Melting Arctic sea ice.
Blue Initiative

Sea-level rise

The ocean is rising about as fast as your fingernails grow, and the pace is speeding up. For a coastline, small numbers repeated for a century are not small.

Since 1993, satellites have measured the height of the ocean surface from orbit to within a few centimetres, over and over, all around the planet. Their record leaves little doubt: global mean sea level has risen by about ten centimetres in three decades, and it is rising faster now than when the measurements began. Over the longer record, from 1901 to 2018, the rise was about 20 centimetres, faster than in any century of the past three thousand years, according to the IPCC.

An artist's impression of a boxy satellite with a gold body and a large solar panel, orbiting above the blue Earth.
An artist's impression of the Sentinel-6 Michael Freilich satellite, which continues the record of sea level measured from orbit since 1993. Credit: NASA

Two engines

Two processes lift the sea, and both are driven by warming:

  • Water expands as it warms. The ocean has absorbed more than 90 percent of the extra heat trapped by greenhouse gases. Warmer water takes up more room. Between 1971 and 2018 this thermal expansion explained about half of the rise.
  • Ice on land melts into the sea. Mountain glaciers retreat, and the great ice sheets of Greenland and Antarctica lose more ice than snowfall replaces. Melting sea ice, which already floats, adds almost nothing; melting land ice adds directly to the ocean.
+20 cmglobal mean sea level rise from 1901 to 2018 (IPCC AR6)
3.7 mmaverage rise per year from 2006 to 2018, nearly three times the rate of 1901 to 1971 (IPCC AR6)
1 billionpeople are projected to live in low-lying coastal zones by 2050 (IPCC SROCC)
The Larsen C ice shelf releasing iceberg A-68 in 2017.
The Larsen C ice shelf in Antarctica releasing iceberg A-68, one of the largest icebergs ever recorded, in July 2017. Credit: NASA/JPL-Caltech/NOAA/GSFC

A slow, long clock

Sea level responds slowly. Heat keeps spreading into the deep ocean and ice sheets keep adjusting for centuries after the air stops warming, so some further rise is already locked in. What we choose now decides how much and how fast. The IPCC’s likely ranges for 2100, relative to 1995 to 2014, run from 0.28 to 0.55 metres in the lowest-emissions pathway to 0.63 to 1.01 metres in the highest. Rises close to two metres by 2100 cannot be ruled out if ice sheets become unstable.

Every centimetre of sea level puts more homes within reach of the next storm.

What matters for people is not only the average. A higher sea turns rare floods into regular ones: in many places, a flood that used to happen once a century is expected to happen every year by 2100 under higher warming. Salt water pushes into rivers and aquifers, and some coasts are also sinking as groundwater is pumped out, which multiplies the effect.

A flooded city street at night in Miami, with cars driving through water that reflects red tail lights and street lamps.
High-tide flooding in Miami during the king tides of October 2016, when rain could not drain into a sea already at its highest. Photo: B137, CC0

Proven solutions

Rising water is not new to people who live by the sea, and some countries have lived below it for centuries. Their experience shows that coasts can be made far safer, by building, by making room for water and by working with nature, as long as the planning starts early and continues for decades.

  • Cut emissions. The single biggest lever: every tonne of CO2 avoided reduces the eventual rise.
Black and white aerial photograph of a Dutch village flooded to the rooftops, with the streets turned into water.
Oude-Tonge on the island of Goeree-Overflakkee after the North Sea flood of 1953, seen from a US Army helicopter. Photo: US Agency for International Development, public domain
  • Protect: the Dutch Delta Works. In 1953 a North Sea storm surge broke Dutch dikes and 1,836 people died. The Netherlands answered with the Delta Works, a chain of dams, sluices and storm-surge barriers that turned about 700 kilometres of ragged coastline into a much shorter, straighter line of about 80. The last great piece, the Maeslant Barrier near Rotterdam, was finished in 1997. Its two gates, each 210 metres wide and 22 metres high, close automatically whenever the water is forecast to rise more than three metres above normal sea level near Rotterdam; they fill with water and sink to the bottom within two hours. It closed for the first time in 2007 (Rijkswaterstaat; Rijkswaterstaat).
The two white steel arms and gates of the Maeslant Barrier swinging out across a wide waterway, under a cloudy sky.
The Maeslant Barrier closing across the Nieuwe Waterweg near Rotterdam. Each of its two gates is 210 metres wide. Photo: Quistnix, CC BY 2.5
  • Protect: the Thames Barrier. London’s barrier spans 520 metres of the Thames at Woolwich and protects 125 square kilometres of central London from tidal surges. Since it began working in 1982 it has been closed 221 times to hold back floods, as of November 2025 (Environment Agency).
The shining steel hoods of the Thames Barrier piers lit up at dusk, reflected in the calm river.
The Thames Barrier at Woolwich at dusk. It has closed more than two hundred times since 1982 to keep tidal surges out of central London. Photo: Robert Burniak, CC BY-SA 4.0
  • Make room for water. After river floods in 1993 and 1995, when about 250,000 people in Gelderland alone were evacuated as a precaution, the Netherlands changed its approach: instead of only raising dikes, it gave rivers more space. The Room for the River programme widened the space for high water, for example by digging new flood channels, at 34 locations along the IJssel, Waal, Lower Rhine and Lek between 2006 and 2019, for €2.3 billion, improving safety for around 4 million people (Rijkswaterstaat).
A long brick arched bridge crossing a new side channel of the river Waal at Nijmegen, with a white arch bridge in the distance.
The Spiegelwaal at Nijmegen, a side channel dug for the river Waal under the Room for the River programme, crossed by the bridge De Oversteek. Photo: Havang(nl), CC0
  • Work with nature. Mangroves, salt marshes, dunes and reefs absorb wave energy and can grow with the sea. Field measurements found wave heights 13 to 66 percent lower for every 100 metres of mangroves (McIvor et al., 2012). Worldwide, mangroves prevent more than US$65 billion of flood damage a year and keep more than 15 million people from being flooded; Vietnam, India and Bangladesh gain the most in people protected (Menéndez et al., 2020). During Hurricane Sandy in 2012, coastal wetlands in the north-eastern United States avoided about US$625 million of flood damage (Narayan et al., 2017).
  • Plan retreat where needed: managed realignment. At Medmerry in West Sussex, the Environment Agency, with the RSPB, stopped patching a shingle bank that needed constant repair and, by November 2013, had built 7 kilometres of new embankments further inland, letting the sea in behind. The £28 million scheme reduced flood risk to hundreds of properties, created 183 hectares of new tidal habitat for wildlife and saves about £300,000 a year in repairs (Environment Agency, 2016; Environment Agency).
Aerial view of a red and white lighthouse standing in a wide green salt marsh cut by creeks, on the edge of tidal mudflats.
Salt marsh around the Westerheversand lighthouse on Germany's Wadden Sea coast. Marshes like this, and the new ones created at Medmerry, absorb wave energy and can build up as the sea rises. Photo: Walter Rademacher, CC BY-SA 3.0
  • Accommodate. Raise buildings and roads, flood-proof ground floors, and design drainage for higher tides.
  • Stop sinking. Limiting groundwater pumping slows the subsidence that makes rising water worse.

None of these places is finished. The Netherlands is already improving its storm-surge barriers to cope with higher seas. But these places show that a country which measures its risk honestly and plans ahead can keep people safe.

Organisations doing this work

These non-profits have a public record of results. Take a look at what they do, and if their work speaks to you, consider donating to them directly through their own websites. The Local Solar System Foundation has no affiliation with any of them and receives nothing from them. Before you give, read each organisation’s latest annual report and accounts, as you would with any charity.

  • Mangrove Action Project is a US charity with 501(c)(3) status, founded in 1992. It trains communities, foresters and conservation groups in Community-Based Ecological Mangrove Restoration, a method that restores the water flows mangroves need so that they return by themselves, rather than simply planting seedlings that often die. It has supported mangrove restoration in more than 20 countries.
  • Wetlands International is a Dutch public-benefit organisation (ANBI) with offices around the world and work in more than 100 countries. In Demak, on the north coast of Java, where the sea has taken more than 3 kilometres of land in places, it led a 2015 to 2020 programme that built permeable brushwood fences to trap mud so that mangroves could grow back, to protect about 70,000 people from erosion and flooding (Wetlands International).
A dense tangle of reddish-brown mangrove stilt roots arching into still, clear water.
Mangrove roots in the Laguna de La Restinga on Margarita Island, Venezuela. The tangle of stilt roots slows the water and traps mud. Photo: Wilfredor, CC0
  • Climate Central is a policy-neutral US non-profit (501(c)(3)) founded in 2008. Its free maps let anyone see which land is projected to be below flood levels as the sea rises, and its maps and analyses are used by news outlets in more than 170 countries. Knowing your risk is the first step to planning for it.

How you can help

  • Know your risk. Look up your home, school or workplace on a public sea-level or flood map, and make a family flood plan if you live near the coast.
  • Protect coastal nature. Volunteer with or support groups restoring mangroves, marshes and dunes.
  • Ask your council about its adaptation plan, and whether new building is still allowed in land likely to flood.
  • Cut your own emissions where it counts most: home heating, travel and electricity.
  • Share the data. Translate this explainer or add local tide-gauge data through the contribution board.

Sources and further reading