‘It’s absolutely guaranteed’: the best and worst case scenarios for sea level rise
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 started slowly but is surely picking up speed: the ocean, long considered so big it couldn't be affected by humanity, is creeping toward us. As the water warms and the ice sheets melt, sea level has risen more than half a foot in the last century and even if we stopped producing greenhouse gases today, a very unlikely scenario, it will only keep coming. With 40% of the Earth's population living in coastal zones, the UN has warned that the accelerating pace of sea level rise threatens a "mass exodus". What we do now matters. While stopping carbon emissions is the most important collective task, individuals around the world are fighting back against the rising waters in their own ingenious ways. As our islands vanish, our shorelines shrink and our cities flood, Guardian Seascape is telling the troubling but inspiring stories of how humanity is facing down The Rising Ocean . Chris Michael, editor 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, Antonio 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, its too late to hold back the ocean. 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 that if you change the thermometer tomorrow, it can take hundreds to thousands of years to reach an equilibrium. Taken together, youre talking multiple centuries to reach an equilibrium with the new temperature weve 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 heatwaves, 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. Its an absolute given of global heating. 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 Nina 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, its 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. 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. 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 doesnt actually mean that theyre going to lose their house right away. So how bad could things get? Again, its 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 1.5C 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: 1.8 C = 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 1.8C by the end of the century, sea levels are predicted to rise 0.320.62m. Regional rivalry: 2.7 C = 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: 3.6 C = 55-90cm A very high emission scenario where CO 2 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 duel development: 4.4 C = 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 2 metres by the end of this century alone. The worst case were looking at is something like more than 2 metres in a century, Bamber says. To put that in context, 2 metres of sea level rise would displace, or would affect or flood on an annual basis, approximately a 10th of the planets population, so about 790 million people. (In 2020, 896 million people lived within the low elevation coastal zone a figure probably rising to 1 billion people by 2050.) In all of these cases, its important to remember that sea levels wont stop there theyll keep rising long beyond 2100. These estimates are just the IPCCs immediate threat assessment for where the world might be by the end of this century. Whats more, the different emission scenarios actually have a relatively small impact over 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 2-3 metres if warming is limited to 1.5C, but it could double to 2-6 metres if the warming is limited to even a slightly higher figure of 2C. At sustained warming levels of 2-3C, 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 9-15 metres, under strong warming. And if global heating advances to 5C, the planet could expect 19-22 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 Ive 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] its going to make a huge difference [is] not for us, but for our childrens children. Thats the difficult thing to get your head around.