant. I'll still read your comments trying to understand.)
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  20. Hansen, in a 2005 editorial essay cited Kienast and said "In melt-water pulse 1A... sea level rose about 20m in approximately 400 years" which "is an average of 1m of sea level rise every 20 years".

    Which is about 0.14 mm per day. This would be a catastrophic rate for people living near the sea. He noted that the ice sheets disintegrating at the time "were at lower latitudes than the ice that remains today and the period of rapid ice sheet disintegration was undoubtedly preceded by a period in which the ice was preconditioned for collapse". But, he points out climate forcing today far exceeds the forcing that drove melt-water pulse 1A. And, he said, sea level rise doesn't have to add up to 20 m to obligate scientists who expect it to happen to warn that an event anything like it would "wreak havoc" on civilization.

    Hansen has said in the past that if the planetary energy imbalance is about 1 W/m2 and all that energy was going into melting ice it would raise sea level by about 1 m per decade. Obviously, an armada of icebergs shooting off Antarctica wouldn't have to melt to raise sea level.

    Pfeffer told a story at the AGU this year about how the sea level rise threat isn't being fully grasped even by those he would have thought must be exposed enough to the data to get it. He said MOMA in NYC exhibited a model showing a New York that by 2100 had protected itself from 2m of sea level rise. He asked the designers, what was the plan for the ongoing steady or possibly accelerating rise after that - how was their design going to cope with another meter and another one after that? Blank stare.
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  21. For what it's worth, the latest data point for sea level was just posted---and it is the highest observation in the period of satellite record. One data point, I know, but we may indeed be heading out of the "pothole".

    [This is my first time posting a hyperlink using HTML. Forgive me in advance if I botched it...]
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  22. Enginerd @72, it's a bit hard picking out that blue dot siting astride the end of the trend line. But yes, it's there, and the highest to date.
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  23. Enginerd at 13:28 PM on 15 February, 2012

    Tom Curtis at 13:31 PM on 15 February, 2012

    Here's the latest from the CU Sea Level Research Group
    CU Sea Level Research Group

    It was updated a week ago.
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  24. It will be interesting to see how much sea level rises over the next few months as conditions in the tropical Pacific move toward neutral. There seems to be greater volatility in the sea level response, but it may be temporary.
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