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The world must stick to its target to limit climate warming to 1.5°C to avoid catastrophic melting of ice sheets and glaciers, according to a report.

The International Cryosphere Climate Initiative (ICCI), a group of scientists who study ice-covered parts of the world, warns that a rise of 2°C would liquidate most tropical and mid-latitude glaciers and set off long-term melting of the Greenland and Antarctic ice sheets, leading to 12 to 20 metres of sea level rise.

In the 2015 Paris Agreement, all countries committed to holding global average temperature to “well below 2°C” over pre-industrial levels and “pursuing efforts” to limit it to 1.5°C. Our still-rising greenhouse gas emissions have already caused almost 1.2°C of warming and put us on track to exceed 3°C.

More than 350 cryosphere scientists have an open letter calling on countries to commit to the 1.5°C limit at the upcoming COP28 climate summit in Dubai.

“From the cryosphere point of view, 1.5°C is not simply preferable to 2°C or higher. It is the only option,” Iceland’s prime minister Katrín Jakobsdóttir said in a statement.

Earth’s regions of snow and ice are melting faster than we expected and already approaching tipping points, says at the University of Bristol, UK, who reviewed the ICCI report.

“We need to put the brakes on, big time,” says Bamber. “Otherwise, we’re going to see irreversible changes in the polar regions that are going to have global consequences.”

In the past two years, Antarctic sea ice has hit back-to-back record lows, Swiss glaciers have lost 10 per cent of their volume and a winter heatwave melted snow up to 3000 metres high in the Andes.

But 2°C of warming would be much worse, the report warns. The Arctic Ocean would be ice-free almost every summer. Annual carbon emissions from thawing permafrost soils would equal those of the European Union today. And absorption of atmospheric CO2 would permanently acidify polar seas and threaten krill, salmon and king crab.

The Himalayas would lose half their ice, disrupting water supplies for agriculture and hydropower and raising the threat of floods caused by glacial meltwater breaking through a barrier of ice or rock. One such flood at least 179 people in Sikkim, India, in October. A this year found that 15 million people are at risk from sudden glacial floods, mostly in India, Pakistan, Peru and China.

“The lakes will start to get larger and larger,” says at the International Centre for Integrated Mountain Development in Nepal, whose hometown of Namche Bazaar, Nepal, was damaged by an outburst flood in 1985. “They’ll be more and more hazardous, and once they get to a point, something can just trigger them, like a landslide.”

Keeping to 1.5°C now requires the world to reach net zero emissions by 2034. Some scientists have argued 1.5°C is dead, while others point to the rapid uptake of solar and wind energy as reason for continued hope.

“It could be that [over 1.5°C] is where we end up,” says at the University of Colorado Boulder, who helped organise the scientists’ letter. “But I think talking ourselves out of rapid change now is selling ourselves short on what is possible because [of] cultural tipping points, social tipping points.”

And even above 1.5°C, “every tenth of the degree counts,” says Bamber.