Climate change may subject nearly three-quarters of the world’s small islands - home to around 16 million people - to reduced fresh water availability by 2050, reports a paper published online this week in Nature Climate Change.
Changes in water balance - the difference between precipitation and evaporation - are expected in many regions as a result of climate change. However, islands smaller than the spatial resolution used in climate models, including French Polynesia, the Marshall Islands and the Lesser Antilles, have been difficult to assess. The standard method for assessing aridity is to calculate the ratio of precipitation to potential evapotranspiration (water loss by evaporation from surfaces and through plant leaves), but this does not work for small islands as global circulation models can only provide estimates of precipitation, not potential evapotranspiration.
To overcome this problem, Kristopher Karnauskas and colleagues used a method for estimating the potential evapotranspiration for small islands, to enable them to calculate an aridity change index (ACI) for 80 globally distributed island groups. They find a tendency towards increasing aridity at over 73% of the island groups. Although they find that about half of the island groups will experience increased rainfall (mostly those in the deep tropics), increases in evaporation were more consistent across the islands, resulting in a shift of global island freshwater balance towards greater aridity.