Sea level rise: Everything you need to know

This article was first published in September 2022 and updated in July 2024.

Homes, livelihoods and ultimately lives are under threat from rising sea levels.

By 2100, up to 410 million people could be at risk from coastal flooding as the climate crisis causes sea levels to rise even higher.

Here's what you need to know.

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 2021, 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 Greenland ice sheet is now losing around 30 million tonnes of ice an hour, reports the Guardian, and a recent study predicts its total collapse could happen by 2025.

With the ice sheet at "a tipping point of irreversible melting", scientists currently expect an unavoidable sea level rise of 1-2 metres.

Global sea levels have already risen by over 10cm between 1993 and 2024, according to NASA, which says sea levels have been rising at unprecedented rates over the past 2,500 years.

The US Space Agency and other US government agencies warned in 2022 that levels along the country’s coastlines could rise by another 25-30cm by 2050.

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 sweep further inland than they would have 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.

Greenland and Antarctica are losing around 270 billion and 150 billion tonnes of ice a year, respectively.

There are also likely negative feedback loops that could speed up glacier ice melt. For example, the Thwaites Glacier in Antarctica is disintegrating more quickly than anticipated. It’s nicknamed the ‘doomsday glacier’ because sea levels could rise more than three metres without it and its supporting ice shelves.

Heat stored in the ocean is responsible for between a third and half of global sea level rise, NASA says. The past decade has been the ocean’s warmest since at least 1800, and ocean temperatures reached a new high in 2023.

Bangladesh, China, India and the Netherlands were singled out by the United Nations last year as being at high risk from rising sea levels, with nearly 900 million people living in low-lying coastal areas in acute danger.

According to Reuters, sea levels around Tonga are rising at almost twice the global average rate.

In Europe, sea level rise is expected to go above 10cm "prior to 2050", says the European Environment Agency.

While recent research in the US has found that almost 1,100 critical buildings in coastal communities could be at risk of monthly flooding by 2050. Some communities could become unliveable within two to three decades, the report says.

In its Global Risks Report 2024, the World Economic Forum added the category 'Critical change to Earth systems' as one of the top two threats to the world in the coming decade - and sea level rise from collapsing ice sheets is identified as a key contributing factor.

Adaptation is vital, it says, but "efforts are falling short", with a finance gap currently estimated at $194 billion to $366 billion a year.

Countries and cities around the world are nevertheless putting 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 several countries including Denmark, Germany and the United Kingdom.

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

More drastic action is taking place in Fiji, where 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.