That romantic hike through a lush tropical island may be an experience for today's couples instead of their kids or grandkids. Climate researchers say that small islands in the Caribbean, Pacific and Atlantic will be drying out as the world's temperatures rise and rainfall patterns shift toward the middle and end of this century.
Some small islands will become wetter, but the majority -- 73 percent -- will become drier. That means less freshwater for local residents, agricultural products that sustain the islands' economies, and vegetation and wildlife that depend on the island's unique ecosystems.
"It's going to be harder to grow stuff because there's not going to be enough water," said Kristopher Karnauskas, assistant professor of atmospheric and oceanic sciences at the University of Colorado Boulder.
"The small island doesn't have a large catchment area," he said. "Unless they are really well developed, they are relying on rainfall."
Karnasuskas and colleagues at the Woods Hole Oceanographic Institution and University of Arizona published their findings today in the journal Nature Climate Change. Their study is a projection of what might happen if current trends in global temperature and rainfall continue.
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.
The Hawaiian Islands are a mixed bag, according to the report's calculations, which call for a slight increase in arid conditions by 2050, followed by slightly wetter conditions from 2050 to 2090.
"Tropical rainfall might just expand northward enough to bring moisture to Hawaii," he said.
However some experts say the rainfall patterns across the world's oceans will be difficult to predict as the world warms.
"The elephant in the room here is the change in precipitation, specifically, the uncertainty in the direction of change," said Alessandra Giannini, a climate researcher at Columbia University in an e-mail to Discovery News.
"The authors correctly point out that there is some coherence in the large-scale pattern, with deep tropical islands trending towards wet, and sub-tropical islands such as Easter Island trending towards dry. But the largest uncertainty in projections remains model disagreement in how physical processes responsible for precipitation are represented, and how they may be changing as the planet warms."
Giannini studies climate change and drought in the Caribbean basin. Previous climate studies predicted that the entire region, including Cuba, Jamaica, Hispaniola (Haiti/Dominican Republic) and Puerto Rico, will become drier.
The region's poverty will make it harder for island residents to adapt by finding new sources of water, according to Marisa Escobar, a senior scientist at the Stockholm Environment Institute.
"It will be more difficult for islands," Escobar told Discovery News. "On continental settings, you could create infrastructure that transports water from one place to another. But some small island have very little in terms of watersheds and reservoirs."