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How much will sea levels rise in the 21st Century?

What the science says...

Select a level... Basic Intermediate

Sea levels are rising faster now than in the previous century, and could rise between 50cm to 1.5 metres by 2100

Climate Myth...

Sea level rise predictions are exaggerated

"Professor Niklas Mörner, who has been studying sea level for a third of a century, says it is physically impossible for sea level to rise at much above its present rate, and he expects 4-8 inches of sea level rise this century, if anything rather below the rate of increase in the last century. In the 11,400 years since the end of the last Ice Age, sea level has risen at an average of 4 feet/century, though it is now rising much more slowly because very nearly all of the land-based ice that is at low enough latitudes and altitudes to melt has long since gone." (Christopher Monckton)

Measuring Sea Levels

Sea levels are rising due to thermal expansion and melting of land-based ice. Global warming is causing the oceans to absorb a lot of extra heat (up to 90%). This makes the volume of water expand, and sea levels rise. The Greenland and Antarctic ice caps, and many of the world’s glaciers, are all slowly melting. The runoff feeds into rivers and directly into the oceans. This too adds to sea levels.

Prior to the use of satellite systems, measurements were taken using tide-gauges, devices that measure the height of a water level relative to a fixed point on land. Global estimates of sea level rise were subject to substantial differences in measurement from different parts of the world.

Sea levels change all the time. They are affected by seasons, astronomical tides, storm surges, currents and density, among other influences. Tidal gauges reflect these short term influences, introducing a large margin of error.

The IPCC Fourth Assessment Report described studies that estimated sea level rise for the 20th century between 0.5 and 3.0 mm a year. The most likely range, according to the IPCC, was between 1.0 and 2.0 mm a year.

Satellite altimetry since 1993 provides a more accurate measure of global sea level rise. Three different satellites take measurements: TOPEX/Poseidon (launched 1992), Jason-1 (launched 2001) and Jason-2 (launched 2008).

Figure 1: Source - CSIRO

The IPCC projections are derived from climate models. Using both tide gauge and satellite data, we can see that sea levels are rising. Unfortunately, sea level rise is already tracking the worst-case projections, as this graph shows:  


Figure 2: 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).

In fact, the climate models underestimated the rate of sea level rise because the rapid melting of the ice sheets and glaciers was not incorporated in the last IPCC report. (It was left out because the data were not considered sufficiently robust).

Damaging Potential

Rising sea levels are widely considered to be the greatest threat posed by climate change. They threaten low-lying countries with inundation, forcing inhabitants to migrate.  Coastal cities and ports could be flooded, as could cities sited near tidal estuaries, like London. Many nuclear installations are built by the sea so they can use sea water for cooling.

The potential for sea level rise is enormous. This is because the ice caps - Greenland and Antarctic - contain huge amounts of fresh water - around 70% of all the freshwater on Earth. Estimates suggest that if the Greenland ice sheet was to melt away to nothing, sea levels would rise around 6 metres. To put that a different way, a loss of just one per cent of the Greenland ice cap would result in a sea level rise of 6cm. 

If the West Antarctic Ice Sheet (WAIS) were to melt, this would add around 6 metres to sea levels. If the East Antarctic Ice Sheet (EAIS) were to melt as well, seas would rise by around 70 metres.

In a process that is accelerating, all three ice caps are losing mass. While nobody is suggesting any of the ice caps will melt away to nothing, only a small amount of melting would cause great problems.

A 1% loss of ice from these three sources would produce a likely increase in sea levels of around 76cm. With the thermal expansion implied by such melting, and contributions from melting glaciers, the oceans would actually rise far more.

Predictions for future sea levels

Future sea level rises depend on a number of factors. The amount of CO2 emitted will determine how much global warming takes place. The amount of ice that melts will vary according to the amount of global warming. The same is true of thermal expansion.

Previous estimates of sea level rise have been based on a set of possible outcomes called emissions scenarios. These theoretical scenarios range from emissions which fall very quickly, to emissions that continue to rise even faster than they have already. Scientists then calculate possible outcomes for each scenario.

In the next IPCC report (AR5), due in 2014, a new method has been used. Emission scenarios have been replaced by Representative Concentration Pathways (RCP). Four trajectories were chosen, based not on emissions, but possible greenhouse gas concentrations in the year 2100. From the concentrations, the RCPs project a ‘forcing’ for each pathway (the amount of warming); 2.6, 4.5, 6.0, and 8.5 Watts per metre squared. Each pathway is named after it’s forcing e.g. RCP4.5. The lowest emission scenario is also referred to as RPC3PD, because it posits a peak warming of 3 w/mby 2070 (~490 ppm CO2 and equivalents), and a reduction to 2.6 w/m2 by 2100. (PD stands for Peak/Decline). 

A draft version of the next report from the IPCC (AR5), due for publication in 2014, was recently leaked. Although the information is subject to change, the draft report says sea levels are likely to rise by between 29 and 82 centimeters by the end of the century, (compared to 18-59 centimeters in the 2007 report).

Other recent studies have projected comparable sea level increases. Jevrejeva (2011) for example modelled sea level rise using RPC scenarios. This table shows best and worst cases (RPC3PD and RCP8.5), with two in between. The figures for each projection are listed in this table:

Table 1: Projected sea level rise (m) by 2100 for the RCP scenarios. Results presented as median, upper (95% confidence interval) and lower (5% confidence interval) limits, calculated from 2,000,000 model runs. Sea level rise is given relative the period 1980–2000. (Jevrejeva 2011)

Another study (Rahmstorf 2011) obtained much the same results:

Figure 4: Sea level hindcasts and projections for different models calibrated with different temperature and sea level data. The error bars on the right indicate 90% confidence intervals (5–95 percentile, using the GISS temperature dataset); for the proxy-based projection the uncertainty is as presented in Kemp et al., 2011. (Rahmstorf 2011)

What's in the pipeline? 

The 'pipeline' is a term used to describe the slow reaction of the oceans to heating (inertia). Even if we were to stop emitting greenhouse gases tomorrow, the oceans would continue to rise, driven by the heat already stored. (90% of all the sun's energy falling on the surface of Earth is absorbed by the oceans as heat). This sea level rise is said to be 'in the pipeline'.

A paper published in PNAS - Levermann (2013) - has found that greenhouse gases emitted today will cause sea levels to rise for several centuries. For every degree of warming, sea levels will rise by more than 2 meters in the next few centuries. The Earth's temperature has already risen 0.8 degrees C over pre-industrial temperatures. 

Jevrejeva (2011) also found increased rates of sea level rise, even if emissions were to stabilise at 490 ppm by 2070 following the scenario in RPC3PD (RPC2.6):

Table 2: Projected sea level rise (m) by 2500 for the RCP scenarios. Results presented as median, upper (95% confidence interval) and lower (5% confidence interval) limits, calculated from 2,000,000 runs of the model. Values of sea level rise are given relative the period 1980–2000.


Based on the new mid-range IPCC  RCP4.5 scenario - around 650 ppm CO2 and equivalents producing a forcing of approximately 4.5 watts/metre - the most likely sea level rise by 2100 is betweem 80cm and 1 metre.  Longer term, sea levels will continue to rise even after emissions have been reduced or eliminated.

Basic rebuttal written by GPWayne

Update July 2015:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial

This rebuttal was updated by Judith Matz on September 13, 2021 to replace broken links. The updates are a result of our call for help published in May 2021.

Last updated on 12 July 2015 by MichaelK. View Archives

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Further viewing

From Peter Hadfield (potholer54 on YouTube) published on Dec 5, 2021

Compare two photos 130 years apart and it looks as though sea levels haven't moved. So why all the fuss about rising sea levels and evacuating islands? This video closes the yawning gap between internet myths and science.


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Comments 51 to 75 out of 102:

  1. Maybe its because the 'soil' stock change isn't displayed? However, it almost looks like the -30 +/ 45 PgC number refers to both Vegetation and Soil. In this case, if I didn't misread the graph otherwise, I lack any explanatory approach :)

  2. Ok, reading more carefully I got the answer: The land net flux was just negative in parts of the past in between pre-industrial and now; in the graph however everything is shown for recent time.

  3. In the graph showing sea level rise data versus IPCC Assessment Three projections ("Sea Level Rise - models & observations"), why do the projections start in 1990, when the report came out in 2001?

  4. Can anyone here comment on this:

    On the surface it looks like more shanigan's have been found, and at least in these locations the rate of sea level rise was exagerated. 


    [PS] No accusations of fraud. Since paper is by "Albert Parker", I would treat it with some strong skepticism.  eg see here. and here

  5. Bill 13 @54: The paper that you have provided a link to demonstrates that science is a continuous process of discovery. 

  6. Ollier is a serial fake-skeptic (SkS did a take-down of his work, here).  Both Ollier and Parker are affiliated with the fossil fuel-mouthpiece Energy and Environment.

    The real travesty is examing just a handful of locations measuring sea levels in just a small part of the world and somehow handwaving away global datasets showing global sea level rise, inluding satellite measurements with global coverage.

    Instead of what science has found, this:

    Actual SLR


    Deniers want us to see this:

    Denier SLR

  7. Bill13 @54.

    The nonsense written by Parker & Ollier (2017) will be celebrated (indeed is being celebrated) by the deluded community of AGW denialists. It is a very silly piece of work that boldly states that the alignment of messy tidal gauge data by others is wrong and then dreams up a fantasy scheme of alignment that suits their won purpose. They couldn't even be bothered to take the annual cycle out of the data which shows how silly their work is. They thus contradict Unnikrishnan & Shankar (2007) and fails to even mention Woodworth et al (2009). It will probably be some time after those deluded denialists have found some other nonsense to cheer about that the eggregious errors perpetrated by Parker & Ollier will be set out within the literature. Proper scientists have better things to with their time than debunk the work of Parker (assuming he's still calling himself that these days) & Ollier.

    It is sad that these two pick on Yemen to do their denialist act as Yemen is a country particularly vulnerable to SLR, "one of the top five most vulnerable low-income countries," according to Al Saafani et al ((2015).

  8. Bob Loblaw, Eclectic and michael sweet,

    This has been an interesting journey, exploring what I will describe as the “conflicting views” on the future sea level rise “predicted” for the remainder of the 21st century. Let me say that I appreciate that my use of the term “predict” is used in a general sense and that many of what I refer to as “predictions” are in fact “projections” because they are predictions based upon certain assumptions relating to a number of things but most importantly, the level of CO2 emissions based upon the various pathways assumed by the IPCC.

    The relevance of the views of a lawyer are on such a technical subject as “sea level rise” is certainly questionable but I suspect the interest of Bob Loblaw is simply because there are a number of legal cases that will be coming before the courts of the United States over the next few years and these cases will be adjudicated by lawyers and not physicists or other scientists. Having said that, there are many lawyers who have an engineering or scientific background before entering law so there may be some hope of having a scientist hear the case. In my case, my undergraduate degree was in the “dismal science”.

    In researching this topic, I have largely focused on Chapter 13 of the IPCC Fifth Assessment (Fifth Assessment) and those portions of Chapter 3 dealing with sea level rise as well as blog information contained on this website on the subject as well as blog information on one other website (which does not carry much weight from most of the commentators on this website). I have also read the US Climate Science Special Report published in late 2017 (US Climate Report) as well as the very good RealClimate article on the Fifth Assessment (suggested by Bob Loblaw).

    But before I delve into my impressions from these sources, I would also like to reference the discussion of “uncertainty” both in the Fifth Assessment and the US Climate Report. In both reports, the extent of understanding (and certainty or uncertainty about that understanding) is based upon levels of confidence (dealing with the consistency of the evidence and degree of agreement within the literature) and likelihood expressed probabilistically (based upon the degree of understanding or knowledge).

    What I want to focus on are the levels of “Confidence”:

    “Medium Confidence” means suggestive evidence (a few sources, limited consistency), competing schools of thought.

    “High Confidence” means moderate evidence (some sources, some consistency) medium consensus

    “Very High Confidence” means strong evidence (established theory, multiple sources) high consensus.

    All of the definitions for uncertainty are found in the US Climate Report in the “Guide to this Report” which is easily located.

    I think it is very important to keep these measurements in mind when analyzing the findings of the Fifth Assessment. When they use “Medium Confidence” they do not mean “medium consensus” because that term is reserved for “High Confidence”. Unless the term “Very High Confidence” is used then there is considerable uncertainty remaining.

    So to commence this research the most logical place to begin is the Fifth Assessment projections found at Section 3.7.6:

    "It is very likely that the global mean rate was 1.7 [1.5 to 1.9] mm yr–1 between 1901 and 2010 for a total sea level rise of 0.19 [0.17 to 0.21] m. Between 1993 and 2010, the rate was very likely higher at 3.2 [2.8 to 3.6] mm yr–1; similarly high rates likely occurred between 1920 and 1950."

    Figure 3.14 of the Fifth Assessment shows the “bump” in sea level rates in the period 1920 to 1950. Given that the accepted view is that the rapid increase in the use of GWG’s only started after 1950, it seems incumbent on scientists to explain the “bump”. The only explanation I could find in the Fifth Assessment was that this “bump” was ”likely related to multi-decadal variability”. See Section 3.7.4. However, the natural question is if “multi-decadal variability” caused the increase in rates in the 1920-1950 period then why cannot the increase in rates found since 1993 of approximately 3.2 mm/yr also be attributed to multi-decadal variability? Or should not at least a portion be attributed to this internal variability, if only a portion, then how much?

    So the Fifth Assessment found that it was “very likely” (read 90-100%) that the average rate of sea level rise since 1901 was 1.7 mm/yr. But before we get into the 3.2 mm/yr rate, we now have a number of papers since the Fifth Assessment that have suggested that the Fifth Assessment’s 90-100% assured estimate is all wrong and the real rate for 1901 to 1990 is 1.1 to 1.2 mm/yr. (Hay 2015 Dangendorf 2017). When asked by others how the IPCC could have got this so wrong, the answer seems to be that everyone is entitled to be wrong, that is science. I fully agree but it does not necessarily engender confidence in other “Very Likely” predictions or projections of the IPCC in the Fifth Assessment.

    Perhaps the IPCC will, in the Sixth Assessment actually maintain its 1.7 mm/yr rate which I understand was similar to the AR4. Why do I say this? Because my understanding is that these “new” lower estimates are largely based upon a reanalysis of VLM. But here is what the Fifth Assessment has to say about VLM adjustments:

    "High agreement between studies with and without corrections for vertical land motion suggests that it is very unlikely that estimates of the global average rate of sea level change are significantly biased owing to vertical land motion that has been unaccounted for. {3.7.2, 3.7.3, Table 3.1, Figures 3.12, 3.13, 3.14}"

    So now on to the $64,000 question as to whether the observed acceleration in sea level rise since 1993 is an increase in the long term rate or is reflective of decadal variability or only reflects “apples and oranges” measurement issues with satellite altimetry compared to tide gauges.

    We have disagreements both on the rate of acceleration and the causes of the acceleration.

    Firstly, we have a disagreement between the Fifth Assessment estimates of what the acceleration rate is and the recent Nerem 2018 paper. From the Fifth Assessment, the acceleration is quite small with Ray & Douglas (2011) at -.002 to .002 mm/y, Jeverejeva (2008) at .012 mm/yr and Church & White (2011) at .012 mm/yr. Then we have Nerem (2018) re-evaluating things and coming up with .084 mm/yr. I do not propose to get into the technical disagreements that I have read on the Nerem (2018) paper but even extrapolating his acceleration, his projected 2100 sea level rise is somewhere around 65 cm close to the low range of the IPCC RCP8.5 estimate. Although I am not qualified to make any judgments, I suggest that anyone who is qualified should at least read the comments made by FrankClimate on the other website under the Part IV discussion on sea level acceleration. Without question, FrankClimate is technical. His comments have now been incorporated into the Part IV discussion. Would be interested to hear from Eclectic as to whether he disagrees with FrankClimate.

    Secondly, we have questions of what is the cause of this recent acceleration since 1993. I had to ask myself why 1993 and not 1990? The obvious answer is that it is in 1993 that satellite altimetry came into the equation with the launch of the TOPEX satellite. Although I think there is general agreement that there are serious questions about whether the data from TOPEX for the first six years should be used at all (or for that matter even the remaining period for that satellite), my sense from looking at the NASA website is that the satellite altimetry is pretty well matching the tide gauges. I think there are a number of people who disagree with me on this but the average rates seem to match. But it is curious that where we see this very large increase in SLR is not at the land-based tide gauges but out in the middle of the oceans. It at least led me to ask myself whether this significant difference between the tide gauge measurements and satellite altimetry measurements in the middle of the oceans would have always been there if we could have measure it with satellites much earlier. I fully appreciate that the tide gauge measurements have shown an upward trend since 1980 (Section 3.7.4) but my understanding is that the large average increase during the satellite era can be attributed to the large increases found in the middle of some of the oceans, especially the Indian Ocean.

    But back to attribution. A number of authors have suggested that the way to reconcile the “bump” in 1920-1950 and the increases since 1990 is to link these climate changes to multi-decadal variability, and specifically the AMO or the PDO. Here is what the Fifth Assessment has to say about this at 3.7.4:

    "Several studies have suggest­ed these variations may be linked to climate fluctuations like the Atlan­tic Multi-decadal Oscillation (AMO) and/or Pacific Decadal Oscillation (PDO, Box 2.5) (Holgate, 2007; Jevrejeva et al., 2008; Chambers et al., 2012), but these results are not conclusive."

    Others have said that the increase in SLR since 1990 is not “statistically relevant” when looking at the long term sea level rise. In that respect, the Fifth Assessment does make the following statement immediately following the above quotation:

    "While technically correct that these multi-decadal changes represent acceleration/deceleration of sea level, they should not be interpreted as change in the longer-term rate of sea level rise, as a time series longer than the variability is required to detect those trends."

    For those who say that the acceleration should be attributed to AGW, they largely point to the increased rates of melting in the glaciers and the Greenland ice sheet and potentially catastrophic impacts relating to the West Antarctic Ice Sheet (WAIS). I cannot obviously get into discussing these topics without clearly being “snipped” for too long a post. In my view, having read the Fifth Assessment, the risk of “dynamic changes” in WAIS (there is virtually no risk with the topography of Greenland bedrock) are minimal. Here is what the Fifth Assessment has to say about the MISI hypothesis relating to WAIS at

    "In summary, ice-dynamics theory, numerical simulations, and paleo records indicate that the existence of a marine-ice sheet instability asso­ciated with abrupt and irreversible ice loss from the Antarctic ice sheet is possible in response to climate forcing. However, theoretical consid­erations, current observations, numerical models, and paleo records cur­rently do not allow a quantification of the timing of the onset of such an instability or of the magnitude of its multi-century contribution."

    As to the evidence of a retreat of WAIS, see Chapter 13 at

    "Although the model used by Huybrechts et al. (2011) is in principle capable of capturing grounding line motion of marine ice sheets (see Box 13.2), low confidence is assigned to the model’s ability to cap­ture the associated time scale and the perturbation required to ini­tiate a retreat (Pattyn et al., 2013)."

    What this tells me is that there is a “theoretical” danger but so far we do not have any evidence of an actual retreat or the time frame over which this could occur. We cannot base our rational responses to AGW based upon theories which have not been supported with observational evidence.

    As for the Greenland ice sheet, we know that the major warming was caused by warm waters appearing around Greenland and the impact that this has had on the melting of the ice sheet in the peripheries around the ocean at least from 1990 to 2012. My understanding is that this has been attributed to a decrease in cloudiness associated with the NAO which would mean that it was the increased insolation which caused the increase in the melting. Here is the discussion in FAQ 13.2 regarding the Greenland ice sheet:

    "Although the observed response of outlet glaciers is both complex and highly variable, iceberg calving from many of Greenland’s major outlet glaciers has increased substantially over the last decade and constitutes an appreciable additional mass loss. This seems to be related to the intrusion of warm water into the coastal seas around Green­land, but it is not clear whether this phenomenon is related to inter-decadal variability, such as the North Atlantic Oscillation, or a longer term trend associated with greenhouse gas–induced warming. Projecting its effect on 21st century outflow is therefore difficult, but it does highlight the apparent sensitivity of outflow to ocean warming. The effects of more surface melt water on the lubrication of the ice sheet’s bed, and the ability of warmer ice to deform more easily, may lead to greater rates of flow, but the link to recent increases in outflow is unclear."

    With the above information, the question that has been posed to me is where would I place the estimate of GMSL at 2100 compared to the Fifth Assessment (RCP 8.5) projection of .59cm to .98cm?

    Firstly, it seems to me that during the 20th Century we had an almost linear rise in sea level as is acknowledged by the Fifth Assessment at 13.3.6 at p. 1159:

    "GMSL rise during the 20th century can be account­ed for within uncertainties, including the observation that the linear trend of GMSL rise during the last 50 years is little larger than for the 20th century, despite the increasing anthropogenic forcing (Gregory et al., 2013b)."

    Here is a larger quote from the same Gregory paper:

    “The largest contribution to GMSLR during the twentieth century was from glaciers, and its rate was no greater in the second half than in the first half of the century, despite the climatic warming during the century. Of the contributions to our budget of GMSLR, only thermal expansion shows a tendency for increasing rate as the magnitude of anthropogenic global climate change increases, and this tendency has been weakened by natural volcanic forcing. Greenland ice sheet contribution relates more to regional climate variability than to global climate change; and the residual, attributed to the Antarctic ice sheet, has no significant time dependence. The implication of our closure of the budget is that a relationship between global climate change and the rate of GMSLR is weak or absent in the twentieth century. The lack of a strong relationship is consistent with the evidence from the tide gauge datasets, whose authors find acceleration of GMSLR during the twentieth century to be either insignificant or small.”

    This is consistent with the “Munk enigma” that he saw a near linear increase in GMSL during the 20th Century notwithstanding the impact of AGW only in the second half.

    The Fifth Assessment RCP 8.5 assumes that in the second half of the 21st Century we will have what at least are “quadratic increases” if not “exponential increases” in the GMSL rate. I have no understanding of how a “quadratic curve” differs from an “exponential curve” and I do not have to notwithstanding all of the debate that I read on this issue on the “other website”. What I do know is that it is much steeper than a linear increase.

    From Table 13.5 the Fifth Assessment has acknowledged that in the case of RCP 8.5 that in the period 2018 to 2100 they project an average sea level rate of 11.2 mm/yr for the mid-case and for the high case of .98 m the projected average rate is 15.7 mm/yr. See Section 13.5.1 at page 1180:

    "The rate of rise becomes roughly constant in RCP4.5 and RCP6.0 by the end of the century, whereas acceleration continues throughout the century in RCP8.5, reaching 11 [8 to 16] mm yr–1 in 2081–2100."

    Notwithstanding this projection, the Fifth Assessment acknowledges that this would exceed the average rate of 10 mm/yr during the deglaciation after the Last Glacial Maximum when there were massive ice caps over North America and Europe and Asia to supply the melt water (Chp13 pg. 1205):

    "For the RCP8.5 scenario, the projected rate of GMSL rise by the end of the 21st century will approach average rates experienced during the deglaciation of the Earth after the Last Glacial Maximum."

    The IPCC clearly understood this but did not explain how this could be achieved given the lack of such volumes of ice now (Chp 13 pg. 1185):

    "The third approach uses paleo records of sea level change that show that rapid GMSL rise has occurred during glacial terminations, at rates that averaged about 10 mm yr–1 over centuries, with at least one instance (Meltwater Pulse 1A) that exceeded 40 mm yr–1 (Section 5.6.3), but this rise was primarily from much larger ice-sheet sourc­es that no longer exist."

    Grammatically, the phrase “but this rise ….” modifies the reference to 10 mm/yr and not 40 mm/yr.

    The IPCC projection of sea level rise attributes the largest rise to thermal expansion, secondly to glaciers, and thirdly to the Greenland ice sheet mass balance loss and with a negative contribution by the Antarctic ice sheet.

    As to the IPCC’s ability to adequately model dynamic changes to the Greenland and Antarctic ice sheets here is what the Fifth Assessment says at pg 1187:

    "As discussed in Sections and, there is medium con­fidence in the ability of coupled ice sheet–climate models to project sea level contributions from dynamic ice-sheet changes in Greenland and Antarctica for the 21st century. In Greenland, dynamic mass loss is limited by topographically defined outlets regions."

    Note the use of the term "Medium Confidence".

    With all of the above research, given that I could not accept some of the projections of the IPCC for RCP8.5 (leaving alone the fact that RCP 8.5 is probably unrealistic given the changes we see in a move to renewable energy sources at least in the developed world) the question came down to what would I guesstimate the GMSL for 2100 if for some reason I was asked my opinion (which I was by Bob Loblaw).

    For me, I would go back to the observations and look at where the sea level has moved since 1900 and assume that it will follow along the same largely linear path that it has pretty well followed since we have kept records in tide gauges. Taking Figure 13.27 of the Fifth Assessment and applying a ruler to the line, it projects out to about .4m by 2100. In other words, whatever impact CO2 emissions have had they are “baked in the cake”. What we see is what we will get.

    Using the most recent date online at NASA, as of December 2017, we have had an 87.5 mm rise since 1993 representing an average rate of 3.2 mm/yr according to the NASA website. If we multiply this figure of 3.2 mm times 82 years, we arrive at around 26.24 cm of further rise if the rise continues to be linear. If you add this 26.24 to the .19 cm for the period 1900 to 1990 it totals 45.24 cm.

    So my guess is that we probably will have a further 21 to 26 cm from now until 2100 representing somewhere around 8 to 10 inches of sea level rise. Unfortunately, I will not be around to see if I am right!

  9. Norrism

    You have decided to go with the lowest estimate that you can find.  You are welcome to choose whatever you want.  

    I compared the projections of the IPCC third assessment (2001), IPCC AR4 (2007), IPCC AR5 (2013) and the 2017 US Climate report.  I notice every 5 years that the estimates go up substantially.  You pick the lowest estimate from the 2013 report, neglecting the 2017 report.  In recent newspaper articles, one of the lead authors of the last two reports testified in court that these reports are writen conservatively ie they give low ball estimates for political reasons.  In 2013 surveys reported at RealClimate reported that a majority of sea level experts estimated likely sea level rise as higher than the IPCC upper estimates. 

    I am surprised that you have decided to ignore the 2017 report and also ignore recent new knowledge about ice sheet instability.  You are allowed to pick and choose as much as you like, but I think we need to consider all knowledge when we make policy choices and we need to apply the Precautionary Principle to preserve a living environment for future generations.  Obviously, you feel no responsibility for the future after you are gone.

    I presume that you do not purchase car, life or house insurance since the chance of you making a claim this year is too small.  No problem there, you are too old to suffer if it turns out that sea level rises 8 feet, an amount that cannot be ruled out with current knowledge.  Perhaps you should study how the Precautionary Principle applies to this type of problem.  I note that in many states engineers are required to design to withstand possible issues like sea level rise so they have to use higher estimates (six feet or more) for their calculations.

    You have decided to go with Curry.   You understand that Curry is held in contempt by the scientiic comunity and that her fame is solely due to her taking a position contrary to world scientific opinion.  I suggest you invest in land near the ocean.  Prices are currently depressed because others think high sea level rise is more likely than you.

    I see no mention of sea level refugees in your post.  Too bad for poor people elsewhere who get screwed, they will just have to move to somewhere that is not near you.

  10. michael sweet@59,

    I shared a different version of the following as a comment on "In court, Big Oil rejected climate denial”. And that comment was an expansion of a comment I made on "Climate scientists debate a flaw in the Paris climate agreement" in response to John Hartz's comment pointing to an article about the IPCC reports already being an outdated understanding when they are published.

    Several years ago I heard a CBC Radio interview of one of Canada's scientist representatives working on writing an IPCC Report explaining that political-minders had influence on what was written by their nation's scientists involved in the IPCC Report process. The only limit on the political push was that the final wording had to be scientifically supportable (scientifically representing the available evidence).

    Political interests wishing to diminish or delay corrective climate action almost certainly abused that process to push for the least disastrous presentation that can be supported by the available information. They pushed as hard as possible toward the 'feel good news' side of how bad things will be. They may have even been the reason for the unclear, able to be abused for political marketing purposes, term 'hiatus' being included in one of the reports.

    As more information is obtained, as climate science is expanded and improved, it gets harder to push that low. An example is that it is harder to exploit the term 'hiatus'. However, much delay of increased public awareness and understanding was achieved. And the trouble-makers care about Winning any extension or increase in their ability to get away with their understandably unacceptable Private Interests.

    I have clarified in other comment threads that it is unacceptable for anyone to benefit from creating challenges/harm/reduced access to resources that Others, including future generations, will have to deal with. The generation that includes people benefiting from creating future challenges owes future generations the neutralization of those damaging unsustainable pursuits, or a building a conservative solution that is almost certain to mean that future generations face no challenge, getting a truly better future (the obligation and expense is on the current generation).

    On the matter of sea ice levels that means the current generation building all of the corrective protective features now, based on a worst case understanding of the possible future result of the unsustainable and harmful activities some people get away with benefiting from. The one way the current generation can reduce that requirement is by reducing activities that generate the need for such a requirement.

    So, 'legal arguments' made by the likes of Chevron in the California case that the earlier IPCC reports were not as adamant about how bad things were, how unacceptable what had developed was, are constructed by abusing the fact that each IPCC Report is subject to political manipulation by the 'powerful likes of Chevron/Exxon/Koch Industries and the elected representatives they can influence' towards down-playing how bad things have developed to be and how much worse they will become. They are trying to manipulate the system to 'not be required to correct the understandably incorrect developments they pursued and benefited from'.

    Increasing awareness that 'legal' does not mean 'ethical or helpful or acceptable' is an important part of the required increased awareness and better understanding that will make a beneficial difference for the future of humanity. And increasing awareness that popularity and profitability have proven to be lousy measures of acceptability, and detrimental to ethics and helpfulness, is also important.

    But the most important increase of understanding is that systems that develop unsustainable activities that are harmful to others also develop resistance to being corrected. And they need to be corrected to become systems that promote the development of sustainable improvements for all of humanity (systems that push to achieve, and improve, all of the Sustainable Development Goals). And the portion of the population that resists correction are understandably 'the core system problem'.

  11. michael sweet @ 59

    The US Climate Report increases the upper level of the very likely range from 98 cm to 130 cm but does not explain its reasons for its difference with the very likely range of the Fifth Assessment.

    It does not deal with my main "common sense" concern that the projected rates for the period 2081 to 2100 of up to 16 mm/yr are unrealistically high in comparison to the average 10-11 mm/yr that was experienced at the height of the melting after the Last Glacial Maximum.  According to the US Climate  Report, the  "pulse water" rate of 40 mm/yr lasted for only about 300 years out of thousands of years of melting after the Last Glacial Maximum.

    Can you point me to any academic discussion of my conundrum that these projected rates do not seem realistic given the ice mass differences that existed during the meltdown compared to now?  Surely someone has had to address this.

  12. Norrism

    Your very long post at 58 boils down to a claim that you think 8-10 inches of sea level rise from the present is all we should expect.  Therefor we do not need to take any action on AGW.

    If we take no action we would have to consider that at least RCP 4.5 although RCP 6.5 seems more appropriate to me.

    This is a scientiic board.  Arguments are supposed to go like this:

    I say according to paper 1, sea level rise is proected to be 2- 5 feet with a median of 3 feet.  

    You say according to paper 2 sea level rise is 1-3 eet with a median of 2 feet.

    Then we discuss the merits of papers 1 and 2 citing additional papers.  More recent papers have more weight and papers written by reliable scientists have more weight than unreliable scientists.

    Here you say acccording to the most recent US Climate change report for sceniero 4.5 sea level rise is expected to be 1.5-3 feet with the possibility of 5 feet but it is very likely it will not be under 1.5 feet and including recent Antarctic estimates the median estimate may be 7 feet.  You have recalculated their numbers and think that 8-10 inches is the best median estimate!!!!!  Your estimate is no more than 50% of scientists minimum estimate and is less than 15% of the median including the Antarctic.  You appear to claim that if scientists are unsure what the Antarctic contribution will be it should be counted as zero.

    Is this really an argument that a laywer would present in court??  Experts project at least 1.5 feet and when including the Antarctic a median estimate of 7 feet and a laywer who is an oil investor reads the internet for a few weeks and estimates 8-10 inches.   Which estimate should we go for, the lawer or the scientists?  While you have cited your references, they are diametrically opposed to your conclusion.  Are you being serious?

    Your citations proove you are incorrect from the start.  You cannot hope to convince anyone who is scientific.  You appear to be taking us on a wild goose chase. 

  13. NorrisM:

    I will try to separate three aspects of scientific study: observations, interpretations, and conclusions. Most of what I wll be disagreeing with in your comments falls into the interpretation and conclusion categories.

    I am not sure what gives you the idea that I  am interested specifically in a lawyer's perspective - what I have been interested in seeing from you is a scientific argument that supports your position. That you tend to take a lawyerly approach to the discussion has been apparent, but I tend to see that as a bug, not a feature.

    Michael Sweet has already pointed out how your argument seems to pick the low end of most available data. It has been pointed out to you in the past that this is not good risk management.

    You comment on "the bump" from 1920-1950 in figure 3.14 of the Fifth Assessment. The figure shows results from three studies. The bump is particularly high in one of those studies: Jevrejeva et al. The RealClimate post comments on this, saying

    "The only outlier set which shows high early rates of SLR is the Jevrejeva et al. (2008) data – and this uses a bizarre weighting scheme, as we have discussed here at Realclimate.

    The RealClimate post's figure 3 provides both the sea level rise rates from the IPCC figure, and modelled values. The models tend to underestimate sea level rise, but have been improving (since previous assessment reports).

    Real Climate Sea Level Rise graph

    Please also note that in the IPCC report, figure 13.12, that different semi-empirical studies on sea level projections tend to give higher values if using the Jevrejeva data, and that even work by Jevrejeva gives results within the IPCC range. You need to have a scientific argument as to why you want to pay attention to the Jevrejeva bump, but discount the Jevrejeva projections. It looks like you are just choosing thw answers you like.

    Even if the 1920-1950 "bump" is not well explained, that is not a scientific argument as to why future projections are therefore wrong. We know a good deal less about past inputs than current, and that limits our ability to be sure of what happened historically. This has been discussed with you in the past. Uncertainty in historical sea level rise itself occurs because of the reliance on tide gauges. You allude to this in your post when you discuss  the mid-ocean data that comes available with sateliite monitoring.

    A lengthly discussion on models, data, etc. is hand-waved away with the paragraph:

    "What this tells me is that there is a “theoretical” danger but so far we do not have any evidence of an actual retreat or the time frame over which this could occur. We cannot base our rational responses to AGW based upon theories which have not been supported with observational evidence.

    This is basically a wholesale rejection of science. You basically seem to be rejecting any projections because they haven't happened yet, as there is no observational evidence. I consider this to be irrational. You may wish to reword this or provide further explanation.

    In quoting p1159 of the IPCC report, you neglect to include the closing statement that says:

    From 1993, all contributions can be estimated from observations; for earlier periods, a combination of models and observations is needed. Second, when both models and observations are available, they are consistent within uncertainties. These two advances give confidence in the 21st century sea level projections. The ice-sheet  contributions have the potential to increase substantially due to rapid dynamical change (Sections, and but have been relatively small up to the present (Sections 4.4 and Therefore, the closure of the sea level budget to date does not test the reliability of ice-sheet models in projecting future rapid dynamical change; we have only medium confidence in these models, on the basis of theoretical and empirical understanding of the relevant processes and observations of changes up to the present (,

    I have chosen to bold parts of the quote.

    • Your interpretation that the "biump" in the 1920-1950 period is a game-ender is not in agreement with the IPCC.
    • Your opinion that historical sea-level data are independent of models ("theory") and are purely observational is not in agreement with the IPCC.
    • Your interpretation that there is too much uncertainty to make projections is not in agreement wiht the IPCC.
    • Your opinion that the only reasonable choice it to linearly-extrapolate the historical trends is not in agreement with the IPCC.

    You also comment about "...the average rate of 10 mm/yr during the deglaciation after the Last Glacial Maximum ...". You appear to think that this places some upper physcial limit on rates of sea level rise. The rate of sea level rise is not a function of ice volume, it is a function of the rate of change of ice volume, which depends on the rate of climate change. The temperature rise projected for the remainder of the 21st century is far higher than anything that occurred at the end of the last glacial maximum.

    Marcott et al temperature reconstruction

    All-in-all, you present little more than an argument from incredulity.

    (Note: in lawyer-speak, I reserve the right to ask further questions regardling NorrisM's posts. This comment is limited by time available today.)

  14. Norrismm

    Here is the most up to date sea level graph:

    sea level rise

    I may have misread the graph and the 90% interval for RCP4.5 may be 5.2 instead of 7 feet.   My argument remains unchanged.  Your estimate of 8-10 inches is in contradiction to your sources.

  15. NorrisM

    "The US Climate Report increases the upper level of the very likely range from 98 cm to 130 cm but does not explain its reasons for its difference with the very likely range of the Fifth Assessment."

    The updated sea level estimate from the most recent reports, since AR5, is primarily due to some key studies that have identified additional mechanisms, poorly considered up till now, that can see ice shelves and marine glacial fronts break up more rapidly than previously considered due to mechanical failures. This could lead for example to 1 meter of sea level rise, just from Antarctica, by centuries end.

    Perhaps the key paper is DeConto & Pollard 2016.

  16. Glenn Tamblyn @ 65

    Thanks.  I have printed it and will read it today.  

    One issue which I did not get into in my post given its length already is the cause of ocean warming in the Amundsen Sea which is obviously impacting the melting and calving of the WAIS, or at least parts of it.

    My understanding is that there is geothermal activity at the bottom of the Amundsen sea caused by about 200 fissures of some sort of which some 90 have only recently been discovered.

    If this is a major cause of the warming of the ocean then it obviously is relevant, not as to how much sea level rise we be caused by the melting of the WAIS but how much of that melting can be laid at the doorstep of AGW.

    Have there been any academic papers that have discussed the geothermal warming and its effect on the Amundsen Sea? Either positive or negative.  


    [DB] Off-topic and sloganeering snipped.

    No more.  Period.

  17. michael sweet @ 62

    There is much to reply to in the responses to date but do not interpret my comments that I do not think AGW is something we have to deal with which will require a move from fossil fuels.  What I am trying to determine is how much time we have to deal with the issue.  There are many potential effects of AGW but obviously one of the most critical is sea level rise.  What I am trying to sort out is how much can we expect and, to a certain extent, how much can be mitigated by a reduction in fossil fuel use.

    But I would like to address one philopsophical point about making contributions to this website.  If your position is that all we can do is exchange academic papers, you would slow any discussion on this blog to a snail's pace.  For example, there could be no references of criticism of the Nerem 2018 paper by other persons until other papers had been published which perhaps disagreed with it.  As you know full well, this could take a year given the process of first writing the paper and then having it go through the peer review process and finally having it published.

    The other philosophical problem I have with your approach is that you then limit any discussion or questions on this website to persons with a technical background.  I highly doubt that the sponsors of this website intend to limit discussion to those persons.  

    If you cannot adequately communicate and discuss these issues with the non-technical public then I have no idea how you expect to get the public onside except on faith.  "Trust me, we know better".  This is obviously a rhetorical statement on my part.

    As promised, I have supported most of my statements in my reply with citations from either the IPCC or peer-reviewed papers. 

    Obviously, my point is that there is more uncertainty in many of these positions than I am fully comfortable with in order to fully accept the projections of the IPCC or the US Climate Report based upon the levels of uncertainty acknowledged, especially when the projection is based upon a statement of "Medium Confidence".  On this point, the US Climate Report was much more "up front" about these uncertainty levels than the IPCC.  I actually did not locate the measurements of Confidence Levels in the early chapter of the Fifth Assessment (I found the probability levels relatively easily but not the Confidence level definitions).

    Glenn Tamblyn has provided me with a paper on the reasons for the increase in the upper level of the sea level rise in the US Climate Report and I intend to read it.


    [DB] Off-topic and argumentative snipped.

  18. Norrism:

    We can discuss the Nerem paper by comparing it to earlier papers, discussing the methods and reliablity of the authors and reviewing the new data they have discovered.  For important papers (like Nerem) other experts will comment on new papers so that the rest of us can get an idea of what is going on.  Sometimes we have to wait for more papers to come out to decide what is correct.  It took about 5 years for scientists to agree that the "pause" was denier bunk.  Curries claims that it was real have been long proven incorrect.

    This contrasts to your approach.  You said that you had read the IPCC and the 2017 US Climate Change reports as your source of information.  Then you said that you think that everything in those reports is bunk and calculated your own value denovo.  Thus your "estimate" had no relation to either of the reports you claimed to cite.  It is only the wild, unsupported idea of an oil investor. 

    It may be allowed for lawyers to make up any old story they want, but it is not allowed in scientific discussions.  You must support your claims by referring to papers that actually support your position.  Merely reading the IPCC report does not make you an expert comparable to those who have devoted decades of their lives studing the issue. 

    Your inability to discern how the Climate Report reached its new expected values demonstrates that you have little comprehension of what you are reading.  How can you calculate a new value when you have no idea how the existing value was determined?  I could find that information on my own.

    You are welcome to give our opinion in some areas, but it has no place discussing facts on a scientific blog.  You must support your claims with something besides "this is what I thought up on my own after reading a few scientific papers".  Some of the posts are on more subective issues and then all can promote their opinion, athough you still should be able to support your claims with some sort of reference.

    When you have little understanding of a subect you are much better off asking for help on some of the issues you do not uderstand.  People  here are happy to help you undersatnd why scientists are worried that sea level might rise 8 feet when you think 8 inches is a better estimate. A group of top experts recently published a paper warning about the possibility of 17 feet of sea level rise by 2100.

    Making your own novel calculations is a waste of everyones time.  I have a Masters in Chemistry, I have followed AGW closely for 20 years and I have decades of scientific experience. I rely on my personal knowledge less than once a year (and only on chemical behaviour).  If challenged I produce references.   In a scientiic discussion you must find papers that support your claims.  If you cannot find papers to support your claims that tells you that the claim is incorrect. 

    Your claim of 8-10 inches is 50% of the lowest estimate of the IPCC which they say is very likely to be exceeded.  The IPCC is well known to be extremely conservative (low) on sea level rise.  Would a judge allow such a wild claim by a novice to be allowed as evidence in court to argue against expert opinion?

  19. Meanwhile, our knowledge about what is really happening is increasing:

    From the abstract: "Between 2010 and 2016, 22%, 3% and 10% of surveyed grounding lines in West Antarctica, East Antarctica and at the Antarctic Peninsula retreated at rates faster than 25 m yr−1 (the typical pace since the Last Glacial Maximum) and the continent has lost 1,463 km2 ± 791 km2 of grounded-ice area."

    This is from Nature Geosciences, so unfortunately behind paywall.

  20. Yesterday a post here (since deleted as spam) claimed that I had not produced a citation to support my claims aout nuclear power.  I cited Abbott 2011 to support my nuclear claims.


    [JH] Your post was deleted because it responded to a deleted post. 

    [DB] Further, the person to which you were responding has permanently recused themselves from further participation in this venue.

  21. Glenn Tamblyn @ 65

    I have now read the De Conto & Pollard paper which clearly has had a major influence on the increase of the upper estimate in the US Climate Report to 130 cm.  This paper is very technical and I will not pretend to be able to evaluate it.  But on a "risk" basis the US Climate Report places a very low percentage on any significant impact at least up to 2100.   

    Perhaps discussion of the WAIS has to be located somewhere else even though it directly impacts sea level rise which is the topic of this blog.  Any suggestions where?  I see Philippe Chantreau has referenced a paper which is paywalled.

    I have now read a couple of papers on possible geothermal impacts in this area and I now see they are only talking about the identification of former volcanoes and do not suggest that there are presently open rifts causing any heating.

  22. Returning to NorrisM's arguments about the unreliability of sea level rise predictions, in comment #58 and follow-ups. NorrisM has argued that there is uncertainty in the predictions, and closes comment #58 with a justification of using a linear extrapolation with the following sentence:

    "For me, I would go back to the observations and look at where the sea level has moved since 1900 and assume that it will follow along the same largely linear path that it has pretty well followed since we have kept records in tide gauges.

    NorrisM also makes the statement:

    "We cannot base our rational responses to AGW based upon theories which have not been supported with observational evidence.

    Let us examine the uncertainty issue, with particular reference to observations, and whether a linear extrapolation is justified.

    In comment 63, I linked to a RealClimate graph (linked again below) that shows historic sea level estimates from several sources. It is obvious that the different sources provide different curves, which indicates uncertainty even in the historical record. Why is there this uncertainty? Well, observing global sea level isn't as easy or obvious as one might think. We need to do a bit of digging to understand why.

    The RealClimate post linked to earlier about the IPCC Fifth Assessment results  includes a link to another RealClimate post. Both posts include a reference to the following paper (which unfortunately is paywalled):

    S. Rahmstorf, M. Perrette, and M. Vermeer, "Testing the robustness of semi-empirical sea level projections", Climate Dynamics, vol. 39, pp. 861-875, 2011.

    That paper incudes a good summary paragraph (p862) about some of the adjustments that need to be made:

    "The various tide-gauge based global sea-level reconstructions differ with respect to the selection of gauges, the correction for glacial isotatic adjustment (GIA), the correction for changes in atmospheric pressure ("inverse barometer") and. most importantly, the method for aggregating worldwide tide-gauges into a synthetic global mean sea-level curve.

    The Rahmstorf et al paper also gives additional details on methodology. The second RealClimate post linked above talks specifically about the odd Jevrejeva method of weighting the tide-gauge data. I mentioned this in comment 63, quoting from the first RealClimate post. In the second RealClimate link, you can see more about why this method is odd. Note that RealClimate does not just say it's odd, they make the effort to understand the method and do some calculations to understand the impact of the methodology. In that post, they also challenge the readers to find the problem (and they do).

    This is what scientists do when faced with uncertainty: they look in detail at the different sources of information and try to determine why they are different. They don't just throw up their hands and say "I don't know". Uncertainty due to different assumptions and availability of data helps us know what we need to do better, but it does not justify ignoring everything that we do know.

    So, in spite of NorrisM's argument that we should use "observational evidence", he is basing his simple linear extrapolation on data values that represent a significant amount of theoretical understanding, analysis, and interpretation of raw tide-gauge data. So much for his rejection of the danger of sea level rise because it is "theoretical".

    Now, using those global sea level reconstructions is not necessarily bad - indeed, it is a useful exercise. But is it appropriate to use a linear extrapolation? In a word, no. I will repeat the figure that I included in comment 63:

    Real Climate Sea Level Rise graph

    A linear extrapolation would be appropriate if the slope of these curves were approximately linear. They are not. Another RealClimate post  talks about acceleration in sea level rise, and points out that even fitting a quadratic is problematic. Tamino has also posted on this  and includes the following graph that shows how the slope of sea level varies with time:

    Sea Level rise rates

    If sea level rise were linear, the slope would be constant. It is not. If it were quadratic (constant acceleration), the slope versus time would be linear. It is not.

    When deciding to use a particular statistic fit, the first step is to graph the data and see if the preferred equation actually resembles the data. For linear fits, I have always like the illustration given by Anscombe's Quartet. It presents four small data sets that have nearly identical simple descriptive statistics (mean X and Y, standard deviations, linear regression results), but only one of the four is suitable for linearization:

    Anscombes Quartet

    Naive statistical extrapolation of the different global sea level reconstructions is not appropriate. The reconstructions already have "theoretical" knowledge in them, deriving a global value from scattered tide-gauge data. Extrapolating those curves into the future also requires strong theoretical understanding of how the factors that have affected historical sea level will play out in a warming world.

    The scientific literature that provides the basis for the IPCC projections has done this analysis. NorrisM has not.

  23. Free copy of "Testing the robustness of semi-empirical sea level projections",

  24. NorrisM, if you believe that climate models are faulty, then why do you believe they will err low rather than err high?  Surely you dont think government policy should be guided by such biases?

  25. Tamino has a new post out on recent sea level changes. It focuses on the US, but has a discussion of why rates vary locally and over time. He is less than charitable on the idea that sea level has been changing at a constant rate - and that extrapolating a linear fit is appropriate.

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