In the 2013 report by the International Panel on Climate Change (IPCC), scientists used global climate models to predict shifts in rainfall and temperature over the world's continents. But the land mass of many small islands is too small for these models to come up with an accurate prediction.
Karnauskas substituted the known evaporation rates over land to fill in the blanks in in the climate equation for these islands, which have a total population of 18 million people.
"If you could magically transport your self into a climate model to where you ought to find an island in French Polynesia, there's only open ocean," he said. "We pretend that there is a land surface and use principles of how evaporation works and calculate aridity."
By 2090, the calculations show that islands with a population of about 9 million people will become 20 percent dryer, while another 6 million will experience 40 to 60 percent less fresh water.
Karnauskas predicts the hardest hit will be the islands of the Lesser Antilles (from the U.S. Virgin Islands south and west to Aruba), the Azores and Canary Islands off Spain and several remote islands in the South Pacific (French Polynesia, Easter Island and Robinson Crusoe Island) and the Marshall Islands.