With global average sea levels rising faster than during any preceding century in the past 3,000 years, a mass movement of entire populations on a "biblical scale" could ensue, UN Secretary General Antonio Guterres warned on Tuesday.
Citing World Meteorological Organisation data, Mr Guterres said even if global heating was “miraculously” limited to 1.5°C above pre-industrial levels, there would still be a sizeable sea level rise.
He said mega cities on every continent would face serious effects. These included Cairo, Bangkok, Dhaka, Mumbai, Copenhagen, London, New York, and Buenos Aires.
“Rising seas are sinking futures," Mr Guterres said at a debate aimed at highlighting the risks to international peace and security posed by the phenomenon. "Sea-level rise is not only a threat in itself. It is a threat-multiplier.
"Low-lying communities and entire countries could disappear forever," he said. "We would witness a mass exodus of entire populations on a biblical scale."
He said Earth's oceans had warmed faster over the past century than at any time in the previous 11,000 years.
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The UN chief said the danger was especially acute for nearly 900 million people who live in coastal zones at low elevations, and triple the average rate of sea-level rise had already occurred on some coastlines.
Nasa has said Antarctica is losing an average of 150 billion tonnes of ice annually, and the Greenland ice cap is melting even faster, losing 270 billion tonnes a year.
Mr Guterres also underlined the need to address the effects of rising seas across legal and human rights frameworks.
“Rising sea levels are literally shrinking land masses, a cause of possible disputes related to territorial integrity and maritime spaces,” he said.
The current legal regime must look to the future and address any gaps in existing frameworks, he added.
The US ambassador to the UN, Linda Thomas-Greenfield, who called the threats of rising seas a “matter of international peace”, urged the Security Council to consider how the problem could complicate peacekeeping mandates, especially near coastlines.
“For our part, the United States is responding to these challenges by working with communities vulnerable to sea level rise at home and engaging vulnerable states abroad," she said.
"The Biden administration has stepped up efforts to reduce greenhouse gas emissions, including through the historic Inflation Reduction Act, which puts us on track to meet our commitments under the Paris Agreement."
Majid Al Suwaidi, the UAE's director general and special representative of Cop28, echoed Ms Thomas-Greenfield's call and recognised the urgent need for a co-ordinated response across the multilateral system.
“Our obligation with each body contributing within its mandate is to reverse the current trajectories with inclusive, effective and responsive climate action,” he said.
He also urged the world body to address "unprecedented" legal and policy challenges which sea-level rise has brought to the fore and prevent insecurity.
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