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AMINATH SHAUNA grew up on the Addu Atoll, a small group of islands in the Maldives whose villages and beach resorts are spread around a central lagoon. When viewed from above, it all looks about as permanent as the ring left by a coffee cup.

Low-lying islands like those of the Maldives, where half a million people live barely a metre above the Indian Ocean, are ground zero when it comes to the threat of rising sea levels driven by global warming. “One of my earliest memories is of a tidal swell and a big breadfruit tree falling down right in front of our house,” says Aminath, now in her 30s. “This is something I have grown up with.”

But the effects of rising seas will be felt far and wide. In the worst-case scenario, average sea level could rise by nearly 2.5 metres this century. Even a fraction of this would be catastrophic. Globally, over a quarter of a billion people live less than 2 metres above sea level, including in cities such as Jakarta, Rio de Janeiro and Miami.

Aminath knows this all too well. As the environment and climate change minister for the Maldives, she is part of a community of politicians and scientists trying to work out how quickly sea levels will rise, if this can be slowed and what it means for us all. In some places, new ways of holding back the tide may buy us a few decades. Elsewhere, this won’t be possible. We are facing a disaster unfolding in slow motion. Responding effectively means a sea change in the way we think.

Earth’s water lives almost entirely in the oceans or in polar ice caps. Sea levels are largely determined by the balance between evaporating seawater and melting ice returning as liquid to the oceans. If evaporation and melting are equal, all is well. “But as the climate warms, evaporation and snowfall increases a little bit, but the amount of water returned goes way, way up,” says glaciologist .

Sea levels are going up, then, but that isn’t as simple as filling a bath. There is no single equation setting out how much a given amount of warming raises sea levels, because multiple factors are at play. Warming water expands, causing levels to rise even without ice melting. The seabed can subside as more water enters the oceans, lessening the effective rise in the surface level. The weakened gravitational pull of melted ice caps can have an impact too, as can the regional effects of prevailing winds and swirling ocean currents.

It isn’t even easy to figure out how sea levels have changed in the deep past – they can vary from place to place and, until recently, there wasn’t a good enough spread of geological samples that record past sea levels from a variety of locations around the globe. In 2016, a team led by Robert Kopp at Rutgers University in New Jersey compiled a database of indicators that tell us the sea level over the past 3000 years in 24 locations and used statistics to calculate the average past sea level. This work showed that the ocean surface went up and down by no more than 8 centimetres for most of those three millennia. Between 1900 and 2016, however, . Estimates suggest it has risen another 8 centimetres since then.

What happens next depends on how much we cut greenhouse emissions in response to the climate crisis, but seas will continue to rise in the medium term no matter what we do. “The best we can hope for in the next 100 years is that the rate of rise remains constant,” says oceanographer .

Our most robust estimates of this rise come from the Intergovernmental Panel on Climate Change (IPCC), which collates published scientific evidence. Rahmstorf says the IPCC has traditionally been accused by climatologists of underestimating future sea level rises. But he says he gasped when he read its latest report, released last August. It sets out the sea level changes we can expect under several scenarios that differ significantly according to what action humanity takes to reduce emissions (see “How much will the seas rise?”). Even under the most optimistic scenario, in which global carbon emissions are cut to net zero by 2050, a further 50 centimetres of sea level rise is predicted by 2100. On the other hand, if little action is taken on emissions and the climate proves highly sensitive to this, we might see a rise of nearly 2.5 metres by 2100 and 5 metres by 2150.

One of the big uncertainties in these predictions is how the Antarctic ice sheets will respond to warming temperatures. If scientists’ worst fears are realised and the vast Thwaites glacier collapses, unleashing a huge tide of ice into the ocean, that could push us towards the more terrifying scenarios (see “The doomsday glacier“). However the future pans out, there will be bad news. “We have more than 150 cities with more than a million inhabitants on coasts around the world,” says Rahmstorf. “Even a 1-metre rise is quite catastrophic. Five metres on that timescale would basically be an unimaginable disaster.”

Precisely how many people will be affected has been tricky to gauge until recently because, strange as it may sound, there was a great deal of uncertainty about the height of the world’s coastal dry land. Without a good handle on this, it is hard to establish which bits of coastline will be affected by each centimetre of ocean encroachment. “There hasn’t been a proper elevation map for the world, which is odd because we have one for Mars and the moon,” says , a water resources expert at Deltares, a research institute in the Netherlands.

Orbiting radar systems are typically used to determine topography, but these use signals that tend to bounce back off the first thing they hit. That isn’t a problem on the moon, but on Earth, the presence of buildings and tree canopies can lead to overestimated elevations. For accurate data, we need lidar, a laser-based measurement system that can penetrate the densest canopy. This was only sparsely available until 2018, when NASA’s ICESat-2 released a global database of elevation data measured with lidar.

Last year, Hooijer and his colleague , combined this information with population data and established that 267 million people are at risk because they live on land that is less than 2 metres above sea level. If sea levels rise, the 2-metre zone of most at-risk land rises with it, putting more people in danger. Taking a reasonable projection of a 1-metre rise over the next 80 years – and assuming no change in the number and distribution of people in lowland areas – the researchers calculated that the population of this 2-metre zone will increase to at least 410 million by 2100, a number greater than the current US population. Of these people, 72 per cent will be in the tropics.

The most pressing problem for coastal communities and cities isn’t the slow incursion of water, however, but what will come first: worsening tidal surges, storms and floods that will eventually make coastal areas unbearable to live in.

Engineering has long been the go-to solution, and projects are getting bigger across the world. In the Maldives, sea walls protect the capital Male, where more than 200,000 people live on an island roughly the size of London’s Hyde Park. In 2020, the UK finished the £100 million Boston Barrier in Lincolnshire, which can be raised to cut off tidal surges along the river Witham, protecting the town of Boston upstream from flooding. It was activated for the first time in November after warnings of a dangerously high tide. In Jakarta, Indonesia, where land is sinking due to groundwater extraction, the Giant Sea Wall is being built. The $40 billion dyke designed to protect the city is due to be completed in 2025.

“A 5-metre sea level rise by 2150 would basically be an unimaginable disaster”

But physical defences like these are expensive, impermanent and, in some cases, unintentionally harmful. Take groynes, structures built perpendicularly out from beaches into the sea. A series of these is good at stopping protective sediment such as sand from being washed away along the coast through a process called longshore drift, but it can actually increase erosion after the last groyne because this area receives less sand than it would otherwise.

A softer solution has begun to catch on in the past decade. In the Netherlands, where a quarter of land lies below sea level, authorities are racing to protect eroding beaches. They typically dredge sand from the bottom of the North Sea once every three years or so and use it to nourish their beaches, staving off erosion. But in 2011, a new strategy was piloted. The Dutch government had a huge amount of sand – some 21.5 million cubic metres – dredged up and deposited in a heap just offshore. This was then left to be gradually blown and washed away to form an offshore sandbar. The hope was that this would protect the coast for longer and reduce how often the bed of the North Sea had to be disturbed. While scientific studies on this “” are still to come, the Dutch government has published . The distributed sand has contributed to the growth of dunes, which provide further protection, while the numbers of feeding birds, including cormorants and terns, has risen. (The kitesurfers who found a playground in the lagoon behind the sandbar were happy too.) It is now expected that the project will last much longer than its intended 20-year lifespan. A similar sand motor was introduced in the UK in 2019, between .

Meanwhile, so-called nature-based solutions are getting more attention too, not least because they help preserve biodiversity as well as protect coasts. to which coral reefs are not only vital marine breeding and feeding grounds, but natural diffusers of wave energy. Aminath says the Maldives has so far given legal protection to 13 per cent of its reef area against future degradation caused by fishing or pollution. Elsewhere, artificial reefs – breakwaters made of rubble that attract marine species – are being considered specifically with erosion prevention in mind. , South Korea, suggested that such a reef would reduce wave height by up to 70 per cent and cause large waves to break before they reach the shore. Mangrove forests are also known to attenuate wave power and there are projects around the world to both protect existing mangroves and plant new ones.

Where coastal protection is needed, decisions about what kind of scheme to implement and whether it is worth it come down to local conditions – and money. “If you can gain 100 years, that’s worth the investment in many places,” says Rahmstorf. “But if we’re talking about 5 metres of sea level rise, that will really hit the limits of what we can do.” Sooner or later, he says, we are going to be in a situation where coastal communities become unsustainable, creating extreme geopolitical problems, including a massive refugee crisis.

This means that, in many places, we are going to have to shift our mindset from what climate scientists call mitigation to adaptation. Instead of relying on technology and engineering to protect us from rising seas, we must change how and where we live.

High-income nations won’t be immune. In the UK, the Environment Agency is still spending on coastal flood defences in some places in England. But in others, such as parts of Norfolk and Yorkshire, where cliffs are crumbling away at more than 1.5 metres a year, that is no longer always a viable option. There is a growing recognition that, in the coming decades, a managed retreat from the coast may be necessary in some places. “We’re updating our understanding of erosion all the time, but some of the options are really difficult for people to comprehend,” says Julie Foley, director of flood risk strategy and national adaptation at the Environment Agency.

The organisation doesn’t have any firm plans for managed retreat yet. But there are already hints that it may be on the cards. In Withernsea in Yorkshire, some 24 homes are at risk of falling into the sea and need to be demolished, at a cost of up to £40,000 each. In the past, this bill would have been partly met by the local council, but it can’t afford to pay for all the work. So there are calls for the UK government to stump up the cash. “We need to be thinking about how communities over a reasonable period of time start to think about how they can transition away from the coastline,” says Foley. “That’s a really difficult conversation.”

In some cases, we have gone as far as moving whole buildings inland, but only where they are historically important. In 2019, engineers laid temporary tracks to slide the 720-tonne Rubjerg Knude lighthouse in Denmark 70 metres away from crumbling cliffs. But even this may prove a short-term solution. The Belle Tout lighthouse at Beachy Head in East Sussex, UK, is now just 20 metres from the cliff edge, 23 years after it was shifted inland.

Back in the Maldives, Aminath is making plans for what she calls “population consolidation”: the further relocation of people to higher ground. By 2019, 50,000 islanders had moved from across the country to Hulhumalé, a bigger, 2-metre-high island to the north of Male built with dredged sand. Since at least 2008, the Maldives has considered purchasing new territory away from its own islands, in India, Sri Lanka or Australia. Aminath clings to the hope that mitigating measures, allied with global action on emissions, may give her fellow islanders a lifeline. The stakes for her personally are now higher than ever: she recently had her first child. “It’s very scary for me,” she says, “to think that, one day, my 1-year-old daughter might not have a home.”