This is the worst-case scenario envisaged by scientists in the latest report produced by the Ipcc. The ice sheets in Greenland and Antarctica are recording rapidly increasing melt rates, and are estimated to have already raised the global sea level by 1.8 cm since the 1990s. The study, published in Nature Climate Change and entitled "Ice-sheet losses track high-end sea-level rise projections", compared the results of ice-sheet mass balance obtained from satellite observations with projections from climate models. The result? The ice sheets are projected to raise sea levels by a further 17 cm and expose an additional 16 million people to annual coastal flooding by the end of the century.

"Although we expected the ice sheets to lose increasing amounts of ice in response to the warming of the oceans and atmosphere, the rate at which they are melting has increased more rapidly than we could have imagined," said Dr Tom Slater, lead author of the study and climate researcher at the Centre for Polar Observation and Modelling (Cpom) at the University of Leeds, in a statement.

The alarm: sea levels are rising too rapidly

What is triggering the alarm is not only large-scale events, such as those at Pine Island and Thwaites, but also systematic satellite monitoring, which began in the early 1990s: over thirty years, Greenland and Antarctica together have lost 6,400 billion tonnes of ice, pushing global sea levels up by as much as 17.8 millimetres (7.2 mm from Antarctica and 10.6 mm from Greenland, to be precise). "The melting is outpacing the climate models we use and we risk being unprepared for the risks posed by sea level rise," Slater added. "Satellites are our only means of routinely monitoring these vast remote areas, so they are absolutely vital in providing measurements that we can use to validate ice-sheet models."

Until now, sea levels have risen largely through a mechanism of thermal expansion, meaning that the volume of seawater expands as it warms, the university explains. But over the past five years, the melting of ice from ice sheets and mountain glaciers has overtaken global warming as the primary cause of sea level rise.

"It is not only Antarctica and Greenland that are causing the water to rise. In recent years, thousands of smaller glaciers have begun to melt or disappear entirely, as we saw with the Ok glacier in Iceland, which was declared 'dead' in 2014," concluded Dr Ruth Mottram, co-author of the study and researcher at the Danish Meteorological Institute. "This means that ice melt is now the primary cause of sea level rise."