AR6 Sea Level Rise

WG1 Technical summary (Box 4.2)  states:

“Global mean sea level (GMSL) increased by 0.20 [0.15 to 0.25] m over the period 1901 to 2018 with a rate of rise that has accelerated since the 1960s to 3.7 [3.2 to 4.2] mm yr-1 25 for the period 2006–2018 (high 26 confidence). Human activity was very likely the main driver of observed GMSL rise since 1970, and new 27 observational evidence leads to an assessed sea level rise over the period 1901 to 2018 that is consistent with 28 the sum of individual components contributing to sea level rise, including expansion due to ocean warming 29 and melting of glaciers and ice sheets (high confidence)”

However if you look at the actual measurement data made by tidal gauges and satellite altimetry , then you find something quite different .

Tide Gauges and satellite data (Topex/Possieden)

You can see 3 distinct trends. Before 1925 a long term trend of increasing sea levels of 1.3mm/year is seen and from 1925 until 1993 the trend is 1.8mm/year. This appears to be  a systematic shift rather than any physical “acceleration” effect,  perhaps due to a more consistent set of gauges after 1925. There is no evidence of an accelerating trend up until 2000. Thereafter the satellite data from Topex/Jason-123 shows a small systematic step increase in trend to ~2.8mm/year. This increase from 1.8 to 2.8mm/year is interpreted as an “accelerating” sea level rise, but it looks suspiciously like yet another instrumental effect. Indeed there is no acceleration visible within the satellite series data itself.

This satellite data with seasonal effects removed shows a stable trend of ~3.3 mm/year. At this rate we would expect a sea level rise by 2100 of  only a further 26 cm. Yet AR6 is implying far larger rises. In fact the rises they project seem much larger because they all  assume  an accelerating rise.

Likely range between 1 and 1.7m rise in sea levels by 2150 assuming accelerating rise.

If instead we extrapolate the satellite data forward to 2150assuming a continuous linear rising trend of 3.3mm/yr than we get a net rise in sea level of 60cm since 1945. This  corresponds to just a 45cm rise on the AR6 baseline (zeroed at 2100)

Baseline is zero in 1950

Sea level rise is probably the most alarming climate change prediction because many major cities are located on coasts or estuaries with protected harbours. Sea levels have been rising continuously since  the last glacial maximum (100m lower than today). There is no firm evidence that convinces me this rise has accelerated significantly in recent decades. So if we assume that the satellite data are the most accurate measure of trends then we can expect a 45cm rise by 2150 relative to todays levels. The daily tidal range in London reaches as much as 7 meters, so the London barrage should easily be able to cope with an extra 45cm in 150 years time. If not we will have plenty of time to upgrade it.


About Clive Best

PhD High Energy Physics Worked at CERN, Rutherford Lab, JET, JRC, OSVision
This entry was posted in AGW, Climate Change, SLR. Bookmark the permalink.

15 Responses to AR6 Sea Level Rise

  1. scott says:

    The celestial cycle is at least 60 years. Any measurement less than 6o years average is not a valid tidal measurement.

  2. Tor Ole Klemsdal says:

    Thank you for pointing out what the observations really should tell us.
    Further, in 2018-19 both ESA
    and WMO declared that sea level increase rate was accelerating in the sattellite era, and had now reached 5 (4.8) mm/year.
    On the WMO page one can read.” Sea level rise:

    Over the five-year period May 2014 -2019, the rate of global mean sea-level rise has amounted to 5 mm per year, compared with 4 mm per year in the 2007-2016 ten-year period. This is substantially faster than the average rate since 1993 of 3.2 mm/year. The contribution of land ice melt from the world glaciers and the ice sheets has increased over time and now dominate the sea level budget, rather than thermal expansion.”

    However, this apparant acceleration almost certainly is a temporary artefact caused by the extreme Nino-event and rainfall in 2011, and then the large NINO-episode in 2015-16, and does not represent a systemic underlying acceleration trend driven by increased greenhouse gas concentration.
    This fact is quite obvious if you check NASA’s pages on sea level rise, which for some strange reason is the only major sea level observation page that has been adequately updated in 2021 (AVISO, COLORADO, CSIRO and NOAA have not been updated since last summer).

    NASA’s observations icluding thos derived from GRACE-2 gravity data, demonstrates vwery clearly that;
    1) Sea level rise have dropped considerally after the last NINO (as expected), and the last 5 years have increased only from 83.9 mm on April 2.. 2016 to 97.6 mm as the latest observed value at March 31 2021, i.e. only 13.9 mm i 5 y, or 2.74 mm/year.
    2. This is reflected in the underlying causes of sea level rise, i.e the changes in ocean mass increase – and again obviuous from the data from the antartic and greenland ice sheets, which are also presented by NASA
    3. There is similarly no acceleration in the steric sea level rise/accumalation of ocean heat, which can also be verified by NOAAs data on OHC-content.

    Regrettably, there is not much hope that ESA or WMO will publish any updates or corrections to the misleading statements made a couple of years ago…

    • Clive Best says:

      Thanks for explaining how ESA/WMO could be fooled by a temporary phenomenon – El Nino. The problem is that only a few of us actually check their claims and it is difficult to be heard. We are living in strange times.

      • Tor Ole Klemsdal says:

        Thanks. Yes, strange times where the former academic ideal of applauding controversy and discussion is more and more replaced by ‘canceling’. How many journalist have been writing critical thaughts about AR6 in your country? Have not seen any in Norway..

  3. Cytokinin says:

    Well spotted Clive. I have seen this happen in other situations when calibration was not spot on with previous equipment. I imagine that the number of measurements per year has also increased, which could also skew results since new measurement points will increase the range and incorporate variation that was not previously measured.

    I read a paper a couple of years ago about sea level rises around Australia – the most geologically stable continent, so not prone to sinking or rising as a land mass. I seem to remember that there was virtually no variation around the continent and that the rise was quite small.

    What is causing the expansion of the ocean?
    Atmospheric heating causing runoff of melting ice.
    Water expansion due to heating, but where does the heat come from?
    Draining of fresh water from land e.g. lakes and aquifers. This water must ultimately end up in the oceans.
    Heating due to UV penetration due to ozone holes?
    Increased volcanic activity?
    Heat transfer from atmosphere to ocean?

    An economist suggested to me that the precautionary principle gives expediency for a mighty fiscal stimulus. Perhaps following this wealth transfer, the oceans will stop rising and the planet will stop heating?

  4. Andrew Carey says:

    It’s uncommon, but not rare for buildings from 150 years ago to be still standing. There are buildings from the 1960s which have since been demolished, and there are odd buildings from 300 years ago which still stand, but they are definitely rare.
    So in round numbers if all buildings that get demolished get replaced on average with an equivalent one on ground around 1-2 metres higher, then we’re sorted. Coastal towns and cities gradually relocate inland a bit driven by market forces. No need to panic, or have a top-down rearrangement of society, just slap on a CO2 tax, relax the planning restrictions on greenfield development and on Nukes, and the so-called climate crisis becomes a low-level manageable problem.

    • Clive Best says:

      I think you have it pretty much exactly right. If you visit Happisburgh in Norfolk you can see half the village has already eroded into the sea. It used to be the estuary of the Thames before the Devonian Ice Age which diverted the Thames south to London. It slowly erodes into the sea just like Doggerland as we rebound from the last Ice Age.

  5. John Payne says:

    So the IPPC have got it all wrong? Mathematically and from a data perspective we can all relax and not worry about the future?
    I am not a expert on Statistics and only have an elementary appreciation of mathematics. However what concerns me is a viewpoint that accepts only linear relationships between Global Heating and sea level rise and also the comfy acceptance of gradualism, the belief that the future can be predicted from past and present data. It fails to include exceptionalism or catastrophe.
    I strongly believe, and here I do use the term belief, that there are non linear factors acting upon the relationship between atmospheric temperatures, carbon dioxide and future sea level rise.
    These are the thermal expansion of the deep ocean, reaching tipping points in continental ice collapse in Antarctica and Greenland, the “Day after Tomorrow “ film collapse of the AMOC, the effect of Wildfires on atmospheric Carbon dioxide and particulates, destruction and collapse of Tropical forest bio systems. And of course releasing methane which has a role in contributing some 30% to global heating.
    Are these taken into account? The natural world cannot be explained or predicted by satellite sensors.

  6. Chubbs says:

    The acceleration is obvious. Very little sea level rise between Roman times and pre-industrial.

    Here is the updated U of Colo chart with the quadratic term included

  7. Bindidon says:

    Clive Best

    Clive, I perfectly remind the impressing accuracy of your evaluations of temperature data sets using icosahedral binning, spherical triangulation and the like.

    You gave me some years ago the opportunity to compare my little layman’s job with yours:

    So I’m wondering that you seem to trust in people telling you that there would be no acceleration in sea level rise.

    Here is for example a paper written by Sönke Dangendorf & alii:

    Persistent acceleration in global sea-level rise
    since the 1960s (revised version 2019)

    which does not show any bit of superficiality, to say the least.

    Though he’s irascible and a little arrogant at times (but … who isn’t), Grant Foster made at that time a wonderful quick shot

    giving results quite near to Dangendorf & alii’s heavy work.

    On the basis of methods used for GHCN V3/daily, I ‘did my very best’ to adapt that stuff to sea levels.

    Here is a graph

    Interesting is that for the period 1993-2015 (begin of sat era / end of Dangendorf’s eval) the accelerations shown by 2nd order polynomials over NOAA altimetry, Dangendorf and Foster are similar (my eval isn’t accurate enough here):

    Of course, the quadratic factor (0.00028 for Dangendorf) looks insignificant, but it is a monthly series, and for 100 years = 1200 months we obtain when squared a factor of 1,440,000.

    This is still no more than 56 cm – only twice as much as in case of linear increase, but this is a worldwide average. On many places on Earth, no one will see that, but on other places, I don’t know…

    The linear estimates in mm/yr for sea level change at a few hundred PSMSL stations having sufficient data for anomaly construction wrt the mean of 1993-2018, go, including a simple, SONEL based VLM correction, from

    – 997 ULSAN: -14
    up to
    – 1364 OFUNATO II: +75

    Median estimate is +3 mm/yr.

    Clive, I would enjoy you having enough time and motivation to adapt your icosahedral binning to this PSMSL gauge context!

    J.-P. D.

    • Bindidon says:

      Unluckily, it seems that Grant Foster aka Tamino possibly has heavy health problems since July of last year: his blog is inactive since then :- (

  8. oldbrew says:

    Everyone agrees that we must eventually stop burning fossil fuels to avoid excessive temperatures in the next 80 years.

    Well, they obviously don’t all agree. Some agree, some disagree, some are undecided and others not interested.

    Natural climate change existed long before humans discovered coal, oil and gas in a big way, and will continue to do so. Attribution remains difficult and wide open to biases.

  9. Ken Gregory says:

    University of Colorado sea level data gives a bogus acceleration of 0.097 mm/yr2 due to the failure to match the satellite trend data over the overlap periods of the four satellites. The trends of the data during the overlap periods should be the same if the data were calibrated correctly. The first three satellites have long overlap periods but the trends were increased by 1.81 mm/yr. The acceleration of tide gauge data is 13% of satellite data.


    This article shows that the Topex and Jason-1 satellite data overlaps 3.7 yrs, but the Jason-1 best-fit linear trend is 1.47 mm/yr larger than the Topex trend over the overlap period of January 2002 to September 2005. The trend of Jason-2 is 0.33 mm/yr larger than that of Jason 1 during the 4.9-year overlap period.The trend of Jason-3 is 2.75 mm/yr larger than that of Jason-2 during the 0.62-year overlap period. The satellite sea level rise acceleration of 0.097 mm/yr2 is 7.5 time the acceleration as measured by tied gauges. The failure to correctly calibrate the satellite data so that the trends match during the overlap periods is the reason that the satellite data reads higher acceleration than the tide gauges.

    • Bindidon says:

      Ken Gregory

      Are you kidding us here?

      How can you compare trends, let alone accelerations in tide gauge data for the period 1950-now with the acceleration in sat data for the period 1993-now?

      Sorry: that is really brazen.

      You must compare data from different sources over the same period. Anything else is a mix of manipulation and folklore.

      By the way: taking in consideration only gauges with 60 year activity is nonsense because you eliminate a considerable amount of sources for unduly reasons, e.g. alleged 60-year cycles which average out everywhere.

      Here is what you seem to deliberately ignore:

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