HOW HIGH CAN WE BUILD THE WALLS?

Sea-Level Rise: The Sleeping Giant That Could Redraw Britain

Britain’s coastline can appear permanent.

We draw it confidently on maps. We build homes, ports, roads, railway lines, airports and businesses beside it. Coastal towns grow around the assumption that the sea will remain roughly where it is today.

Yet coastlines have never been fixed. They move as tides, storms, currents, erosion and changing sea levels reshape the boundary between land and water.

Now a major UK study has attempted to look much further ahead than most climate assessments. Instead of stopping at 2050 or 2100, researchers from the University of Bristol and Fathom have modelled possible coastal flooding outcomes all the way to the year 2300.

Their conclusion is deeply uncomfortable: sea-level rise may be one of the slowest-moving consequences of climate change, but it could eventually become one of the largest.

Professor Matt Palmer, who led the research, described it as the “sleeping giant” of climate change:

“The oceans and ice-covered parts of the world respond very slowly to global warming, which makes sea level rise the ‘sleeping giant’ of climate change. The result is that sea-level rise is locked-in for centuries to come.”

That does not mean the worst outcome is inevitable. It does mean that some further rise is already unavoidable—and that decisions made during the next few decades could help determine how far the sea eventually rises.

A Climate Problem That Does Not Stop in 2100

Most public discussions of climate change focus on 2050, when the UK aims to reach net zero, or on 2100, the conventional finishing point for many scientific projections.

But the climate system does not recognise the end of a century.

The oceans absorb enormous quantities of heat and expand slowly as they warm. Glaciers and the Greenland and Antarctic ice sheets also respond over decades, centuries and sometimes millennia. Even after atmospheric temperatures begin to stabilise, the oceans and ice sheets can continue responding to the warming that has already occurred.

Global mean sea level rose by about 20 centimetres between 1901 and 2018. Around the UK, average sea level rose by approximately 18 centimetres between 1900 and 2022, although the amount varies regionally because some areas of Britain are still rising while others are gradually sinking following the last Ice Age.

This delayed response is why sea-level rise is “locked in”. It is rather like turning down an oven after a large joint has been heating for hours: the temperature control may change immediately, but the heat already stored within the system continues moving and producing effects.

The question is therefore not whether sea level will rise further.

The questions are:

How much will it rise? How quickly? Which places will be protected? Which places will have to adapt? And how much of the worst outcome can still be avoided?

What the New Research Found

The researchers combined UK-specific sea-level projections with a detailed national flood model. They explored several physically plausible “storylines”, ranging from strong climate action to a reasonable worst-case outcome involving major ice-sheet instability.

The model examined the area potentially exposed during a severe coastal flood event with an annual probability of around 1 in 200. The main population calculations used present-day population locations and an undefended flood extent, rather than predicting exactly where people will live or what defences will exist centuries from now.

By 2100, the different storylines remain relatively close together. The final published study estimates that at least another 500,000 people could be exposed to the modelled coastal flood extent—a rise of approximately 25% compared with the present day.

Under the central storyline highlighted by the University of Bristol, UK sea levels rise by approximately 0.4 to 0.6 metres by 2100. The area classified as exposed to coastal flooding increases by about 13%.

After 2100, however, the pathways begin to separate dramatically.

Under a storyline broadly reflecting current global emissions pledges, approximately 1.7 million additional people could be exposed by 2300. Meeting the goals of the Paris Agreement could avoid up to one million of those additional exposures.

At the other end of the range is the reasonable worst-case storyline.

This is not presented as the most likely future. It is a high-impact possibility involving severe instability in the Greenland and Antarctic ice sheets. The researchers say a robust probability cannot currently be assigned to it, but it cannot be ruled out under high emissions and warming.

In that storyline, local sea levels around the UK rise by approximately 16.5 to 17.5 metres by 2300, and as many as 13 million additional people are exposed to the modelled severe coastal flood extent. Present-day coastal defences would be largely irrelevant at that scale.

Thirteen Million Is Not a Prediction

The 13 million figure is understandably alarming, but it must be interpreted properly.

It does not mean scientists are predicting that 13 million named individuals will definitely be flooded or forced to leave their homes.

The research holds present-day population patterns constant and asks what would happen if those people and settlements remained in the same places while the sea rose. It primarily models an undefended 1-in-200-year coastal flood event.

In reality, populations will change. New defences may be built. Buildings may be raised or relocated. Some land may become wetlands. Communities may move long before permanent inundation occurs.

The study also does not fully model coastal erosion, changing sediment movement or the future development and loss of salt marshes. These processes could substantially alter real coastlines and local flood exposure.

The number should therefore be treated as a national-scale warning about the potential size of the problem, not as a precise forecast of how many people will lose their homes.

However, that does not make it unimportant.

Governments routinely prepare for reasonable worst cases in national security, disease control, energy supply and financial regulation. Climate adaptation should be no different. A low-probability event can still demand serious preparation when its consequences would be enormous.

Which Parts of Britain Are Most Exposed?

The most vulnerable places are unsurprisingly low-lying coastal plains and estuaries.

The study identifies The Wash and the Humber Estuary as major hotspots under every storyline. These regions contain extensive areas of low-lying land, communities, transport links and productive farmland.

By 2300, substantial changes could also affect:

• the Thames Estuary;

• eastern Norfolk;

• North Somerset;

• Gloucestershire;

• Liverpool and the north-west coast;

• Cardiff and the Severn Estuary;

• ports and airports around Britain.

Under the reasonable worst-case storyline, the modelling places parts of the M25, Heathrow’s surroundings, the City of London and large areas of west and north London within the severe flood zone. The Lincolnshire Wolds could become an island during extreme flooding, while floodwater could extend inland towards places such as Cambridge and York.

Again, these are not forecasts of what will definitely happen. They demonstrate how profoundly Britain’s geography could change if high-end ice-sheet instability were allowed to develop.

Flood Defences Matter—but They Cannot Solve Everything

It is easy to respond by saying: “We will simply build higher sea walls.”

In many places, improved defences will indeed be essential.

The Thames Barrier demonstrates what major engineering can achieve. Sea walls, surge barriers, raised embankments, pumping stations, tidal gates and improved drainage can protect homes and infrastructure for decades.

The research suggests that many existing defences could probably be upgraded to manage the central levels of sea rise expected during this century, although this would require significant investment. Under the reasonable worst-case storyline, however, nearly all modelled English defences would be substantially exceeded by 2200.

Every defence also carries costs.

It must be designed, funded, built, inspected, repaired and eventually replaced. Protecting one section of coastline may alter erosion or sediment movement elsewhere. Higher walls can separate communities from the sea and damage coastal habitats.

There will therefore be places where defending the existing shoreline remains practical, places where land must be allowed to flood in a controlled way, and places where buildings and infrastructure eventually need to move.

The difficult challenge will be deciding which is which.

Managed Retreat Must Not Mean Abandonment

“Managed retreat” is an uncomfortable expression.

For people whose homes, businesses and communities are affected, it can sound like a technical phrase for being abandoned.

Yet unmanaged retreat would be worse.

Imagine being told that your home may become indefensible only after its value has collapsed, insurance has disappeared and the next storm is approaching. Moving communities takes decades of consultation, planning, construction and financial support. It cannot be organised after the water arrives.

A responsible national policy would need to address difficult questions early:

Who pays when homes can no longer be defended?

Should homeowners receive compensation?

Where will replacement communities be built?

How will schools, roads, hospitals and businesses be relocated?

How do we preserve the identity and social connections of communities that have existed for generations?

How do we avoid leaving poorer households trapped in the highest-risk areas?

The movement of communities should never be treated simply as lines changing on a map. It involves memories, livelihoods, family histories and a sense of belonging.

My Own Reflection: Water Makes Climate Change Feel Less Abstract

Living inland can make sea-level rise feel like somebody else’s problem.

My own time around boats and the River Thames continually reminds me that water does not behave according to convenient administrative boundaries. Rivers, estuaries, rainfall, groundwater, tides and the sea form interconnected systems.

The Upper Thames is not the coast, but changing water levels, strong flows and unusual weather quickly affect what can safely happen on the river. They turn an abstract weather forecast into a practical decision.

Coastal climate change will be similar, but on a much larger scale.

A flooded port can disrupt national supply chains. A damaged railway affects travellers far inland. Saltwater entering farmland can reduce food production. Relocating a coastal community creates demand for housing, schools and services elsewhere.

Even households hundreds of miles from the sea may eventually share the cost through taxation, insurance, food prices, infrastructure spending and population movement.

The coastline belongs to the whole country because what happens there affects the whole country.

What Should Britain Be Doing Now?

The year 2300 sounds remote, but many of the decisions that shape it begin much sooner.

1. Reduce Emissions as Rapidly as Possible

Adaptation is necessary because some sea-level rise can no longer be avoided. But adaptation alone is not enough.

The difference between the study’s storylines is enormous. Strong global emissions reductions cannot prevent every centimetre of sea-level rise, but they can reduce the risk of triggering far more destructive ice-sheet changes over future centuries.

Solar power, wind generation, energy storage, efficient homes, cleaner transport and lower industrial emissions are not separate from coastal protection. They help determine how difficult coastal protection will eventually become.

2. Plan Beyond 2100

A bridge, railway, nuclear facility, port, reservoir or new settlement may shape land use for a century or longer.

Major developments should not be approved solely because they appear safe on a map ending in 2100. Planning must consider longer time horizons, alternative sea-level storylines and the possibility that infrastructure will need to be adapted or relocated.

The study notes that current national guidance largely stops at the end of this century, despite the much larger divergence in sea-level outcomes after 2100.

3. Decide Where Britain Will Hold the Line

Not every kilometre of coastline can be protected indefinitely.

National and local government must identify which communities, industries, ports, transport routes and heritage sites justify major long-term defences. These decisions need transparent criteria and proper public debate.

Large coastal infrastructure can take decades to plan and build. Delaying difficult choices does not remove them; it merely reduces the available options.

4. Give At-Risk Communities Financial Certainty

People should not be left unable to sell, insure, repair or replace homes because risk has increased.

Long-term adaptation may require new compensation systems, relocation grants, property-purchase schemes and support for affected businesses. These policies must be designed before large numbers of households need them.

5. Work With Nature Where Possible

Salt marshes, dunes, wetlands and restored floodplains can absorb wave energy, store floodwater and provide valuable wildlife habitat.

They will not replace engineered barriers in every urban area, but they can form part of a wider defence system while creating space for coastlines to change more naturally.

6. Monitor the Ice Sheets

The largest uncertainty comes from the future behaviour of Greenland and Antarctica.

Improved satellite measurements, ocean monitoring and ice-sheet research could provide earlier warning if observed sea-level rise begins following a more extreme pathway. Earlier detection would give Britain more time to upgrade defences, alter planning decisions and prepare communities.

The Message Is Not That Nothing Can Be Done

Climate warnings are sometimes presented so dramatically that people conclude the problem is hopeless.

That is not what this study says.

Some further sea-level rise is unavoidable, but the difference between the possible futures remains enormous. Under strong emissions reductions, the UK still faces a serious adaptation challenge. Under severe ice-sheet instability, it could face the reshaping of much of its coastline and the movement of millions of people.

Our choices still matter.

Cutting emissions matters.

Protecting wetlands matters.

Building appropriate defences matters.

Avoiding unsuitable development matters.

Giving communities decades of warning rather than a few years matters.

Conclusion: The Coast of 2300 Is Being Shaped Today

Few of us will ever see the British coastline of 2300.

That can make it tempting to leave the problem to future generations. But those future generations will live with consequences created partly by the energy, planning and infrastructure decisions we make now.

Sea-level rise is a sleeping giant because it moves slowly, stores the effects of past warming and continues responding long after emissions enter the atmosphere.

Sleeping, however, does not mean harmless.

Britain must prepare for the rise that is already unavoidable while doing everything possible to prevent the most extreme outcomes. That means combining emissions reduction with honest adaptation planning—not choosing one instead of the other.

The future coastline has not yet been decided.

But the longer we wait to act, the fewer choices those who live beside it will have.