How will sea level rise affect flood risk to populations and critical infrastructure in the UK in the next few centuries? A pioneering study by scientists from the UK Met Office, Bristol University and Fathom has found that the number of people exposed to flood by 2300 could rise dramatically – but global reductions in greenhouse gas emissions could reduce these numbers.
Global sea levels are rising at an accelerated rate, posing a near-term and long-term threat to communities and ecosystems the world over. Several scientific studies have also shown how climate policy decisions will determine how fast these sea levels will rise in the future.
According to the latest findings of the Intergovernmental Panel on Climate Change (IPCC), the global mean sea level (GMSL) will rise by 0.3 to 3.1 meters by 2300 under a low emissions scenario, and 1.7–6.8 m under high emissions. This increases substantially – by up to 16m under high emissions – if ice sheet instability processes are taken into account.
In the UK, however, planning guidance rarely extends beyond a 21st-century time-horizon. Estimates do not take into account the uncertainty of ice sheet loss and are often based on flood model techniques that ignore important factors such as topography, water pathways and defences.
In this new study, scientists from the Met Office Hadley Centre, Fathom and the University of Bristol address this critical gap and quantify future flood risk based on the IPCC’s recent findings.
The research: What will flood risk look like under best and worst case scenarios?
To model long-term coastal inundation, the research team deployed Fathom’s UK hydrodynamic model operating at a fine 1 arcsecond (~25 m) spatial resolution, simulating the physical behavior of water moving across the terrain during extreme events.
The models were used to simulate flooding resulting from a 1-in-200-year storm surge event – the standard design threshold for major urban flood protections in the UK.
Rather than predicting a single definitive future, the study explores five different sea-level rise “storylines”, or scenarios, based on the range of outcomes identified in the IPCC’s Sixth Assessment Report. These scenarios reflect the uncertainty around how quickly and how much sea levels could rise.
The three scenarios modeled were:
- Storyline A: Strong emissions reductions over the 21st century and a low warming scenario. Plausible“best case scenario” for informing minimum adaptation requirements.
- Storyline B (Baseline): A moderate warming scenario consistent with current emissions pledges.
- Storyline H2 (Reasonable worst case): A low-likelihood, high-impact scenario under high emissions that includes poorly understood ice sheet instability processes in Antarctica.
Key findings: Near-term certainty vs. multi-century divergence
The research highlights a stark contrast between near-term climate commitments and major long-term path dependencies:
1. Shifting risk in 2100
- By 2100, in all five storylines at least an additional 0.5 million people become exposed to the 1-in-200-year undefended flood extent – a 25% increase compared to the present day.
- Under the baseline scenario (Storyline B), real-world impacts to defended areas remain limited by 2100, though southern UK ports located outside defenses will face disruption.
- Under the Worst Case (Storyline H2), 2100 sea-level rise reaches the upper limit of what present defenses could plausibly handle, placing west London, central Cardiff, and infrastructure such as Belfast Airport within the flood zone.
2. The picture in 2300
In the worst case, high emissions scenario, 20% of the populations of England, Scotland and Wales will be at risk of coastal flooding by 2300.
- Under current pledges (Storyline B): Sea-level rises increase by 1.2m – 1.8 m across the UK by 2300. The 1-in-200-year storm surge flood area will grow by 56%, and 1.7 million more people will be exposed.
- The benefit of global action (Storyline A): Swift global action to meet Paris Agreement targets limits 2300 exposure increases to just 28%, avoiding up to 1 million exposed populations compared to the baseline.In the worst case scenario (Storyline H2): If significant Antarctic ice sheet instabilities are triggered, the UK faces catastrophic relative sea-level rises of 16.5 – 17.5 m by 2300. This expands the 1-in-200-year storm surge flood area by over 300%, exposing an additional 13 million people and putting over 20% of the total populations of England, Scotland and Wales at risk.
3. Tipping points for infrastructure and defenses
Between 2150 and 2200 under high-impact pathways, the City of London financial district, West London and the M25 motorway near Heathrow will be permanently inundated.
- The study also intersects flood maps with the UK Environment Agency’s asset database, identifying the thresholds where existing flood defenses fail by 1 meter or more.
- If current pledges are met (Storyline B), less than 1% of defenses face severe overtopping by 2100, rising to 27% by 2200 and 35% by 2300, showing a clear need for sustained long-term structural investment.
- Under the worst case scenario (Storyline H2), 10% of defenses are severely overtopped by 2100, skyrocketing to nearly 100% by 2200 – effectively pushing the country past its hard engineering adaptation limits.
- In Storyline B by 2300, vital public and commercial infrastructure like Glasgow Airport, the Teesside chemical works, and the Port of Dover move into the active flood zone. Flooding propagates deep inland; the Lincolnshire Wolds becomes an isolated island and central areas of historic cities such as Cambridge and York submerge to depths exceeding 1 meter.
- In London, water levels would surpass the 5-meter engineering limit of the Thames Estuary system between 2150 and 2200 under high-impact pathways, permanently inundating the City of London financial district, West London, and the M25 motorway near Heathrow.
Implications for long-term adaptation planning
The findings indicate that evaluating coastal hazards solely on a 100-year horizon creates a false impression of the true risk landscape. Because sea-level rise accelerates heavily post-2100 under high-emissions pathways, long-lived infrastructure and future coastal settlements must be designed with multi-century commitments in mind.
While near-term policies must focus on reinforcing defenses out to 2100, long-term safety under severe climate scenarios requires adaptation strategies and a planned, large-scale managed retreat of populations and strategic settlements away from vulnerable coastlines.
Fathom’s commitment to open research
Supporting scientific advancement, peer review, and transparency is a core pillar of Fathom’s mission. By publishing our core methodologies and collaborative studies alongside world-leading academic institutions, we ensure that water risk intelligence remains robust, transparent, and trusted. This collaborative research helps underpin the next generation of climate-conditioned flood maps, equipping the financial, insurance, and public sectors with the rigorous foresight needed to build true long-term resilience.
Want to learn more about this innovative research? Download the full paper in Nature Communications and dive into the details.
Quantifying UK coastal flood exposure under future sea-level rise to 2300
How will sea level rise affect flood risk to populations and critical infrastructure in the UK in the next few centuries? A study by the UK Met Office, Bristol University and Fathom has found that the number of people exposed to flood by 2300 could rise dramatically – but global reductions in greenhouse gas emissions could reduce these numbers.
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