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Predicting future sea level rise

Posted on 18 December 2009 by John Cook

The two main contributors to sea level rise are thermal expansion of water and melting ice. Predicting the future contribution from melting ice is problematic. Most sea level rise from ice melt actually comes from chunks of ice breaking off into the ocean, then melting. This calving process is accelerated by warming but the dynamic processes are not strongly understood. For this reason, the IPCC didn't include the effects of dynamic processes, arguing they couldn't be modelled. In 2001, the IPCC Third Assessment Report (TAR) projected a sea level rise of 20 to 70 cm by 2100. In 2007, the IPCC Fourth Assessment Report (4AR) gave similar results, projecting sea level rise of 18 to 59 cm by 2100. How do the IPCC predictions compare to observations made since the two reports?

Figure 1: Sea level change. Tide gauge data are indicated in red and satellite data in blue. The grey band shows the projections of the IPCC Third Assessment report (Allison et al 2009).

Observed sea level rise is tracking at the upper range of model predictions. Why do climate models underestimate sea level rise? The main reason for the discrepancy is, no surprise, the effects of rapid flow ice changes. Ice loss from Greenland, Antarctica and glaciers are accelerating. Even East Antarctica, previously considered stable and too cold, is now losing mass. Considering the importance of rising sea level to a human population crowded around coastlines, how can we predict sea level with greater accuracy?

An alternative way to predict future sea level rise is a semi-empirical method that uses the relationship between sea level and global temperature (Vermeer 2009). Instead of modelling glacier dynamics, the method uses model projections of global temperature which can be calculated with greater confidence. Sea level change is then derived as a function of temperature change. To confirm the relationship between sea level and temperature, observed sea level was compared to reconstructed sea level calculated from global temperature observations from 1880 to 2000. Figure 2 shows the strong correlation between observed sea level (red line) and reconstructed sea level (dark blue line with light blue uncertainty range).

Figure 2: Observed rate of sea-level rise (red) compared with reconstructed sea level calculated from global temperature (dark blue with light blue uncertainty range). Grey line is reconstructed sea level from an earlier, simpler relationship between sea level and temperature (Vermeer 2009).

The historical record shows the robustness of the relationship between sea level and global temperature. Thus, global temperature projections can be used to simulate sea levels into the future. A number of different emission scenarios were used, based on how carbon dioxide emissions might evolve over the next century. Overall, the range of projected sea level rise by 2100 is 75 to 190 cm. As you get closer to 2100, the contribution from ice melt grows relative to thermal expansion. This is the main difference to the IPCC predictions which assume the portion of ice melt would diminish while thermal expansion contributes most of the sea level rise over the 21st Century.

Figure 3: Projection of sea-level rise from 1990 to 2100, based on IPCC temperature projections for three different emission scenarios. The sea-level range projected in the IPCC AR4 for these scenarios are shown for comparison in the bars on the bottom right. Also shown in red is observed sea-level (Vermeer 2009).

Figure 3 shows projected sea level rise for three different emission scenarios. The semi-empirical method predicts sea level rise roughly 3 times greater than the IPCC predictions. Note the IPCC predictions are shown as vertical bars in the bottom right. For the lowest emission rate, sea levels are expected to rise around 1 metre by 2100. For the higher emission scenario, which is where we're currently tracking, sea level rise by 2100 is around 1.4 metres.

There are limitations to this approach. The temperature record over the past 120 years doesn't include large, highly non-linear events such as the collapse of an ice sheet. Therefore, the semi-empirical method can't rule out sharp increases in sea level from such an event.

Independent confirmation of the semi-empirical method is found in a kinematic study of glacier movements (Pfeffer 2008). The study examines calving glaciers in Greenland, determining each glacier's potential to discharge ice based on factors such as topography, cross-sectional area and whether the bedrock is based below sea level. A similar analysis is also made of West Antarctic glaciers (I can't find any mention of calculating ice loss from East Antarctica). The kinematic method estimates sea level rise between 80 cm to 2 metres by 2100.

Recent observations find sea level tracking at the upper range of IPCC projections. The semi-empirical and kinematic methods provide independent confirmation that the IPCC underestimate sea level rise by around a factor of 3. There are growing indications that sea level rise by the end of this century will approach or exceed 1 metre.

UPDATE 18/12/2009: A new paper has just come out looking at sea levels during the last interglacial around 120,000 years ago (Kopp 2009). Polar temperatures were around 3 to 5 degrees hotter than now, which is the amount of warming expected for some of the lower emission scenarios. Sea levels at that time were between 6 to 9 metres higher than today. This is not to say sea levels will reach these levels by 2100 - it would take longer for the climate to reach equilibrium. What it does show is that ice sheets are vulnerable to sustained warming.

In a future post, I will look at impacts of rising sea levels. If anyone can suggest recent peer reviewed papers on this area, particularly on a global scale, it would save me a lot of leg work and be very deeply appreciated (Chris, I'm looking at you). Links to full PDFs earn you an extra gold star.

Also, many thanks to Charles Fletcher whose advice and suggestions are an immense help. His paper Sea Level by the end of the 21st Century: A Review was the original inspiration for this post. 

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Comments 51 to 69 out of 69:

  1. batsvensson, I apologize if I misunderstood you comment. But from the first paragraph it looked to me that you were pointing to reaction to weather events more than climate changes: "The reason the Dutch taking this very seriously is because they already had a break in the sea walls a fee decades ago due to an Atlantic storm [...]"
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  2. SNRatio, fast and slow are relative to the ocean response time. Using only the first linear term in temperature you cannot follow the trend even over several decades. You also need to smooth the temperatre data up to when dT/dt is also smooth (i.e. a "nice" regular trend), otherwise you would force the second term to follow temperature interannual variability which would in turn produce highly amplified and unphysical fluctuations.
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  3. @SNRatio in #50, "Sea level rise is not constant over the globe" I did not claim that either, and I am fully aware of the variability of sea levels, my claim was that the Dutch keep track on sea level changes simply because it is of great concern for them.
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  4. @Riccardo in #51, my intention was to support what you said earlier, but also to underline that the Dutch government spends a lot of tax money on this issue, not just because of some academic research interest in the sea and/or weather but because of pure self preservation. Therefore, one can suspect that the Royal Dutch Metrological Institute, KMNI, will taking any climate change issue based on sound scientific research very very very seriously - after all they are responsible for the well being of about 16 million humans lives.
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  5. batsvensson, oh, ok, now I see your point. I'd like people living in the big river deltas of our planet could have similar attention, money and technology from their governments to plan their future.
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  6. Some don't have that money Riccardo (Bengladesh). Some could but it gets lost in corruption and all that (Nigeria). If only the use of money by governments was based on rational choices...
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  7. Definitely. Were governments able to easily take rational choices and had people and countries enough money, why there should be meetings and discussions in Copenhagen and the like? ;) Back in topic, whatever the world will do, sea level won't stop rising any soon and adaptation is going to be essential in many places. And money, a lot indeed, is required.
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  8. RE post 56 "Some ( countries ) don't have that money Riccardo (Bengladesh). Some could but it gets lost in corruption and all that (Nigeria). If only the use of money by governments was based on rational choices..." Phillipe ....A very perceptive observation. And what is the UN's answer to global warming ? To give developing nations billions of dollars! Now, when openly corrupt governments of countries like Zimbabwe, Angola, Sierra Leone, Indonesia, to name a few, get their hands on this money, where do you think the money will go ? I have lived and worked in several of these countries for a total of nearly 15 years and the corruption has to be seen to be believed. These governments have little regard for the well being of their citizens in terms of food, housing, health, education etc, and much less interest in the alledged effects of global warming. As usual, most of the money will be siphoned off into the pockets of corrupt politicians, their family, friends and hangers-on.
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  9. You have nothing to teach me on the subject, unfortunately. I grew up in Africa and worked there too. My parents lived and worked there for 17 years. A closer look at European countries and the US taught me that no category of country has a monopole on corruption, which is simply a feature of Human nature. In Africa, it is more obvious because they don't care about appearances as much and the underlying poverty is, by our standards, extreme. I'm not sure your characterization of the compensation intended is really accurate, but in any case, what else do you propose be done? Nothing? Integrate the real price of carbon in the world's economy without softening the impact in some way to the most vulnerable, who will also likely be the most affected by climate change? These countries have benefited from assistance before, even if the benefits were reduced by corruption. Should nothing have been done then? That would have been a more rational thing to do? In any case, this is OT. I apologize to John for starting the discussion and won't pursue it.
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  10. Phillipe...... I find your reply rather baffling. Of course corruption happens in every country of the world. Where did I state otherwise ? Although in my case it is only in African countries that I have had to pay bribes to leave the country and have my money mysteriously disappear from my bank account. Situations which I have yet to experience in Europe. Also, where did I state that we do nothing with regards to global warming ? If burning coal to produce power causes global warming, then one obvious solution is to go nuclear. But over the years environmental groups have fought against using nuclear power to generate electricity and the result was that in Australia, for example, we have had to rely mainly on coal fed power stations. And now the same environmental groups are complaining that this causes global warming. Yes, my "characterization of the compensation intended" is accurate - developing nations around the world are demanding hundreds of billions of dollars per annum. Introducing a world-wide ETS will inflate the cost of just about everything in the Western world. And when you consider that much of the goods and services consumed in developing countries are imported, this will mean that the increased cost of these goods and services will be passed on to these countries. So while the developing countries will get billions of dollars in compensation, they will also be paying billions of extra dollars in the form of higher prices for goods and services.
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  11. Bern @46 Potential crop failures,famines, severe water shortages, wars, diseases etc are things which mankind has had to battle in the past and no doubt these phenomena will occur in the future with or without alleged man-made global warming. With regard to sea level rise, we have yet to see the "catastrophic" rise in sea level forecast by climate researchers. And if sea level does rise, I don't think it will rise so fast that nothing can be done to help mitigate the effects. By the same token, when the next ice-age comes this will also cause mankind some hardship. Even Hansen discusses the possibility of a mini ice-age in the coming decades. One of the big challenges that mankind faces is the population explosion - about an extra 160 million extra people every year as I understand. All of these will require energy for warming, cooling, cooking etc etc.
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  12. Please excuse me, but am I missing something? Does not most of the discussion on sea level rise miss the most important point? As the Koop article in Nature, 17DEC09, points out, the maximum sea level during the previous interglacial was about eight (8) meters above the current sea level. Does this not suggest the following: 1. Future sea levels will likely be 8 meters higher than we have now - with no help from man. (Things are NOT different, this time.) 2. Don't worry about the Maldives, etc. They will be gone, whatever we do. 3. Don't waste current resources (yen, eruos, dollars, etc.) on wishful-thinking projects. 4. With regard to Climate Change, do only science and do no politics, i.e., if you are a politician, butt-out. P.S. Please don't think that fast climate change must be man-made. Don't believe me, read "The Two Mile Time Machine".
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  13. rajpe, to be in topic i will consider just #1 and 2. During the last interglacial sea level was several meters (4-8) above current level and Greenland was about 3 °C warmer and with less ice than today. That makes sense. What does not make sense is to assume that we are naturally heading there again. Indeed, sea level has been almost constant in the last 2000 years and from about 7 to 2 thousands years ago it rose only a little and very slowly. There is no indication at all that we were heading toward the same state as the last interglacial. Maybe you are right and it's too late to save the Maldives whatever we may try to do starting from now. But it could be worse, much worse than just the Maldives.
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  14. It seems that we all agree that sea level rise depends mainly on global warming. So will the sea level descend with cooling, I assume. Therefore it is very interesting to read the article “The mini ice age starts here” from David Rose in the Daily Mail, link: Professor Mojib Latif, a leading member of the IPCC, together with Professor Anastasios Tsonis explain that we are entering a mini-ice-age for the next 30 years. Thus the sea level will even descend the next 30 years. Climategate and the above mentioned story will make the discussions easier about what is going to happen the next decades!
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  15. No, A. Bakker, that claim of impending global cooling by the Daily Mail "reporter" has been vehemently rejected by Latif--the very scientist it misquoted.
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  16. This common misinterpretation was worth a climate crock of the week.
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  17. Satellite sea level data are valuable in determining short time and/or regional changes, but they are absolutely infeasible to draw secular trends because of orbital drift. The problem is overcome by calibrating satellite measurements against tide gauges (just sixty four of them). The moral of the story is that sea level trends measured by satellites are not better than those measured by a few gauges at unspecified locations. On the other hand, gauge data are of course measured relative to shore elevation which is also subject to regional change. Crustal segments have different and considerable vertical movement relative to both eachother and sea level. It is easy to see that the more gauges are used, the smaller the error gets. However, satellites are only calibrated against a restricted set. I could not find documentation about geographic distribution of tide gauges used in calibration at the UCB site. Reliable GPS calibration is not done yet, although it is said to be in the pipeline.
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  18. Hi John, Great site, but I've just noticed something that doesn't, at first sight, seem right. The graph is taken from the Copenhagen Diagnosis report (fig 16) but that report says that the satellite data is taken from the Cazenave et al 2008 paper (Cazenave, A., et al., Sea level budget over 2003–2008: A reevaluation from GRACE space gravimetry, satellite altimetry and Argo, Glob. Planet. Change (2008), doi:10.1016/j.gloplacha.2008.10.004) But that paper only covers the 2003 - 2008 period. Nor does it indicate the bounds of the observational error.
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    Response: I imagine the authors took the analysis from Cazenave 2008 and updated it with the latest data. You'd need to contact the authors to confirm this (and please report back here to let us know what they say if you do).
  19. Thanks for the response, I e-mailed the authors and got the following back this morning (I haven't been able to look at the paper yet though). Dear Mr McC******, Thank you for your query. You are right that the cited paper discusses only the recent part of the satellite altimetry data shown in Fig. 16. The full data set is discussed in: Cazenave, A. and W. Llovel, 2010: Contemporary sea level rise. Annual Reviews of Marine Science, 2: p. 145-173. There, and in the references therein, you will also find a discussion of the uncertainties in those data. Best regards, Patrick Eickemeier
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    Response: Thanks for following this up and reporting back, much appreciated.

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