“Limiting global warming to 1.5°C would be a major achievement” and avoid many dire climate impacts, lead author Chris Stokes, a professor at Durham University in England, told AFP. “But even if this target is met,” he added, “sea level rise is likely to accelerate to rates that are very difficult to adapt to. “Absent protective measures such as sea walls, an additional 20cm of sea level rise by 2050 would cause some $1 trillion in flood damage annually in the world’s 136 largest coastal cities, earlier research has shown. Some 230mn people live on land within 1m (3.2ft) of sea level, and more than a billion reside within 10m.
Sea level rise is driven in roughly equal measure by the disintegration of ice sheets and mountain glaciers, as well as the expansion of warming oceans, which absorb more than 90% of the excess heat due to climate change. Averaged across 20 years, Earth’s surface temperature is currently 1.2°C above pre-industrial levels, already enough to lift the ocean watermark by several metres over the coming centuries, Stokes and colleagues noted in the journal Communications Earth & Environment. The world is on track to see temperatures rise 2.7°C above that benchmark by the end of the century.
In a review of scientific literature since the last major climate assessment by the UN-mandated Intergovernmental Panel on Climate Change (IPCC), Stokes and his team focused on the growing contribution of ice sheets to rising seas. In 2021, the IPCC projected “likely” sea level rise of 40 to 80cm by 2100, depending on how quickly humanity draws down greenhouse gas emissions, but left ice sheets out of their calculations due to uncertainty. The picture has become alarmingly more clear since then.
“We are probably heading for the higher numbers within that range, possibly higher,” said Stokes. The scientist and his team looked at three baskets of evidence, starting with what has been observed and measured to date. Satellite data has revealed that ice sheets with enough frozen water to lift oceans some 65m are far more sensitive to climate change than previously suspected. The amount of ice melting or breaking off into the ocean from Greenland and West Antarctica, now averaging about 400bn tonnes a year, has quadrupled over the last three decades, eclipsing runoff from mountain glaciers.
Estimates of how much global warming it would take to push dwindling ice sheets past a point of no return, known as tipping points, have also shifted. “We used to think that Greenland wouldn’t do anything until the world warmed 3°C,” said Stokes. “Now the consensus for tipping points for Greenland and West Antarctica is about 1.5°C.” The 2015 Paris climate treaty calls for capping global warming at “well below” 2°C, and 1.5°C if possible. “If you want to slow sea level rise from ice sheets, you clearly have to cool back from present-day temperatures. To slow sea level rise from ice sheets to a manageable level requires a long-term temperature goal that is close to +1°C, or possibly lower,” Stokes added.
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