by Ray Bates1, Peter O’Neill2 and Fintan Ryan3
1Emeritus Professor of Meteorology, University of Copenhagen
2 Lecturer in Engineering, UCD (retired)
3 Senior Captain, Aer Lingus, and Senior Engineer, Inmarsat (Retired)
21 June 2021
We here re-examine our analysis of the information on sea level rise and extreme weather events given in Dublin City Council’s Climate Change Action Plan 2019-2024, in response to the reply to our analysis received from Dublin City Council. Our re-examination leads to no change in our original conclusions. The information on sea level rise and on extreme weather events given in the DCC Plan and in DCC’s reply is shown to be deficient. We remain concerned about the possible uses of this deficient information to advance unjustified actions that could have negative consequences for the residents of Dublin.
Links  to  in this blog post are links to the Dublin City Council’s Climate Change Action Plan 2019-2024, our document “Sea Level Rise and Dublin City Council’s Climate Change Action Plan 2019-2024” and DCC’s reply to our document. These links appear below multiple times and will open the corresponding document in a new browser tab (for links  and , if your web browser supports PDF documents)
Links  to  are links to notes at the end of this blog post. As these links each appear only once in this blog post, you will jump to the corresponding note in this same browser tab, which is then followed by a “back to text” link which will return you to the paragraph you left.
We here re-examine the comments we made on Dublin City Council’s Climate Change Action Plan 2019-2024  in our document “Sea Level Rise and Dublin City Council’s Climate Change Action Plan 2019-2024”  submitted on 12 April 2021, in response to DCC’s Reply of 18 May 2021 .
We wish to reiterate, as stated in the covering letter accompanying our original document, that we welcome the measures DCC has been taking to protect Dublin residents from flooding. We also welcome DCC’s measures currently in the planning stage to improve sea defences. We fully support making allowance for the sea level rise that has taken place since Dublin’s sea walls were originally constructed and for the further rise estimated to take place by the end of the century using reliable science-based projections.
In our comments , we pointed out a number of deficiencies in the information on sea level rise and extreme weather events given in . Our re-examination of the issue here in response to DCC’ Reply  does not lead us to modify our original conclusions. Our motivation in carrying out this analysis is to serve the public interest.
As in , we present the results of our analysis here under the headings of global mean sea level rise, sea level rise in Dublin, and sea level rise and extreme weather events.
2. Global mean sea level rise
DCC’s Reply  lays much stress on a claimed acceleration in the rate of rise of global mean sea level (GMSL). This is evidenced by the following quotes from the document:
– “Sea level rise and increased wave heights appear to be an accelerating phenomenon”
– “The general average [GMSL] is showing an upward trend that is increasing, rapidly as per the global climate models”
– “Sea level rise is likely to be exponential rather than linear”.
This claimed acceleration of GMSL rise is not consistent with information from the NOAA website that the DCC Reply  cites. The actual graph of GMSL rise for the period 1880-2020 shown in  (NOAA Climate Dashboard, top panel) shows little sign of any significant acceleration. The graph is fully consistent with our analysis of global mean sea level rise as presented in . In contrast to the DCC viewpoint, the NOAA website states: “The pace of sea level rise accelerated beginning in the 1990s, coinciding with acceleration in glacier and ice sheet melting. However, it’s uncertain whether that acceleration will continue, driving faster and faster sea level rise, or whether internal glacier and ice sheet dynamics (not to mention natural climate variability) will lead to “pulses” of accelerated melting interrupted by slowdowns.” .
We have done a search of the IPCC reports from AR5 WG1 (2013) onwards and have been unable to find any reference to the “exponential” rate of GMSL rise cited above.
Regarding model projections of GMSL rise, the DCC Reply  gives a range of 0.59m – 0.81m rise by the year 2100. We have searched the IPCC reports and have been unable to find where this range comes from. The figures on GMSL rise given in the IPCC’s AR5 and SROCC reports are given in Table 1 below.
Table 1. GMSL rise in 2100 (in metres relative to 1986–2005). Projections using CMIP5 models with the four Representative Concentration Pathways (RCPs) of greenhouse gas concentration used by the IPCC. Brackets denote likely range of GMSL rise.
|IPCC AR5a||IPCC SROCCb|
|RCP4.5||0.53 [0.36–0.71]||0.55 [0.39-0.72]|
|RCP8.5||0.74 [0.52–0.98].||0.84 [0.61-1.10]|
a IPCC AR5, Chapter 13, p. 1180. b IPCC SROCC, Table 4.4, p. 352.
From this table it is seen that the lower limit (0.59m) of GMSL rise given in  corresponds to the upper limit for the optimistic RCP2.6 scenario in SROCC and exceeds the mean values for the medium RCP4.5 and RCP6.0 scenarios. The upper limit (0.81m) of GMSL rise given in  does not lie within any of the ranges given the optimistic or medium (RCP2.6, RCP4.5 or RCP6.0) scenarios, but requires the extreme RCP8.5 greenhouse gas scenario to bring it about.
Results depending on the extreme RCP8.5 scenario are again given in the second figure in . This figure is reproduced from Figure SPM.1 of SROCC. It gives projections of GMSL rise out to the year 2300 based on CMIP5 models that use the RCP2.6 and RCP8.5 scenarios. Over this figure lies the statement “The cost of inaction today will be at a greater cost to future generations.”
It is invalid to take model projections using the RCP8.5 scenario as a basis for deciding what climate actions should be prescribed today. In relation to this scenario, Dr. R. Pielke has stated:
“The decision by the IPCC to centre its Fifth Assessment Report on its most extreme [RCP8.5] scenario has been incredibly consequential. Thousands of academic studies of the future impacts of climate change followed the lead of the IPCC, and have emphasized the most extreme scenario as ‘business as usual’ which is often interpreted and promoted as where the world is heading. For instance, so far in 2019 two new academic studies have been published every day that present this most extreme scenario as ‘business as usual’ and predict extreme future impacts. Journalists promote these sensationalist findings, which are amplified by activists and politicians and as a consequence climate change becomes viewed as being more and more apocalyptic” .
Drs. Z. Hausfather and G. Peters have similarly argued that the use of the RCP8.5 scenario is misleading . They state that:
“RCP8.5 was intended to explore an unlikely high-risk future. But it has been widely used
by some experts, policymakers and the media as something else entirely: as a likely ‘business
as usual’ outcome…. Happily – and that’s a word we climatologists rarely get to use – the world imagined in RCP 8.5 is one that, in our view, becomes increasingly implausible with every passing year. Emission pathways to get to RCP 8.5 generally require an unprecedented fivefold increase in coal use by the end of the century, an amount larger than some estimates of recoverable coal reserves.”
The DCC Reply  further states: “The rapid increase in concentrations of CO2 to their highest levels in over 800,000 years (estimated at 412ppm globally) driven by anthropogenic activities which is correlated with increased rates of sea level rise, wave heights and increased rates of glacial melting in the albedo effect, and the resulting impacts must also be responded to as per available data information.”
It is true that current atmospheric CO2 levels are at their highest level in over 800,000 years. However, the implications of this are nothing like as definite as the DCC Reply implies. Figure 1 below puts the situation in perspective.
Figure 1. Global atmospheric carbon CO2 in parts per million (ppm) for the past 800,000 years. https://www.climate.gov/news-features/understanding-climate/climate-change-atmospheric-carbon-dioxide
The peaks and troughs in the graph indicate ice ages (low CO2) and warmer interglacials (higher CO2). In these glacial-interglacial cycles, during which very large changes in global temperature and GMSL occurred, CO2 was never higher than 300 ppm. The cycles were not driven by the CO2 variations, but by variations in the Earth’s orbital parameters. These led to very large seasonal and latitudinal variations in the solar radiation reaching the Earth’s surface. In periods when solar radiation was low at northern high latitudes in summer, the ice formed in the previous winter was able to persist over the summer. Over periods of tens of thousands of years, ice sheets were then able to form over the continents. Variations in CO2 did not lead the consequent variations in temperature, but followed them .
No reliable estimate of the effect of increasing CO2 in today’s climate can be gained from studying variations of CO2 during the ice ages. There is no analogy with current conditions. Estimates of the contemporary warming effects of CO2 depend primarily on climate models. The most basic question that is studied by climate scientists is: how much warming would eventually occur if atmospheric CO2 were doubled relative to its pre-industrial value and then held fixed? The answer to this question as presented in IPCC AR5 (2013) varies by a factor of 3 (from 1.5°C to 4.5°C). It has not been possible to arrive at a more precise estimation than this despite 40 years of model development. Estimates from the most recent generation of climate models diverge even further. The whole area of the climate system’s response to greenhouse gas increase in today’s climate remains unsettled  . Estimates that use satellite observations rather than model calculations to assign values to the Earth’s radiative characteristics indicate a considerably smaller warming response to increasing CO2 .
3. Sea level rise in Dublin
In this section, we discuss Dublin sea level rise in a European context, which gives a broader perspective than our discussion in . We examine the period over which it is appropriate to calculate sea level trends for Dublin for use in longer term projections, e.g., to the end of the present century. We show that average trends over a decade, over a 16-year period, or even over a 30-year period are unsuitable, as all of these can seriously misrepresent the longer-term rate of sea level rise. This is due to the fact that sea level varies on multiple time scales as a result of a wide variety of physical processes.
Figure 2 shows sea level data from two European stations, Cuxhaven and Brest, which have long records. NOAA has fitted a linear regression line to these data, which represents relative sea level rise , shown in the figure, which dampens short term annual and semi-annual cycles and noise. From these data, it can be seen that there is a long-term trend of rising sea level with large residual variability superimposed.
Figure 2. Relative Sea Level for two European tide gauges (Cuxhaven and Brest) with long records.
Figure 3 shows moving 50-year trends from these two stations over the full record. Clearly, there is much variability even on this timescale, with a quasi-periodicity of 60–70 years.
Figure 3. Variation of 50-year Relative Sea Level Trends for the same two gauges
[https://tidesandcurrents.noaa.gov/sltrends/sltrends_station.shtml?id=140-012 (Cuxhaven; for Brest, substitute id=190-091)]
Figure 4 again shows the 50-year trends (black line), but now with shorter-term trends also included. Relative to the 50-year trend, the 30-year trend (blue line) shows increased volatility but remains relatively close to the 50-year trend. Extreme volatility arises from using shorter-term trends. The 16-year (in red) and decadal (in brown) trends display repeated excursions to plus or minus 10–15 mm/year, nearly five times the long-term trend of 2.11 mm/year for Cuxhaven or fifteen times the long-term trend of 1.01 mm/year for Brest.
Figure 4. The volatility of short-term and longer-term trends at Cuxhaven and Brest.
The Dublin Port tide gauge has too short a record to have been included by NOAA among the European tide gauges with variation of 50-year sea level trends such as shown in Figures 3 and 4, but the pattern of longer-term trends should not be expected to differ greatly from that of other tide gauges located at the eastern side of the North Atlantic.
Figure 5. Relative Sea Level for Dublin Port
Figure 6. The volatility of short-term and longer-term trends at Dublin Port
The interannual variability of mean sea level is coherent (having a constant phase relationship) along the European coastline, as exemplified by the data from Cuxhaven and Brest shown above. There is no reason to believe that the recent behaviour of Dublin mean sea level is somehow different from that along the rest of the European coastline.
The European results given above reinforce our earlier conclusion that short-term sea level trends in Dublin, which show extreme variations, should not be regarded as a suitable basis for longer-term planning. Even a 50-year trend for Dublin is unreliable for projection into the future, because stations with longer records show that 50-year trends are quasi-periodic with 60-70 year periodicities. These limitations hold true even if the measurements themselves are accurate. As discussed in our earlier document , there are additional questions to be raised regarding the accuracy of the recent Dublin measurements.
4. Sea Level Rise and Extreme Weather Events
The DCC Reply contains the following statement in relation to extreme weather events:
“With regard to monitoring storm frequency and intensity, in the last 20 years Dublin City has experienced an increase in both; we now have 4-8 storms per annum and some of the strongest storms are accompanied by largest rainfall events and highest waves, in the last 100 years. Additionally, some of these storms originate from the South Atlantic, which is unprecedented.”
We examine this statement under separate headings below. We find in all cases that the statement is not in agreement with the evidence.
(a) Wind Speed
An analysis of the Met Éireann station data reveals that storms in Ireland and Dublin are getting less frequent and less severe since records began. This decrease in storminess is in accord with what is to be expected on physical grounds as a result of the pattern of warming observed to have occurred over the past half-century. The primary energy source for storms at Irish latitudes is associated with the equator-to-pole temperature difference. This difference has decreased because temperatures have risen more in the Arctic than at lower latitudes in the Northern Hemisphere. Therefore, there is less energy available for storms in Ireland.
As regards Dublin, Figure 7 illustrates that the maximum speeds in the gusts at Dublin Airport have been decreasing at a rate of about 0.5 knots each decade since 1945. Moreover, the last 13 years have been unusually quiet. In fact, there have been no storms at the red warning level during that period, and only three storms barely over the orange warning level.
Figure 7. Dublin Airport maximum monthly gust > 35 in knots from June 1945 to April 2021. The warning levels (Red: 70 knots. Orange 60 knots. Yellow: 50 knots) are shown. There is a downward trend at the rate of about 0.5 knots per decade.
As regards the statement that in the last 20 years, Dublin City has experienced an increase in both frequency and intensity, Figure 8 speaks for itself with an obvious negative trend in wind speeds of about 1.4 knots per decade. Moreover, these intensities are dwarfed by storms in the previous 55 years.
Figure 8. Dublin Airport gusts greater than or equal to the yellow warning level of 50 knots over the period 2001 to 2021. In this period, there is a downward trend at the rate of about 1.4 knots per decade.
(b) Wind Direction
According to the Royal Haskoning Final Report, commissioned by DCC in 2007 :
Southern and eastern winds contribute to water level rise, the opposite directions cause reduction in water level. The effect of southerly wind is much stronger than the effect of easterly wind.
Figures 9 and 10 show the wind direction frequencies for the two periods 1945 – 1981 and 1982 – 2021. It is seen that the wind directions in the earlier years were more in the direction south and east than in the later years. This indicates that the recent wind directions contribute less to increased wave heights and flooding in Dublin.
Figure 9. Frequency of wind direction at Dublin Airport (in degrees from true north) for gusts over 35 knots for the period 1945-1981.
Figure 10. Frequency of wind direction at Dublin Airport (in degrees from true north) for gusts over 35 knots for the period 1982-2021.
(c) Atmospheric Pressure
A local drop of 1 hPa in atmospheric pressure causes local sea level to rise by about 1 cm. There has been very little change in the range of atmospheric pressure recorded in Dublin since 1944. The lowest pressure recorded at Dublin Airport was 949.6hPa on 18 December 1945. The trend in low pressures below 980 hPa is positive, i.e., the lowest values occurred in the past. This can be seen in Figure 11. Therefore, over the past 76 years there has been no contribution to any increased flooding in Dublin by any tendency towards lower atmospheric pressures.
Figure 11. Pressure (hPa) at Dublin Airport when less than 980 hPa and gusts are greater than 35 knots, for the period Feb 1944 to April 2021. There is a very slight trend in the direction of increasing pressure.
(d) Storms from the South Atlantic
Mention is made in the DCC statement under discussion of storms approaching from the South Atlantic. However, no storms cross the equator, so perhaps the reference is to storms affecting Ireland that originate in the tropical Atlantic north of the equator and intensify in the eastern Atlantic.
In October 2017 Ireland was affected by ex-Hurricane Ophelia and in October 2019 by ex-Hurricane Lorenzo, both of which began as normal tropical depressions moving westward from the coast of Africa around 15°N. Subsequently these two depressions behaved in an unusual manner, curving northwards and intensifying to hurricane strength in the Eastern Atlantic. This behaviour differs from the usual pattern in which such depressions move westwards towards the Caribbean before intensifying.
Such hurricanes of eastern Atlantic origin are not unprecedented. A notable example was Hurricane Debbie, which affected Ireland in September 1961. Our ability to observe hurricanes in the eastern Atlantic was quite poor before the 1960s, when regular satellite observations began . Before that, observations in the relevant areas depended on limited ship traffic and sporadic aircraft reconnaissance. There is therefore no long-term database from which to conduct a statistical study of the frequency of hurricanes of Eastern Atlantic origin affecting Ireland.
In relation to Ophelia in October 2017, the maximum speed of the wind at Dublin Airport was 56 knots. This wind speed was equalled or exceeded at Dublin Airport 181 times since 1944.
Some Irish media reports at the time of Ophelia and Lorenzo attributed these storms to warming of Atlantic sea surface temperatures due to human activities. The evidence for such an attribution was examined in  and . Both studies concluded that these storms’ unusual tracks and intensities were more likely to be due to anomalous wind and sea surface temperature patterns associated with natural variability rather than to human-induced warming of the Atlantic. Anthropogenic warming of the Atlantic is small and greatly outweighed by local (in space and time) variations of up to ±2°C and more widespread variations of ±0.2°C on a timescale of 60-70 years associated with the Atlantic Multidecadal Oscillation.
(e) Changes in Wave Heights
The DCC Reply refers to increased wave heights affecting Dublin. The size of a wave depends on three factors: the distance over which the wind blows across open water (the fetch), the relative strength of the wind, and the duration of that wind. For the Irish Sea in the area of Dublin the wave heights are measured by buoy M2. But this buoy has been operating only since 2001, so any records claimed would be over a period of about 20 years only.
Regarding rainfall events, it is reasonable to assume that the maximum flow of the Liffey would be a good indicator of rainfall trends in the catchment. Figure 12 shows the maximum annual flow rate at Leixlip over the past 20 years. This is seen to have been decreasing, at a rate of about 10 m3/s per decade (OPW data, waterlevel.ie).
Figure 12. Annual maximum discharge rate of the Liffey at Leixlip, 2000-2019.
Regarding GMSL rise, the strong stress of the DCC Reply on a claimed acceleration of this quantity is lacking in context and is unjustified. The model projections of GMSL presented in the DCC Reply give undue emphasis to results derived using the unrealistic RCP8.5 greenhouse gas concentration scenario. Such projections should not be used as a basis of policy. The inferences on GMSL rise drawn from the fact that the current CO2 level is greater than during the glacial-interglacial cycles of the past 800,000 years are also lacking in context and are unjustified.
Regarding sea level rise in Dublin, the DCC Reply states that, in the context of assessing the effects of climate change, Dublin City Council has been monitoring sea level rise over the last 20 years. We have shown in our original comments that sea level trends on such a short time scale are not reliable for making future projections. Here we expand on this by examining the long-term sea level data from the European ports Cuxhaven and Brest. An examination of these data confirms that trends over such short periods are very volatile. Even 50-year trends are not reliable for making projections because they have quasi-periodic variations with periods of 60-70 years. The long-term sea level trend for Cuxhaven over the period 1843-2018 is 2.11 mm/year while the long-term trend for Brest over the period 1807-2018 is 1.01 mm/year.
In relation to extreme weather events in Dublin, all the evidence we have examined is in disagreement with the claims made in the DCC Plan and the DCC Reply. In the last 76 years at Dublin Airport, the strengths of the gusts over 35 knots have been decreasing, the wind directions have been less in the S and E directions (which increase the water level) and more in W and N directions (which decrease it), the minimum pressures measured during storms have been, if anything, increasing. Thus, there is no upward trend in sea level extremes in Dublin from any of these causes. Our examination of storms from the South Atlantic, wave heights and rainfall also provide no support for claims made in the DCC Reply.
In conclusion, we adhere to the analysis presented in our document of 12 April 2021.
 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
 “Sea Level Rise and Dublin City Council’s Climate Change Action Plan 2019-2024” by Ray Bates and Peter O’Neill. 12 April 2021. https://oneillp.wordpress.com/2021/04/06/dublin-sea-level/
 DCC’s Reply, 18 May 2021. https://oneillp.files.wordpress.com/2021/06/dublin-city-council-reply-18.05.2021.pdf
 Pielke, R. Jr. The incredible story of how climate change became apocalyptic. Forbes, December 2019. https://www.forbes.com/sites/rogerpielke/2019/12/06/the-incredible-story-of-how-climate-change-became-apocalyptic/?sh=3b76c8c0789d
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 Hausfather Z and Peters G. ‘Emissions – the “business as usual” story is misleading’. Nature, 30 January 2020. https://www.nature.com/articles/d41586-020-00177-3.
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 Roe, Gerard (2006). In defense of Milankovitch. Geophysical Research Letters. https://doi.org/10.1029/2006GL027817 https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2006GL027817
For further discussion see Happer, W., Koonin, S. and Lindzen, R. (2018):
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 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. https://www.ipcc.ch/report/ar5/wg1/ See in particular the discussion of equilibrium climate sensitivity on page 16, including its important footnote.
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 Steven E. Koonin (2021) “Unsettled: what climate science tells us, what it doesn’t, and why it matters”. BenBella Books, 306pp. . (Prof. Koonin is a physicist and member of the U.S. National Academy of Sciences who served as Undersecretary for Science in the U.S. Department of Energy during the Obama Administration.)
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 Bates, J. Ray (2016). Estimating climate sensitivity using two-zone energy balance models. Earth and Space Science, 3, 207-225. Open access at http://onlinelibrary.wiley.com/doi/10.1002/2015EA000154/epdf
This paper indicates an equilibrium climate sensitivity to doubling CO2 of about 1°C, an estimate that is consistent with the satellite temperature observations of Christy and McNider (2017) as adjusted for the effects of volcanic eruptions:
Satellite Bulk Tropospheric Temperatures as a Metric for Climate Sensitivity. Asia-Pac. J. Atmos. Sci., 53(4), 511-518. DOI:10.1007/s13143-017-0070-z
The validity of the satellite temperature observations is supported by their close agreement with radiosonde balloon observations:
Christy et al. (2018). Examination of space-based bulk atmospheric temperatures
used in climate research. International Journal of Remote Sensing. Vol. 39, 3580-3607.
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 The sea level trends measured by tide gauges that are presented here are local relative sea level (RSL) trends as opposed to the global sea level trend. Tide gauge measurements are made with respect to a local fixed reference on land. RSL is a combination of the sea level rise and the local vertical land motion [https://tidesandcurrents.noaa.gov/sltrends/sltrends.html].
Local relative sea level change directly determines the impact on local coastal land, Global sea level change is only indirectly reflected in changes affecting the local coastal land. Local relative sea level may for example fall – as in most of Norway, Sweden and Finland – if the rate of local vertical land motion due to post-glacial isostatic rebound (land slowly rising following an ice age after the weight of ice is gone) exceeds the change in global sea level.
 “Exceptional Weather Events: Hurricane Debbie”, Met Eireann, September 1961. https://web.archive.org/web/20131112040048/http://www.met.ie/climate-ireland/weather-events/Sep1961_hurricane-Debbie.pdf
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 “What Caused Storm Ophelia?” by Ray Bates and Ray McGrath. Royal Irish Academy blog post, 25 October 2017.
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 “What Caused Hurricane Lorenzo?” by Ray Bates. GWPF Note 19, Global Warming Policy Foundation, London. 28 October 2019.
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