Impact Of Sea Level Rise On The Dawlish Line
By Paul Homewood
Last month I reported on a study which investigated the potential effects of sea level rise on the Dawlish rail line in Devon. For those who missed it, my post is here.
One of the authors was Roland Gehrels, and I must thank him for taking the time and trouble to comment and explain much of the detail from the study.
There was one point which I took particular issue with at the time. This was the claim that during the lifetime of the Great Western Railway, which was completed in 1846, there has been a 20cm rise is sea levels in the English Channel. However, almost half of that occurred in the last 40 years.
My view was that this gave the misleading impression that recent sea level rise had been much greater than in the past. This certainly does not tally with NOAA tide gauge records, which show the current rate of rise less then it was up to the 1960’s.
Roland correctly states that the rise in the last forty years has been about 100mm, compared to 176mm in the last century. We have no tide gauge data at Newlyn, or anywhere else nearby, before 1916, so we don’t know what sea level rise was beforehand. (The study uses data from Brest for this earlier period). Nevertheless, it would come as little surprise to learn that sea level rise was minimal until late in the 19thC.
But why use this particular 40-year period? As the above graph shows, sea levels were falling during the 1960’s and 70’s, a period of falling global temperatures, partly tied into the cold phase of the AMO. We would not measure temperature trends from winter to summer, so why try to measure sea level trends during only half of the AMO?
As the AMO lasts about 60 years, let’s look at the trend at Newlyn between 1950 and 2009, so we can see the true trend across the whole cycle. (The study itself uses data up to 2009).
This is not cherry picking on my part. It is widely accepted that trends need to be measured over at least 60 years for them to be meaningful, as, for instance, leading oceanographer Bruce Douglas maintained. Also the paper by Chambers, Merrifield and Nerem, Is there a 60-year oscillation in global mean sea level?” found that there is a significant oscillation with a period around 60-years in the majority of the tide gauges examined during the 20th Century, and that it appears in every ocean basin.
We find that the rate of rise is 1.78mm/year, almost identical to the trend of 1.76mm since 1916. Put simply, there has been no long term acceleration in sea level rise.
This is not just some academic point scoring. As I commented before, there may be all sorts of good reasons to reroute the Dawlish line. And as Roland correctly points out, sea level rise is cumulative, meaning that gradually it will impact more and more on the railway, a problem that is compounded by the fact that the land there is sinking by about 0.5mm/year.
But it is important to base such decisions on realistic and robust data. Which brings me onto the real reason for this post!
The paper uses sea level predictions from DEFRA’s UK Climate Projections on which to base estimates of line disruption and financial impact.
Even on the lower emissions scenario, the assumption is a rise of 4.7mm/year up to 2020, and 5.2mm from 2020 to 2040. This clearly is not supported by real world evidence of what has actually been happening.
The analysis contained in the study may be used to assess the economic viability of rerouting the Dawlish line. However, no decision of this magnitude should be made on the basis of such wholly unrealistic, airy fairy data.
I have asked Roland to contribute to this discussion.
The original paper, “Sea-level rise impacts on transport infrastructure: The notorious case of the coastal railway line at Dawlish, England” is available here.