by Ray Bates1 and Peter O’Neill2
1 Emeritus Professor of Meteorology, University of Copenhagen
2 Lecturer in Engineering, UCD (retired)
12 April 2021
We examine the information on sea level rise given in Dublin City Council’s Climate Change Action Plan 2019-2024 (Dublin City Council, 2019) in the light of current understanding of this topic in the relevant scientific literature and with the use of tide gauge data for Dublin. We present our analysis under the headings of global mean sea level rise, sea level rise in Dublin, and sea level rise and extreme weather events. We point out a number of deficiencies in the information on these subjects given in the DCC Plan and conclude that this information does not provide any valid justification for the drastic measures DCC is seeking to implement.
2. Global mean sea level rise
Global mean sea level (GMSL) has been rising since the end of the last ice age, having risen by about 130m between 20,000 and 7,000 years ago (Figure 1). Subsequently, the rate of rise became slower, with intermissions during cold periods such as the Little Ice Age (approx. 1350-1850). The current period of GMSL rise began around 1850, as the Little Ice Age came to an end and the melting of alpine glaciers became evident (Figure 2).
Fig. 1.Estimated global sea level changes since the Last Glacial Maximum (~21,000 years ago), reproduced from Fig. 3.1 of Curry (2018). This figure was prepared by Robert A. Rohde from published data, and is incorporated into the Global Warming Art project. https://commons.wikimedia.org/w/index.php?curid=479979
Fig. 2. Global mean sea level anomalies (mm; blue) and carbon emitted (millions of tonnes; red) since the early 19th century. Reproduced from Fig. 4.1 of Curry (2018). [Sea level from Jevrejeva et al. (2014), carbon from Carbon Dioxide Information Analysis Center (CDIAC, 2014)].
It is clear from Figure 2 that the GMSL was rising long before 1950, when fossil fuel emissions began to be appreciable. The rate of rise since 1850 is not constant, but shows substantial multi-decadal variability. Douglas (1992) has argued that acceleration in observed GMSL rise needs at least 50 years of observations to be significant; otherwise, it is merely short‐term variation. The sea level reconstruction of Jevrejeva et al. (2014) shown in Fig. 2 indicates a linear trend of 1.9 ± 0.3 mm/yr during the 20th century.
The Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC, 2013; page 11) states “It is very likely that the mean rate of global averaged sea level rise was 1.7 [1.5 to 1.9] mm/yr between 1901 and 2010, 2.0 [1.7 to 2.3] mm/yr between 1971 and 2010, and 3.2 [2.8 to 3.6] mm/yr between 1993 and 2010.” On page 290 the report states: “The trend in Global Mean Sea Level observed since 1993 is not significantly larger than the estimates of 18-year trends in previous decades (e.g., 1920-1950).”
Recently, a NASA-led study has resulted in a state-of-the-art framework that brings together advances in sea level models and satellite observations to improve our understanding of sea level rise for the past 120 years (Frederickse et al. 2020). The GMSL rise over the period 1900-2018 obtained in this study are shown in Figure 3.
This figure again shows multi-decadal variability. An analysis of the data shows that, relative to the values given above, the average rate of GMSL rise during the 20th century needs to be revised downward to 1.4 mm/yr. The rate of GMSL rise shown by the study’s reconstructed tide gauge data in the period 1993-2018 is seen to be in good agreement with the satellite altimetry data available during the same period, both rates of rise being about 3.3 mm/yr.
Fig. 3. Global sea level rise, 1900-2018, as measured by tide gauge after correcting for the imprint of Glacial Isostatic Adjustment, the effects of Gravity, Rotation and Deformation, and the effects of Vertical Land Movement. Source: Frederickse et al. (2020). Reproduced from https://climate.nasa.gov/vital-signs/sea-level/. Data at https://opendap.jpl.nasa.gov/opendap/allData/homage/L4/gmsl/global_timeseries_measures.nc.html
The Frederickse et al. data also shows that the rate of GMSL rise in the 20-year period 1934-1953 was 3.3 mm/yr, about the same as in the period 1993-2018. According to Frederickse et al., the above-average rate of GMSL rise in that period is attributable to above-average contributions from glaciers and the Greenland Ice Sheet, with the Greenland contribution around 1935 being slightly greater than in 2018 (see their Fig. 1d).
The rate of GMSL rise quoted in the DCC Plan, 3 – 4 mm/yr, is close to the current rates as given above. However, the DCC Plan does not mention that similar rates of rise occurred before 1950, or that significant sea level rise started around 1850, well before greenhouse gas emissions were of any significance.
3. Sea level rise in Dublin
On page 30 and elsewhere in the DCC Plan, the value 6 – 7 mm/yr is given as the rate of sea level rise in Dublin between the years 2000 and 2016. This is about twice as large as the current GMSL values given by the IPCC and NASA, as quoted above. A graph of sea level rise in Dublin port for the period 1938 to 2018 is given in Fig. 4. Just as in the case of the GMSL, it exhibits significant inter-decadal variations.
Fig. 4. Sea level at Dublin Port (Alexandra Basin tide gauge), 1938 to present. MSL refers to the Mean Sea Level datum, based on an arithmetic mean of regularly sampled measurements between high and low water; the Mean Tide Level datum is based on an average of mean low water and mean high water. Note the DCC analysis within the box to the upper right. The 16-year linear trends centred at earlier times (approx. 1985 and 1975) have been inserted independently. (A larger image will open when opened in a new web browser tab)
Data sources: Permanent Service for Mean Sea Level (1938-2009), Marine Institute (2009 and after); for further detail see  below.
The linear trend over the period 2000-2016 in this graph is 7.64 mm/yr. However, by going back a few decades a negative rate of sea level rise of comparable magnitude over a similar period can be found. For example, in the period 1966-1981 there was a falling trend of -5.79 mm/yr. This kind of variability in the Dublin record was ignored in the DCC Plan.
Another study of sea level rise in Dublin Bay from Maynooth University (Nejad et al., 2020) finds a rate of sea level rise of 10 mm/year over 2003-2015 (a period still shorter by four years than that used by Dublin City Council). Our estimate over this shorter period would also increase to almost the same value — illustrating the variability of estimated sea level rise over short periods. Nejad et al. indeed found rates of sea level rise during the 1980s “of up to 20 mm/year” in Dublin Port, a rate which if extrapolated from that time would have already given a sea level rise of at least 600 mm (two feet). The fact that no such rise has occurred should serve as a cautionary warning against extrapolation from short term trends, as should the observation of Nejad et al “that the rate of sea level rise in Dublin is cyclic with some extreme upward and downward trends”. The fact “that the amplitude of the cycles increases after 1980 and it has never become negative since 2003” does not necessarily indicate “a continuous upward trend” in perpetuo.
Nejad et al. find that the rate of rise in Dublin over the longer period of 1938-2015 was 1.67 mm/yr which is in good agreement with the global average.
They also point out that the current elevated rate of rise is greater than the rates of rise at three other locations on the east coast and that “this raises the question of whether other factors might have been involved in this higher rate of SLR in Dublin Port such as tidal changes in the Liffey estuary or local land motion”.
The observation by Nejad et al. that “the recordings of all four tide gauges were found to be in good agreement between 2003-2015, though this was markedly less so from 2016 to the present” also points to the need for further investigation of the quality of these recordings, certainly for the period from 2016 to the present, but possibly before 2016 as well.
On page 38 of the DCC Plan, in a section headed “Future Risks”, the following sentences occur:
“In terms of relative land vulnerabilities, Dublin, Louth and Wexford are at highest risk. Under a projected sea level rise of 6m, it is estimated that close to 1,200 km2 of land area would be at risk.”
To reach the 6m rise in sea level cited here would take about 800 years if the 2000-2016 rate of Dublin sea level rise of 7.64 mm/yr were to continue, and 4,200 years if Frederickse’s average 20th century GMSL rate of 1.4 mm/yr were used. In referring to future sea level rises of 6m, Dublin City Council appears to be looking into the very distant future.
4. Sea level rise and extreme weather events
The DCC Plan places much emphasis on a claimed increase in the occurrence of weather extremes (the term “extreme weather event” occurs 30 times in the text). This is linked with sea level rise and an increased risk of flooding, as in the sentence “The slow, gradual increase in temperatures and sea level rise will contribute to the increased frequency and intensity of extreme weather events and flooding.” (p. 29).
Neither meteorological theory nor empirical evidence support a link between anthropogenic global warming and an increase in extreme weather events such as suggested in the DCC Plan. Dynamic meteorology informs us that the main energy source for storms in middle latitudes is the horizontal temperature gradient between equator and pole. As global warming increases, the temperature gradient in northern middle latitudes decreases, because the Arctic warms faster than latitudes to the south. Therefore, there is less available potential energy to cause mid-latitude storminess.
The IPCC’s Fifth assessment Report (IPCC, 2013) is in general agreement with this viewpoint. In relation to storminess it states: “Over the last century there is low confidence of a clear trend in storminess due to inconsistencies between studies or lack of long-term data.” [p. 113]. In relation to floods it states: “There continues to be a lack of evidence and thus low confidence regarding the sign of any trend in the magnitude and/or frequency of floods on a global scale over the instrumental record.” [p. 112]. The winter of 2013-2014 was held to be the stormiest on record for Ireland and the UK (Matthews et al., 2014). However, a more recent review of storminess over the North Atlantic (Feser et al., 2015) suggests that decadal variability dominates for the last 100-150 years and that there is no evidence of a sustained long-term trend. Kelly (2016) states: “A survey of official weather sites and the scientific literature provides strong evidence that the first half of the 20th century had more extreme weather than the second half.”
As regards projections for the future, the IPCC’s ‘Special Report on Extremes’ (IPCC, 2012) states (Summary for Policymakers, p. 9): “Projected changes in climate extremes under different emissions scenarios generally do not strongly diverge in the coming two to three decades, but these signals are relatively small compared to natural climate variability over this time frame. Even the sign of projected changes in some climate extremes over this time frame is uncertain.”
5. Concluding remarks
Dublin City Council’s Climate Change Action Plan 2019-2024 states that the current rate of sea level rise in Dublin is 6-7 mm/yr while the global mean rate of rise is 3-4 mm/yr. This information is provided without appropriate context. The DCC Plan does not mention that sea level rise, both globally and in Dublin, shows strong multi-decadal variability, present rates of rise having been equalled by, exceeded by or being of opposite sign to rates in the recent past. Nor does the DCC Plan mention that the current period of global mean sea level rise began around 1850, long before atmospheric greenhouse gas increase became significant.
Consideration of the following details is important to gain a realistic perspective.
• Globally, a recent NASA-led study has led to a downward revision of the linear trend of global mean sea level rise over the 20th century to 1.4 mm/yr. The linear trend since 1993 as given by both satellite altimetry and reconstructed tide gauge measurements greatly exceeds this, being 3.3 mm/yr, but this recent value is not unprecedented, having been equalled in the 1940s.
• In Dublin, sea level rise over the period 1938-2015 had a linear trend of 1.67 mm/yr. The current 16-year trend greatly exceeds this value, but multi-decadal variability in Dublin is so marked that in the 16-year period 1966-1981 there was a falling trend of -5.79 mm/yr. Forward projection using trends over short periods does not provide a reliable guide to the future.
• The DCC Plan refers to a projected sea level rise of 6m. Such a figure is not realistic in the context of contemporary circumstances.
• In relation to the consequences of sea level rise, the DCC Plan places a marked emphasis on a claimed increase in the occurrence of extreme weather events. This claimed increase is not in accord with IPCC assessments or with independent studies in the peer-reviewed scientific literature.
With these facts taken into account, it can be seen that the information on sea level rise given in the DCC Plan does not provide any valid justification for the drastic measures DCC is seeking to implement.
References and Footnote
CDIAC (2014) Annual Global Fossil Fuel Emissions. Carbon Dioxide Information Analysis Center. http://cdiac.ess-dive.lbl.gov/trends/emis/glo_2014.html
Curry, Judith (2018) Special Report: Sea Level and Climate Change. https://curryja.files.wordpress.com/2018/11/srepial-report-sea-level-rise-3.pdf
Douglas, B.C. (1992). Global sea level acceleration. J. Geophys. Res., 97(C8), 12,699-12,706.
Dublin City Council (2019). Climate Change Action Plan 2019-2024. https://www.dublincity.ie/sites/default/files/2020-07/2019-dcc-climate-change-action-plan.pdf
Feser, F. et al. (2015), ‘Storminess over the North Atlantic and northwestern Europe – a review’, Q.J.R. Meteorol. Soc., 141, 350–82, DOI:10.1002/qj.2364
Frederikse, T. et al. (2020). The causes of sea-level rise since 1900. Nature, 584, 393–397. https://doi.org/10.1038/s41586-020-2591-3 Commentary in The Earth Observer, Sept-Oct 2020, pp. 34-35. https://eospso.nasa.gov/earthobserver/sep-oct-2020
IPCC (2012) Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation [Field, C.B. et al. (eds.)]. A Special Report of Working Groups I and II of the Intergovernmental Panel on Climate Change. Cambridge University Press, 582 pp. https://www.ipcc.ch/report/managing-the-risks-of-extreme-events-and-disasters-to-advance-climate-change-adaptation/
IPCC (2013). Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Stocker, T.F. et al. (eds.). Cambridge University Press, 1535 pp. https://www.ipcc.ch/report/ar5/wg1/
Jevrejeva, S., JC Moore, A Grinsted, A.Matthews, G Spada (2014) Trends and acceleration in global and regional sea levels since 1807. Global and Planetary Change, 113. 11-22. https://doi.org/10.1016/j.gloplacha.2013.12.004
Kelly, Michael J. (2016). Trends in Extreme Weather Events since 1900 – An Enduring Conundrum for Wise Policy Advice. Journal of Geography & Natural Disasters, 6:1. http://dx.doi.org/10.4172/2167-0587.1000155
Matthews, T., Murphy, C., Wilby, R.L. and Harrigan, S. (2014), ‘Stormiest winter on record for Ireland and UK’, Nature Climate Change 4, 738–40, DOI:10.1038/nclimate2336
Nejad, A.S., Parnell, A., Greene, A., Kelleher, B.P. and McCarthy, G. (2020). Recent rapid sea level rise in Dublin based on tide gauge analysis. https://eartharxiv.org/repository/view/203/, https://doi.org/10.31223/osf.io/z9hk2
 Data from PSMSL is supplied as Mean Tide Level data up to the end of 2001 and as Mean Sea Level data from 2002 to 2009. There is a small difference between these two. PSMSL data is available as monthly or annual values: https://psmsl.org/data/obtaining/stations/432.php
Data downloaded from the Marine Institute is provided at approximately 5 minute intervals, relative to one or both of two datums, the Ordnance Survey Malin datum and the Lowest Astronomical Tide datum. The user must reduce this data to monthly or annual values where needed. All data in Figure 4 has been adjusted where necessary to Mean Sea Level , as used by PSMSL from 2002 to 2009. Gauge data for Dublin Port may be downloaded from the Marine Institute either as a table displayed in the web browser: https://erddap.marine.ie/erddap/tabledap/IrishNationalTideGaugeNetwork.htmlTable?time%2Caltitude%2Clatitude%2Clongitude%2Cstation_id%2Cdatasourceid%2CWater_Level_LAT%2CWater_Level_OD_Malin%2CQC_Flag&time%3E=2001-01-01T00%3A00%3A01Z&station_id%3E=%22Dublin%20Port%22&station_id%3C=%22Dublin%20Port%22
or as a CSV file to be saved: https://erddap.marine.ie/erddap/tabledap/IrishNationalTideGaugeNetwork.csv?time%2Caltitude%2Clatitude%2Clongitude%2Cstation_id%2Cdatasourceid%2CWater_Level_LAT%2CWater_Level_OD_Malin%2CQC_Flag&time%3E=2001-01-01T00%3A00%3A01Z&station_id%3E=%22Dublin%20Port%22&station_id%3C=%22Dublin%20Port%22
Note that there will be more than 1.4 million records and 140 megabytes, which may be slow to download, and may exceed the row limit if opened in a typical spreadsheet. Currently the earliest data downloaded from the Marine Institute will be from March 2007.
The present document can be downloaded at https://oneillp.files.wordpress.com/2021/04/sea-level-rise-report-5.pdf