US coastal areas occupied by more than 13 million people will be at risk of being completely swamped by the sea under a worst-case climate change scenario, new research predicts, potentially leading to a population upheaval comparable to the Great Migration of the 20th century.

Population growth in coastal areas over the course of this century, particularly in vulnerable areas of Florida, is likely to collide with the reality of rising seas caused by melting glaciers and thermal expansion as the planet warms.

Research led by the University of Georgia has provided the first glimpse of how demographic changes in America will place greater numbers of people at the frontline of sea level rises. In a severe scenario involving a 6ft (1.8m) rise in sea levels by 2100, fuelled by the gradual collapse of the Antarctic ice sheets, a total of 13.1 million people would risk seeing their homes inundated.

A less dramatic sea level rise of 0.9m, still considered at the upper end of IPCC projections, would risk land occupied by 4.2 million people being claimed by the sea.

The projections are based on demographic changes in the US population over this century, as well as areas forecast by the National Oceanic and Atmospheric Administration as being at risk of inundation.

Florida would bear the brunt of this potential population displacement, with more than six million people affected by a 1.8m sea rise by 2100. In Miami alone, nearly two million people would either have to be protected or displaced under this worst-case scenario.

Other areas where significant numbers of people would be at risk include Long Island in New York; New Orleans; Charleston, South Carolina and San Mateo, California.

Should these people need to move away from the coast, rather than be protected by new sea defenses, the population movement would be on a scale similar to the Great Migration, a long-term shift of African Americans from southern to northern states during the 20th century.

“Sea level rise is widely recognized as one of the most likely and socially disruptive consequences of future climate change,” states the study, published in Nature Climate Change.

“Florida accounts for nearly half of the total at-risk population. Whereas other south-eastern states have substantially fewer people at risk, states such as Georgia, South Carolina and Louisiana have over 10% of future coastal populations at risk under the 1.8m scenario.

“The south-eastern US alone represents nearly 70% of the entire projected populations at risk, suggesting the impacts of sea level rise will be highly regionalised in nature.”

The paper warns that finding new land for displaced people will be “problematic” as well as expensive, citing relocations of Alaskan villages that have cost as much as an estimated $1m per resident.

“We knew that the number of affected people had been underestimated until now, but the extent of that underestimation surprised us,” said Mathew Hauer, coauthor and demographer at the University of Georgia.

“Adaption will be an option, such as deploying critical infrastructure, raising buildings and roads. If we don’t do anything, the migration will mimic the Great Migration from the south to the north, over similar timescales.

“At the moment, roads and hospitals are typically being built in areas without these future projections in mind. In the future, this will need to be a factor in where we deploy roads, hospitals, people and so on.”

The global sea level has risen, on average, by around 8in (20cm) since 1880, according to Nasa. This increase is uneven around the world, however, and predicting future sea level rise is complicated by a range of factors.

The rate of disintegration of the polar ice sheets is a key variable, as are changes to influences such as the Gulf Stream, which may be causing water to “pile up” along America’s east coast.

Sea levels are rising several times faster than they have in the past 2,800 years. German researchers recently warned that the rate of increase from thermal expansion – in which the ocean expands as it warms – has been underestimated.

Last month, a Climate Central study found that coastal flooding days have more than doubled in the US since the 1980s. Climate change driven by human activity has been blamed for three-quarters of the flooding events that have occurred over the past decade.