This story was originally published by The Guardian and appears here as part of the Climate Desk collaboration.

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 is already being felt.

Since the 1880s, the mean sea level globally has already risen by 16 centimetres to 21 centimetres. Half of that rise has happened over the past three decades.

It is accelerating, too: the ocean rose more than twice as fast (4.62 millimetres 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 2.77 millimetres 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 4.62 millimetres 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?

A centuries-long time lag

Part of the problem is 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.

‘It’s absolutely guaranteed’: The best and worst case scenarios for #SeaLevelRise. #ClimateChange #ClimateCrisis #RisingSeas #GHG #GlobalHeating

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.

“The atmosphere changes quite rapidly, but deep ocean circulation takes centuries,” says Prof. Jonathan Bamber, director of the Bristol Glaciology Centre at the University of Bristol.

“As the heat sinks into the deep ocean, it takes centuries to be moved around and for a new equilibrium to be reached. Ice sheets also have a response time, so if you change the thermometer tomorrow, it can take hundreds to thousands of years to reach an equilibrium.

“Taken together, you’re talking multiple centuries to reach an equilibrium with the new temperature we’ve established.”

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 heat waves, droughts, floods and more extreme weather events. But there is a certain apocalyptic inevitability to a rising ocean.

“The thing about sea level rise is that it is absolutely guaranteed,” Bamber says. “If you warm the planet, sea level is going to go up, period, no caveats. The oceans warm up and the ice melts. It’s an absolute given of global heating.”

So far, the ocean has acted as a buffer against global heating. About 90 per cent 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 per cent — of the ocean surface last year experienced at least one marine heat wave.

But heat is just one factor in the rising sea. Thermal expansion explained about 50 per cent of sea level rise from 1971 to 2018 — the other components are glacier melt (22 per cent), ice-sheet melt (20 per cent) 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 is boosted by storm surges and tidal variation, as happened during hurricane Sandy in New York and cyclone Idai in Mozambique.

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 about 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.

“A headline that says, for example, more than a quarter of the Netherlands will be underwater by 2100 — that sounds very dramatic,” says Prof. Gerd Masselink, an expert in coastal geomorphology at the University of Plymouth. “But at the moment, people in the Netherlands are walking around and riding their bikes below sea level. There are coastal defence structures in place. And if you say 200 million people are going to be affected by rising sea level: well, anyone who lives on the coast is affected in some way, but it doesn’t actually mean that they’re going to lose their house right away.”

Best and worst case scenarios by 2100

So how bad could things get? Again, it’s hard to predict exactly, but the IPCC has tried its hand at modelling different scenarios for how high sea levels will rise by 2100, based on how well humanity succeeds in mitigating the climate crisis. Each scenario is the result of complex calculations (“shared socioeconomic pathways”, or SSPs, which get expressed as a number) that take into account likely emissions, but also consider potential socioeconomic changes such as population, urban density, education, land use and wealth — which also affect fossil fuel use.

  • Most optimistic: 1.5 C heating = 28-55cm sea level rise
    If the world shifts towards a more sustainable future, sticks to development targets and meets the Paris climate goal of keeping global heating to 1.5 C by 2050, in the near term — ie., over the next century — the likely global mean sea level is predicted to rise by 0.28-0.55m. The IPCC calls this its “sustainability” scenario (an SSP rating of 1-1.9).

  • Middle of the road: 1.8C = 32-62cm
    Assuming socioeconomic and technology trends do not shift markedly, inequality persists but we can meet a “low” emissions target (SSP 1-2.6) that keeps global heating to within 1.8 C by the end of the century, sea levels are predicted to rise 0.32-0.62m.

  • Regional rivalry: 2.7C = 44-76cm
    An intermediate greenhouse gas emission scenario (SSP 2-4.5) where net zero is not reached by 2100, this scenario assumes a resurgent nationalism that makes environmental concerns a low international priority as states seek development over sustainability.

  • Inequality: 3.6C = 55-90cm
    A very high emission scenario where CO2 doubles from current levels until 2100 (SSP3-7) because of highly unequal investment in human capital, increasing inequalities across and within countries, and more investment in coal.

  • Fossil fuel development: 4.4C = 63-101cm+
    Social and economic development is coupled with exploitation of fossil fuel resources to run energy-intensive lifestyles around the world. The IPCC also warns of a “low-likelihood high impact” scenario (SSP5-8.5) in which ice sheet instability drives sea levels above two metres by the end of this century alone.

“The worst case we’re looking at is something like more than two metres in a century,” Bamber says.

“To put that in context, two metres of sea level rise would displace, or would affect or flood on an annual basis, approximately a 10th of the planet’s population, so about 790 million people.” (In 2020, 896 million people lived within the “low elevation coastal zone — a figure probably rising to one billion people by 2050.)

The end of the beginning

In all of these cases, it’s important to remember that sea levels won’t stop there — they’ll keep rising long beyond 2100.

These estimates are just the IPCC’s immediate threat assessment for where the world might be by the end of this century.

What’s more, the different emission scenarios actually have a relatively small impact on sea level rise in the short term — but they begin to diverge dramatically in the longer term. Over the next 200 years, global mean sea level will rise by about two to three metres if warming is limited to 1.5 C, but it could double to two to six metres if the warming is limited to even a slightly higher figure of 2 C.

At sustained warming levels of 2 C to 3 C, the Greenland and West Antarctic ice sheets will be irreversibly gone. The collapse of major Antarctic ice shelves at the end of the century, followed by increased discharge of ice, could lead to catastrophic sea level rise by 2300 of nine to 15 metres under strong warming. And if global heating advances to 5 C, the planet could expect 19 to 22 metres of sea level rise, wiping out entire cities and countries by the year 2300.

Masselink says he is struck by the timescale of global heating impact — the delay between our actions now and future repercussions.

“What I’ve always found remarkable is that, while the difference between ‘no more greenhouse gas emissions’ and ‘keep burning’ is significant, it is not going to make that big an impact in the next few decades,” he says.

“[Where] it’s going to make a huge difference [is] not for us, but for our children’s children. That’s the difficult thing to get your head around.”