Sea level rise: Everything you need to know

Lost homes, lives and livelihoods are among the worst impacts of rising sea levels.

And by 2100, up to 410 million people could be at risk from coastal flooding as the warming climate expands the ocean, causing sea levels to rise even higher. Building and development in coastal areas, driving the expansion of coastal communities, also puts more people at risk.

Sea level is the measurement of the sea’s surface height. Between the 1800s and early 1990s, tide gauges attached to structures such as piers measured global sea level, as research organization the Smithsonian Institution explains. Now satellites carry out this task by bouncing radar signals off the ocean’s surface.

Because local weather conditions and other factors can affect sea level, measurements are taken globally and then averaged out.

In September, scientists discovered a sea-level "fingerprint" from the Greenland ice sheet, confirming fears about the extent to which ice is melting.

These fingerprints are "detectable patterns of sea level variability around the world resulting from changes in water storage on Earth’s continents and in the mass of ice sheets", according to NASA.

The scientists in Greenland used gravity data collected for more than 30 years by two satellites. Lead researcher Sophie Coulson then put the team's observations on changes in ice thickness into a computational model to create sea-level predictions for 1993 to 2019. She then compared these predictions to new satellite data, according to NBC.

“Scientists have a solid understanding of the physics of sea-level fingerprints, but we’ve never had a direct detection of the phenomenon until now,” says Isabella Velicogna, a professor of Earth system science at the University of California, Irvine.

The rapid melting of the Greenland ice sheet is the cause of 20% of current sea level rise, according to Woods Hole Oceanographic Institution.

Sea levels reached a record high in 2021, and NASA says sea levels are rising at unprecedented rates in the past 2,500 years.

The US space agency and other US government agencies warned this year that levels along the country’s coastlines could rise by another 25-30 centimetres (cm) by 2050. This 30-year increase would match the total sea level rise over the past 100 years.

The global sea level has risen by about 21cm since records began in 1880. While measuring in centimetres or even millimetres might seem small, these rises can have big consequences. This is particularly true where storm surges end up sweeping further inland than they would have done previously.

Two main factors cause sea level rise. These are melting ice from glaciers, and seawater expanding because of rises in global temperatures, explains the US National Oceanic and Atmospheric Administration.

NASA says melting ice from Antarctica and Greenland have caused about a third of the global average sea level rise since 1993.

Antarctica is losing about 150 billion tons of ice a year, while Greenland is losing about 280 billion tons a year.

There are also likely negative feedback loops that could speed up glacier ice melt, which were not previously accounted for, explains Elena Perez, Environmental Resilience Lead at the World Economic Forum. For example, the Thwaites Glacier in Antarctica is disintegrating more quickly than anticipated. It’s been nicknamed the ‘doomsday glacier’ because without it and its supporting ice shelves, sea levels could rise more than 3-10 feet.

Heat stored in the ocean is responsible for between a third and half of global sea level rise, NASA says. The last decade has been the ocean’s warmest since at least 1800, and ocean temperatures reached a new high in 2021.

Scientists link accelerated global warming over the past 200 years to growing levels of carbon dioxide in the atmosphere, the Smithsonian Institution explains.

Carbon dioxide is the main greenhouse gas produced from burning coal and other fossil fuels. Sea levels began rising at faster rates when people began burning coal in around 1850.

Sea levels are thought to have risen at about double the rate they would have without the increasing levels of greenhouse gases caused by human activity like burning fossil fuels, the Smithsonian says.

China, Bangladesh, India, Egypt, the Netherlands, the United States, Brazil, Australia, New Zealand and Pacific islands such as Tonga and Fiji are among the countries most at risk as sea levels rise, according to data content site Visual Capitalist.

In Egypt, about 95% of the population live along the River Nile, where some areas are located below sea level.

In the Netherlands, about half the population lives below sea level.

Tonga and other Pacific islands are at risk as ocean warming raises sea levels, Visual Capitalist says. According to Reuters, sea levels around Tonga are rising at almost twice the global average rate.

Many large cities are also at risk.

In the Chinese city of Shanghai, the number of people who live on land at risk of flooding from rising sea levels could almost double to 22.4 million if global warming rises from 2°C to 4°C, according to an interactive sea level tool from data visualization site EarthTime.

In its Global Risks Report 2019, the World Economic Forum identified three key strategies for adapting to rising sea levels. These are:

1. Engineering projects that keep water out.

2. Growing natural defences, like mangrove forests and wetlands, which help protect coasts from rising seas and erosion.

3. Moving homes and businesses to safer ground, away from flood-risk areas.

Countries and cities around the world are putting these strategies into action. In New Zealand, climate adaptation policies are being designed to ensure public housing is not built near areas prone to climate hazards.

Sea walls, surge barriers and other coastal defences are being built and strengthened in countries including Denmark, Germany and the United Kingdom. However, these solutions are not without their challenges, warns Perez. Such approaches can end up causing erosional issues, such as sea walls, where wave energy – rather than dissipating in the sand – rebounds off the hardened structure and scours sand from the beach, exacerbating erosion. And, if they’re built on coastal wetlands, this habitat destruction can release carbon dioxide into the atmosphere – therefore exacerbating climate change.

South Korea and the islands of the Maldives in the Indian Ocean are experimenting with floating homes, while countries including China and India are finding ways to absorb and store storm water for reuse.

In Fiji, government officials are making plans to relocate whole villages because of rising sea levels - 42 villages have been recommended for relocation in the next five to 10 years, while six have already been moved to safer ground, The Guardian reports. The plan, which is in the final stages of consultation and will soon go before Fiji’s cabinet for approval, will be no easy task. Churches, houses and even burial grounds will need to be transported across Fiji's mountainous terrain.

“These are questions that so many governments around the world are going to be asking in the next 10 years, 20 years, 50 years,” Erica Bower, an expert on planned relocations, told The Guardian.

In San Francisco in the United States, tidal marshes are being restored around the coast to help reduce the impact of erosion and storm surges. Perez cautions, though, about the risk of coastal squeeze. Sometimes coastal wetlands cannot move fast enough to outpace the rising sea, she says. And, sometimes they can’t move at all because they’re blocked by coastal development.