Even if the world stopped emitting greenhouse gases tomorrow, ocean levels would continue to rise. Not only is dangerous sea level rise "absolutely guaranteed", but it will keep rising for centuries or millennia even if the world stopped emitting greenhouse gases tomorrow, experts say.
Rising seas are one of the most severe consequences of a heating climate that are already being felt. Since the 1880s, mean sea level globally has already risen by 16cm to 21cm (6-8in). Half of that rise has happened over the past three decades. It is accelerating, too: the ocean rose more than twice as fast (4.62mm a year) in the most recent decade (2013-22) than it did in 1993-2002, the first decade of satellite measurements, when the rate was 2.77mm a year. Last year was a new high, according to the World Meteorological Organization. It is no coincidence that the past eight years were the warmest on record.
The numbers might seem small. Even 4.62mm is just half a centimetre a year. So why did the UN secretary general, António Guterres, warn in February that the increase in the pace of sea level rise threatens a "mass exodus" of entire populations on a biblical scale?
Part of the problem is the that even if the world stopped emitting greenhouse gases immediately - which it will not - sea levels would continue to rise. Even in the best-case scenario, it's too late to hold back the ocean.
The ocean rose twice as fast between 2013-22 as it did between 1993-2002. Photograph: Nasa
The reason for this is not widely known, outside the science community, but is crucial. The systems causing sea level rise - specifically, the thermal expansion of the ocean and the melting of glaciers and ice sheets due to global heating - have a centuries-long time lag.
To stop the acceleration of sea level rise over the past century, Bamber says, we would have to go back to pre-industrial temperatures.
But under any temperature rise scenario, countries from Bangladesh to China, India and the Netherlands, all with large coastal populations, will be at risk. Megacities on every continent will face serious impacts, including Lagos, Bangkok, Mumbai, Shanghai, London, Buenos Aires and New York.
The climate crisis has many other hazards, of course: blistering heatwaves, droughts, floods and more extreme weather events. But there is a certain apocalyptic inevitability to a rising ocean.
So far, the ocean has acted as a buffer against global heating. About 90% of the energy trapped in the climate system by greenhouse gases goes into the ocean as heat - keeping the planet cooler than it otherwise would be, but threatening marine life. Even though the world has been experiencing a cooler period over the past few years (known as La Niña conditions), more than half - 58% - of the ocean surface last year experienced at least one marine heatwave.
But heat is just one factor in the rising sea. Thermal expansion explained about 50% of sea level rise during 1971-2018 - the other components are glacier melt (22%), ice-sheet melt (20%) and changes in land water storage.
The impact is hard to gauge because the ocean does not rise at the same speed uniformly, it's not like a bath. For one thing, Earth is not a perfect sphere; temperatures are also different across the planet, and are affected by ocean currents. The impact of sea level rise are boosted by storm surges and tidal variation, as happened during Hurricane Sandy in New York and Cyclone Idai in Mozambique.
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What we do know, of course, is that the first impact of rising seas will be on coastal communities worldwide, especially densely populated, low-lying urban areas. Major cities on all continents are at risk and it is an existential threat for countries such as Tuvalu and other small island developing states.
Making predictions over populations at risk, however, is not straightforward either. While the Netherlands, which has one of the lowest elevations in the world, is at risk from sea level rise, it also has gone to great lengths to build defences to protect itself.
