A new climate change study offers possibly the grimmest outlook yet for U.S. coastal residents.

Up to 13.1 million Americans could be forced from their homes by the end of the century because of global sea level rise, warns a paper published in the journal Nature Climate Change this week. That's a three-fold increase over previous projections.

And if protective measures are not taken, researchers said the mass departure from impacted areas could rival that of the Great Migration of more than 6 million African Americans from the rural South during the 20th century.