We have passed the point of no return: rising seas will soon directly affect hundreds of millions of people around the planet. They will indirectly affect many millions more as transport connections, water supplies and factories in low-lying areas are lost or have to be relocated. What exactly are we facing? The latest research suggests we are likely to see a rise of a metre by the year 2100. Given how much carbon dioxide is already in the air and oceans, as much as three metres may be baked in over the 200 years or so that follow. And while that might seem like a long way off, coastal groundwater levels will rise much sooner, wrecking infrastructure and causing toxic pollution well before cities such as Miami, New York and San Francisco are permanently inundated. Where will all those people, warehouses, water treatment plants and rail lines move to, given that the interior of large land masses will have become drier? The forced migration of hundreds of millions of people will undoubtedly lead to serious international conflict over space and basic resources like fresh water. Conflict is another word for war. So can we adapt instead of migrating, by learning to live with higher water? It’s an ironic question to ask in English, since the language arguably owes its origins to coastal flooding. Angles, Saxons and Jutes moved to Britain from a flat area of what is now Germany and Denmark during a period of extreme flooding from about AD400 to AD800. Wealthy families escaped to Britain from around the river once called Fifeldor, or the “door of monsters”, which is an apt name for a river that floods catastrophically. When these elite families arrived, some historians believe that as much as a third of the British population was enslaved. We can probably agree that migration of elites and enslavement of indigenous people is not a model that should be emulated. The Fifeldor river is now known as the Eider, and is equipped with a massive storm surge barrier. Major coastal engineering works have been part of the response to flooding since the 1960s, including the Thames Barrier, the Rotterdam Barrier, and the new concrete and steel floodgates in New Orleans, designed by Dutch engineers. These gates are the most visible pieces of complex machines that coordinate pumps, tide gates and flood storage systems intended to control monster floods. But these systems will require increased funding and concentrated political will to scale them up as rising tides and extreme storms become more common. Walls, gates and pumps are also fundamentally brittle, in the sense that the consequences of their failure is catastrophic. It would be smarter to have systems that are “safe to fail”, as urban planner Nina-Marie Lister has proposed, meaning systems designed to fail gradually or in ways that avoid harm. Otherwise, coastal defence machines may themselves become the new sea monsters. While we’re talking monsters, coastal regions are often contaminated, having hosted military bases, waste disposal sites, chemical plants or petroleum refineries. The old ones may be buried under various kinds of “caps”, but are typically not protected from flooding that comes up from below or from the sides, as groundwater and rainwater does. Many of these industrial and military sites are already leaking into local groundwater. As sea and coastal groundwater levels rise, these toxins can move in unexpected directions. If a volatile chemical such as vinyl chloride comes into contact with a cracked sewer line, or simply enters the gravel around a pipe, vapour can rise into homes, schools or workplaces – causing cancer and other illnesses. If such contaminants enter shallow tidal waters, they will kill fish, shellfish and birds. This is already happening in San Francisco Bay, where chemicals known as persistent organic pollutants were buried at the shore, at a site where thousands of houses are now planned to be built. Fish living in the adjacent mud have been found with multiple tumours and reproductive changes. So how could we live with all this if, on a global scale, we simply must? One possible strategy is to dig out the contaminated soils and make them inert, by combining them with other chemicals, baking them, or filtering out the worst. We might also sequester them by binding the scary materials in blocks of concrete, compacted dirt or glass. Those digs would leave us with extensive – and useful – ponds near areas where the sea and the groundwater are rising, and where rivers will flood more often. The Dutch have pioneered the use of floating urban districts in ponds that are protected from waves and tides, using flexible infrastructure connections and “slip-collars” on hefty columns that tether them to land while allowing them to rise and fall with small tides. Such a sea-city could be built in some of the newly dug ponds, while others could be used for recreation, or to support coastal wildlife. If we surround those ponds with low dikes, they could stay in place longer as seas continue to rise over the centuries, forming part of a “managed retreat” strategy. Most of the hundreds of millions who will be affected by rising seas live in less wealthy countries, of course, not in Europe or the US. To avoid forcing an unimaginable number of people to move when most have no viable destination, strategies for living with water need to be cheap. Ideally, people in the developing world would be able to build and maintain their chosen strategy on their own – without hiring European or US companies or buying equipment they can’t afford to replace. If their strategy for living with higher water is to move dirt around to create ponds and dikes, local leaders who can direct labour and equipment must be able to accomplish and maintain these independently. But, since adaptation in place prevents conflict, making everyone safer, we all have an interest in helping pay for it around the world. Floating cities come with many challenges: they will need new water supply and sewage infrastructure that are more local, and not so centralised. It’s also true that they can’t match the density of high-rise towers. And in some places they just won’t be suitable or appropriate. But despite these issues, they offer a template for thinking about one of the biggest problems our civilisation faces. For the next 200 years, every person on this planet who values peace, health and political stability must pay attention to the waters rising around us. Kristina Hill is an associate professor of environmental planning at the University of California, Berkeley. Further reading The Water Will Come: Rising Seas, Sinking Cities and the Remaking of the Civilized World by Jeff Goodell (Back Bay, £17.99) All We Can Save: Truth, Courage, and Solutions for the Climate Crisis, edited by Ayana Johnson and Katharine Wilkinson (Oneworld, £13.99) New York 2140 by Kim Stanley Robinson (Orbit, £9.99)