Rising Sea

Not long ago Watson et al. compared satellite sea-level data to estimates from tide gauges. They concluded that the satellite data show systematic drift, which is satellite-specific, most strongly affecting the first six years’ observations.

Of course land-based tide gauges require correction for the rise and fall of land itself, vertical land movement (VLM), and they used two methods to do so. One is based on a model of Glacial Isostatic Adjustment (GIA model based), another uses GPS estimates (GPS based). Both models suggest satellite drift, strongest early.

On this basis they estimated an adjustment to correct for the systematic bias. Here are the revised data sets (in red and blue) compared to the un-adjusted (in black):

The revisions lower the overall rate of sea level rise; without them the estimate is about 3.4 mm/yr, but with GIA model based adjustment it’s only 3.1 mm/yr, with GPS based adjustment merely 2.8 mm/yr.

Just a few months ago Chen et al. applied more detailed trend analysis to the new data sets, using EEMD (Ensemble Empirical Mode Decomposition). They detect acceleration of sea level rise in the revised satellite data, and note that known sources of sea level rise have increased, especially melt from the Greenland Ice Sheet.

What I find fascinating is to remove the estimated linear trend from these data and look at what’s left over — the residuals. If I do this with the un-adjusted data, I see this departure from its straight-line pattern:

I could make a good argument that these data show both acceleration (recently) and deceleration (earlier), while I’d also argue they could be random flucutations at longer time scales. But the same analysis — subtract the linear trend to examine the residuals — using GIA model based bias adjusted data shows this:

Acceleration is still there but the early deceleration isn’t. Here’s the GPS based bias adjusted data:

It tells the same story: strong acceleration recently but little if any deceleration early on.

One thing we know affects global sea level is the el Niño phenomenon (ENSO). I took the GPS-based data and fit el Niño as estimated by MEI, the Multivariate El Niño Index, with a 2-month lag. This gave me an estimate which slightly lowers the sea level rise rate, but the sudden and sharp upturn that happened around 2011 is still there:

From these data it seems the current rate of sea level rise is substantially higher than others would suggest. I would estimate, based on the GPS based data, somewhere around 5.5 mm/yr.

That this is anything other than “one of those fluctuations,” I can’t deny. But neither can I deny that it could be real; this is what acceleration of sea level rise can look like. Given what we know about the speedup of Greenland melt and thermal expansion, it’s not impossible this brief trend will persist, and coastal cities will be hard hit sooner than expected. Draw your own conclusions.


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21 responses to “Rising Sea

  1. Over the last several years I ask that that question in a lot of the comments I make at CargoCult Etc., “What would an acceleration look like?” There is a paper with that approximate title. And I always suggest it might look like what has been happening over the last several years. Always silence. They think La Niña will take it away, and it won’t. AVISO 10-year rate is 4.24 mm/yr; 5-year rate is 4.54 mm/yr.

  2. Looks like it would fit quite snugly with the apparent acceleration in the CERES-EBAF TOA net flux data over 2000-2017.

  3. Again you’re revealing some great truth in the data, I had read that sea level rise was exponential anyway – we just the start as linear.

    [Response: I don’t think it qualifies as “great truth.”]

    • David B. Benson

      One expects a sigmoid shape, sometimes called “S” shape, to the sea level curve over time. An example is the sea level curve from Last Glacial Maximum to Present.

  4. Like you, I think the sea level stuff is still waiting on more data.
    I have a suggestion for a piece you might look at, re Cliff Mass’s blogspot which comments on whether a recent paper on high pressure ridging off the W coast is likely to be changing with climate change in a direction which might increase the off-shore Calif winds, leading to more severe fires. I don’t have the data, but the graphs he cites from the paper don’t look as necessarily flat as Mass states, although there is obviously a LOT of noise in terms of year-to-year fluctuations.
    At any rate, as a retired statistician, I do love your posts.

  5. This is a good site to download sea level station reports from:
    http://www.psmsl.org/data/obtaining/
    I live in New Zealand and downloaded station data for us. Due to the fact that many stations have sketchy data with many holes but many stations data improve in reliability for the past 10 years or so I focussed on the period from 2008 to today. Most stations in and around New Zealand show a trend of about 10 to 20mm per year of sea level rise over that period.
    These data are not corrected for land movement which could be up or down. But I was very surprised by the large trend over the past 10 years in the data of tidal stations. The data from Tamino above are global averages and include areas with falling SL due to land rebound and gravitational rebound from ice mass losses. Locally in areas not affected by this, SLR would arguably be well above that average. Perhaps New Zealand is a point in case.

    • Thomas Everth
      New Zealand is not the best place to pick to judge sea level rise. Both on general history
      “23 million years ago, when it was mostly under the sea.
      Then the tectonic situation changed yet again and instead of extension there was a return to compression, plate collision and subduction. The modern plate boundary developed along the Kermadec Trench and southwards under the North Island, and the Southern Alps began to be uplifted along the Alpine Fault. Modern New Zealand has since been rising slowly out of the sea, with the much larger underwater portion of Zealandia still hidden below sea level.”

      and on continuing earthquake and volcano action
      “Aerial photographs show the seabed uplift north of Kaikoura – estimated to be between 2 – 2.5 metres. Earthquake Nov 15, 2016 · Auckland, New Zealand data from various GPS stations have revealed that Cape Campbell in the Marlborough region of the South Island of New Zealand moved horizontally north-east by 2 to 3 metres, and the tide gauge at Kaikoura rose a whopping 90 centimetres.”

    • Tamino has touched on stations in Australia. In NZ the 5 longest and most reliable stations according to Denys and Pearson 2014 show an average rise of 1.7 mm a year, a long way short of a trend of about 10 to 20mm per year of sea level rise over that period.
      Auckland, Wellington, New Plymouth, Dunedin and Lyttelton,
      All except Auckland appear to have fallen from 2000.
      Is this correct?

      • No that’s not correct. Nz’s stations show a strong already trend over the last decade. You can access a collection of sleep data from 2008 to 2017 below from my Dropbox link.
        NZ is certainly not ideal to measure global sleep trends. But living there makes it interesting to look at local data.
        You remark that NZ is rising. Well then we should see a falling local bias in SLR data. Interestestingly however, our local trends over the last decade show a bias towards a strong rising trend.

    • Isn’t this a part off what is going on there? I would guess this pattern is basically ENSO neutral (exaggerated during La Niña events,) and only reverses across the Pacific during El Niño events.

      sea level trends from 1993 to 2017

  6. Is there a database of global land precipitation and reservoir storage? Has anyone tried to correlate the dips and spikes in SLR with floods/droughts over continents?

  7. Yes, there are such databases. There is a paper by Ben Chao et al (http://science.sciencemag.org/content/320/5873/212) summarising the effect of changes in dam storage on sea level. There are also a number of papers looking at the various contributions (including dams and changes in groundwater storage) to sea level. One such is: http://onlinelibrary.wiley.com/doi/10.1029/2011GL048794/abstract.

    There are a number of more recent articles on this subject.

    One particular paper that could be of interest is: doi:10.1002/grl.50834 – this discusses the large drop in GMSL in around 2011 due to a very strong La Nina event.

    Neil

  8. Thomas, it would be interesting to see that data, but I would also note that there has been the odd bit of seismic movement here. Stations around Christchurch would be suspect. In such an active landscape, I think you need to do GPS adjustments to make any sense of it.

    • Hi Phil, nice to meet you here.

      Yes, I have Bevan Litchfield GNS 2012 on ground movements but I am not sure if this is detailed enough to account for the land movement of the actual stations. Here are some data and some overview stats I made:

      https://www.dropbox.com/sh/p05amonad6nmdhs/AABgREqxao-ZK_NGohJ5Yg67a?dl=0

      The pdf in the root folder gives you an overview.

      Some data were provided to me by LINZ directly, others I got from http://www.psmsl.org/data/obtaining/

      I teach ESS and statistics and my course next year will focus on tidal time series. I was initially just aiming for letting students investigate seasonal variations and compare various places to each other globally and locally but I got intrigued by the significant SLR trend over the 2008-2017 period in the NZ stations.

      email: evertht@mbas.ac.nz

  9. The paper underlying the OP (Watson et al 2015) did note that its results were “in closer agreement with the rate derived from the sum of the observed contributions””, these being as per IPCC AR5 Ch13 Table 13.1 which totted up SLR components to +2.8mm/yr over 1993-2010. And many of the Watson et al (2015) authors have since report on acceleration in Chen et al (2017).
    Elsewhere there is examination of the early satellite (TOPEX) callibration issues although these researchers were not impressed with the use of the word SNAFU in this Nature article‘s title.

  10. This reminds of the joke.
    A duck is about to cross the road “Don’t do it!” says the chicken “You’ll never hear the last of it!”

    That came to mind because you appear to have created a hockey stick.

  11. I have a question I hope someone can answer.
    To mee it seems as though the wiggles around the trendline for sea level (first graph) are getting larger since ca 2010. Is this just an eyeball error or is there a real physical reason like eg. the enhanced hydrological cycle?

  12. “Given what we know about the speedup of Greenland melt and thermal expansion, it’s not impossible this brief trend will persist, and coastal cities will be hard hit sooner than expected. Draw your own conclusions.”

    When we are convinced that warming is real, increasing, with significant consequences and inevitable then your conclusion is quite apt.
    I cannot quibble with one and 4 but feel there is still some wriggle room in 2 and 3 to allay your concerns.

    [Response: Living in a fantasy world allows you plenty of wiggle room.]

  13. Of course everyone knows well in the land of the Red Team that all those measurements are incorrect the land is sinking not the water rising.
    Well in fact with ICE loss the land will rise in some places, however that is how they will paint it, not reading the fact that every aspect of the movement of a land mass is taken into account.