Scientists believe that global sea levels could rise far more than predicted, due to accelerating melting in Greenland and Antarctica.
The long-held view has been that the world’s seas would rise by a maximum of just under a metre by 2100.
This new study, based on expert opinions, projects that the real level may be around double that figure.
This could lead to the displacement of hundreds of millions of people, the authors say.
The question of sea-level rise was one of the most controversial issues raised by the Intergovernmental Panel on Climate Change (IPCC), when it published its fifth assessment report in 2013.
It said the continued warming of the planet, without major reductions in emissions, would see global waters rising by between 52cm and 98cm by 2100.