Human-driven sea level rise is increasing coastal flooding: what it means for UK flood risk

The research made headlines worldwide, including coverage in The Independent. For anyone working in UK development, planning or property, the findings are worth more than a passing glance, because they speak directly to how we assess and design for flood risk on coastal and tidal sites.

What the research found

The first paper, led by scientists at Climate Central and published in Science Advances, analysed 519 tide gauges around the world. It found that a human-caused sea level rise signal is now detectable at 97% of those sites. More striking: climate change was responsible for 58% of the days with extreme water levels between 2000 and 2018, and has nearly tripled the number of days exceeding extreme thresholds since the 1970s.

The second paper, from a Tulane University-led team in Nature Climate Change, looked at the question a different way, through the lens of the “1-in-100-year” flood that engineers and planners know well. It found that a coastal flood expected once a century in 1900 is now, on average, about twelve times more likely. Roughly a quarter of that increase is down to human-caused climate change alone, which has quadrupled the frequency of these extremes since 1900. At nearly half the sites studied, the old once-a-century event now turns up at least once a decade.

The two studies were built on different methods and different metrics, which is partly why they are persuasive together. As Robert Kopp of Rutgers University, a co-author on the Climate Central paper, put it: sea level rise is making both tidal and storm-driven flooding “more frequent, extensive and expensive.”

The mechanism that matters

The headline numbers are arresting, but the underlying mechanism is the part planners should hold on to.

Extreme coastal flooding happens when three things line up: a high tide, a storm surge, and the baseline sea level beneath them. Raise that baseline by even a modest amount and you change the maths for every storm that follows. A surge that would once have stayed within the defences now tops them. As Sönke Dangendorf, the Tulane study’s lead author, explained, smaller storms can now produce flooding that previously required far more severe conditions.

In other words, the storms don’t have to get worse for the floods to get worse. The water is simply starting from a higher point. Ben Strauss of Climate Central put it more bluntly to the Associated Press: “Essentially every coastal flood today has human fingerprints on it through climate change.”

Why this lands differently in the UK

Neither study singles out British tide gauges, so it would be wrong to pin a specific UK figure to either paper. But the wider picture for these islands is well established by the National Oceanography Centre: UK sea level is rising faster than the global average, and around two-thirds of the rise observed since 1900 has happened in just the last three decades or so.

That matters because a great deal of England’s population and infrastructure sits behind defences in exactly the places this dynamic bites hardest: the Thames Estuary, the east coast and the wider North Sea corridor that flooded so catastrophically in 1953, the Humber, and the Severn Estuary. These are tidal systems where surge plus a rising baseline, not rainfall, drives the worst events.

What it means for flood risk assessments and planning

The practical consequence of all this is straightforward: the historic baseline is moving, and assessments that quietly assume it is fixed will understate risk over a development’s lifetime.

UK practice already anticipates this, which is the reassuring part. The Environment Agency’s climate change allowances, underpinned by the UKCP18 projections, exist precisely so that a Flood Risk Assessment accounts for sea level rise across the design life of a scheme, often well into the next century for residential development. The new research is, in effect, observational evidence that those allowances are pointing in the right direction, and that the sea level component of coastal and tidal risk deserves to be treated as a primary driver rather than a footnote.

For anyone preparing or commissioning a coastal or tidal FRA, a few things follow:

• Treat sea level rise as central, not peripheral. On tidal sites the allowance for rising sea level usually matters more than peak rainfall, and it compounds over the design life.
• Test the lifetime, not just today. The Sequential and Exception Tests turn on a development being safe for its whole lifetime. A site that looks marginal now can look very different against a 2125 sea level allowance.
• Don’t read defences as a fixed guarantee. Standards of protection are defined against a baseline that is shifting beneath them; the residual risk behind a defence grows as the sea rises.

A measured reading

It is worth keeping perspective. Both studies attribute the rising-baseline component of flooding to human activity, not the storms themselves, and the Tulane analysis stops at 2005 because of the climate models available, which means its findings are, if anything, conservative. Local conditions also vary enormously; ground movement and subsidence can dominate at individual sites in ways that have nothing to do with climate.

None of that softens the central message. The frequency of damaging coastal floods is rising, the human fingerprint on that rise is now measurable, and the assumption of a stable sea level, the quiet premise behind a lot of older risk thinking, no longer holds. For developers, planners and the consultants advising them, that is less a reason for alarm than a reminder to assess coastal and tidal flood risk on the basis of the sea we will have, not the one we used to have.