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Sea level in Dublin Bay is rising, even when you take into account the ups and downs that there have been over past decades, a team of researchers at three universities in Ireland have found.
Over the years, the favoured tools for measuring sea level have evolved, from soldiers sticking poles in the sand during the 1842 Ordnance Survey of Ireland, to float gauges – buoys on a wire that talk to a paper and pen that trace the patterns – and later to electronic monitoring.
For recent research, scientists worked with measurements from 1938 onwards, drawing and meshing together, and adjusting, five different datasets.
And basically “looking at that long Dublin record”, says Gerard McCarthy, an oceanographer at the Irish Climate Research and Analysis Unit (ICARUS) at Maynooth University, and one of the researchers.
Overall, the trend shown through the data matches what was expected, with an estimated sea level rise between 1953 and 2016 of 1.08mm/yr in Dublin, he says.
“The thing about Dublin that was very strange is that in recent years, the sea level has been rising a lot faster than we would expect it to be,” says McCarthy.
“That is something we would say we still don’t understand the reasons behind,” he says. “But we’re sure that is what the data is saying, at least.”
The ups and downs are also surprising. Sea level for Dublin rose at a rate of 3.41mm/year between 1975 to 1985, the study found.
But it fell over the next decade, at a rate of 9.8mm/yr from 1985 to 1996, before rising again at a rate of 6.48mm/yr from 1997 to 2016.
“I think the fairest thing to say is that we don’t know what that is about,” says McCarthy. “We’re still working on that.”
It could be related in part to slow cycles in the Atlantic, he says, that seem to relate to the strengthening and weakening of the Gulf Stream. But they don’t know for sure, he says.
If you look at too short a timeframe, those fluctuations over decades could impact estimates of trends. But they’ve taken a longer view and what researchers are most confident about from looking at the stretch of years, says McCarthy, is the overall rise.
Why It Matters
The mass of the Irish Sea had been drawn away to the horizon on Tuesday morning at Sandymount Strand where, just south of Sean Moore Park, a strong southeast wind was whipping the sands along the beach, up into the yellow and green grasses of the dunes and onwards along the path into the park.
A stone wall shields the road from the beach.
Dublin City Council already has plans to build up this flood defence, maybe adding a tumble of boulders to the sea side and raising the wall, or building out a wide promenade too.
Gerry O’Connell, a senior engineer in the council’s flood projects and river restoration division, says that the council follows the Office of Public Works’ guidelines when building its flood defences.
Currently, they build it up high enough to protect against a once-in-200-years flood event, he says, adding on 500mm for defences that protect houses and industrial areas, and 1m for key infrastructure such as hospitals, he says.
Then, there’s a safety margin added on of 300mm for a wall and 500mm for an embankment, he says.
Talk of millimetres might make it all sound low stakes.
When people hear that between 1997 and 2016, the sea level rose at a rate of 6.48 mm/year, it might seem inconsequential, says McCarthy, the oceanographer. “A lot of people react by thinking mm per year is a very small number.”
It is, compared to tides, which rise and drop a couple of metres every couple of hours, he says. “But that small number is very important in terms of frequency of flooding.”
Sea level generally around the globe has risen about 20cm over the 20th century, he says, which transforms a once-in-50-years flooding event – so how frequently you expect to get flooded – to maybe less than once in five years.
“It’s a real transformation in terms of how frequently you get flooded,” says McCarthy.
“Flooding is kind of a lottery in some ways,” he says. Particularly in Dublin, he says.
On the west coast of Ireland, the pattern of storms barreling across the Atlantic and hitting land is straightforward enough, he says. “You get the biggest storms and you get the biggest storm surges.”
Dublin’s storm surges and flooding are a bit more complicated. “When you get the biggest surges tend to be very related to the dynamics of the Irish Sea, so, more local kind of seiching,” says McCarthy.
To understand seiching, says McCarthy, think about sloshing water back and forth in a bathtub. Now imagine the Irish Sea is the bathtub.
“You can pile up the water over Liverpool and it seiches back over to Dublin 12 hours later,” he says.
For flooding, a few things need to come together. You need a high tide, and tides don’t just rise and fall over a day, they also cycle over a month, he says.
There are spring tides, neap tides, and king tides – the last being when the sun and moon are lined up, and the moon is a super moon.
Add a storm on top of a king tide, say, and you’re in trouble, says McCarthy. “If this does all line up, then you do have problems.”
Also, storms are behaving differently to the past, says O’Connell, the senior council engineer.
“We’re finding that storms are worse than they were in the past, though that analysis is going on,” he says. There seem be more, he says, and “they’re coming from directions we’ve never seen before”.
Once you take into account this seiching, the changing tides, and unpredictable storms, sea level rise from climate change makes flooding a higher risk still, says McCarthy. “It kind of weights the dice a bit.”
“As it slowly rises, it means that what would have been a smaller storm surge before, is now a much higher storm surge,” says McCarthy.
There are other looming catastrophes that have the potential to disturb sea levels around Ireland. “This is the weirdest thing I’m going to tell you about sea level rise,” says McCarthy.
“If Greenland melts, sea level in Ireland will fall,” he says. “And if Antarctica melts, sea level in Ireland will rise much higher than it does in the southern hemisphere.”
That’s because Greenland and Antarctica are huge lumps of ice, so big that they change gravitational fields. “Because they are so big, they’re actually attracting the sea up towards them,” he says.
If Greenland melts, it will pour more water into the sea, but less water will be drawn towards that mass and up around Ireland too, he says.
But if Antarctica melts? “It’s the opposite unfortunately,” says McCarthy.
In December 2021, researchers reported alarming cracks and fractures on top and underneath the Thwaites Glacier, a giant ice shelf in Antarctica.
Many of the glaciers on the West Antarctic ice sheet are held in by small bits of ice sheet, that kind of plug it in, says McCarthy.
“And that stops the glacier just careening into the sea and melting,” he says. “And that’s what’s cracking.”
“The doomsday scenario is that if the ice-sheet disintegrates, then it exposes the big body of the glacier to the warmer marine water,” he says.
The only way to prevent that is action on climate change, he says, and reducing greenhouse gases. “When you take action like that, you are kind of reducing that uncertainty and that risk.”
After floods hit Dublin, and Clontarf in 2002, it prompted Dublin City Council and the Office for Public Works to approve flood defences in Clontarf and Dollymount Promenades.
The council has its own monitors in the Liffey estuary, and is monitoring sea-level rise and wave heights and using software to model and predict tides too, says O’Connell, the council engineer.
Says McCarthy, of the council’s defences so far: “It’s been very successful actually.” There were higher waters in 2014 than in 2002 but without the same flooding, he says.
But people still do need to understand that greater sea level rise is ahead, he says. “Eventually you are going to get that sea level rise, you know, it’s going to keep going for 100s of years.”
And the measures the council are taking probably have a shorter lifespan than it is predicting at the moment, he says.
In August 2021, the Intergovernmental Panel on Climate Change report laid out scenarios of how much sea-level rise the world will suffer, depending on how much it reduces emissions.
By 2150, the “2.6” scenario – which is basically if the Paris Agreement is followed, says McCarthy – would mean between 0.46m and 0.99m of sea-level rise. The “4.5” scenario – the path we are on, he says – would mean 0.66 to 1.33m of sea level rise.
O’Connell, the council engineer, says that the council has a programme of works planned to protect the city.
It’s looking to build up the wall along the promenade in Sandymount and to bolster the low defences near Sean Moore Park. It is considering also whether to build up the sand dunes there too, and on Bull Island, where the groundwater is becoming contaminated by sea water, he says.
The work is kind of piecemeal at the moment, O’Connell says, and he thinks the city can cope with 1m of sea level rise. “But in the future we may have to think about something more substantial.”
That might include revisiting the idea of a tidal barrage – a dam-like structure – across Dublin Bay.
That idea was ruled out due to the estimated cost of about €1 billion, said O’Connell. “Currently, damages are only in the hundreds of millions, so it’s not viable currently, but it may become viable in the future.”
McCarthy, of Maynooth University, says he completely understands how people’s attachments to places might provoke them to oppose flood-defence measures that redraw or block the scenery around them, as with the Clontarf flood wall.
But it is important to think about what is down the road and to take sea level rise and climate change seriously, he says, so “it’s not seen as something that may or may not happen”.