Many Low-Lying Atoll Islands Could Be Uninhabitable by Mid-21st Century

Published: 25 April 2018

Sea level rise and wave-driven flooding will negatively impact freshwater resources on many low-lying atoll islands in such a way that many could be uninhabitable in just a few decades. According to a new study published in Science Advances, scientists found that such flooding will not only impact terrestrial infrastructure and habitats but also, and more importantly, make the limited freshwater resources non-potable. This represents a threat to the sustainability of human populations.

Credit: Peter Swarzenski , U.S. Geological Survey

Most of the world’s atolls are in the Pacific and Indian oceans. The scientists focused on Roi-Namur Island on Kwajalein Atoll in the Republic of the Marshall Islands for their site study, which was conducted between November 2013 and May 2015. The Republic of the Marshall Islands has more than 1,100 low-lying islands on 29 atolls and it is home to numerous island nations and hundreds of thousands of people.

Variety of climate-change scenarios

Scientists from the U.S. Geological Survey, Deltares, National Oceanic and Atmospheric Administration, and the University of Hawaiʻi at Mānoa used a variety of climate-change scenarios to project the impact of sea level rise and wave-driven flooding on atoll infrastructure and freshwater availability. The approach and findings in this study can serve as a proxy for atolls around the world, most of which have a similar morphology and structure, with even lower land elevations on average. ‘The tipping point when potable groundwater on the majority of atoll islands will be unavailable is projected to be reached no later than the middle of the 21st century,’ said Curt Storlazzi, USGS geologist and lead author of the new report.

Impacts on freshwater

Sea levels are rising fastest in the tropics, where thousands of low-lying coral atoll islands are located. Previous studies on the resilience of these islands to sea level rise projected minimal inundation impacts until at least the end of the 21st century. However, those previous studies did not take into account the additional hazard of wave-driven overwash (storm waters and waves that wash up and over the low-lying island) nor its impact on freshwater availability. ‘The impact of the waves was not taken into account until now because we were unable to calculate accurately enough and the computing costs were too high. Today’s software allows us to make the calculations and the results are shocking: the critical point is reached decades before we thought it would be. So this problem will affect the generations living on the islands now unless we can combat climate change,’ says co-author Ap van Dongeren, who completed the wave modelling with colleagues from Deltares.

Annual flooding

These findings have relevance not only to populated atoll islands in the Marshall Islands but also to those in the Caroline Islands, Cook Islands, Gilbert Islands, Line Islands, Society Islands, Spratly Islands, Maldives, Seychelles, and Northwestern Hawaiian Islands. The scientists working on the study therefore project that, based on current global greenhouse gas emission rates, the interactions between sea level rise and wave dynamics over coral reefs will lead to an annual wave-driven overwash of most atoll islands by the mid-21st century. Such annual flooding would make the islands uninhabitable due to frequent damage to infrastructure and the inability of their freshwater resources to recover between overwash events.

Loss of freshwater

The primary source of freshwater for populated atoll islands is rain that soaks into the ground and remains there as a layer of fresh groundwater that floats on top of denser saltwater. With average annual overwashing of atoll islands in the next few decades (assuming current greenhouse gas emission rates), flooding impacts to infrastructure and the loss of freshwater resources would make human habitation difficult in most locations from the 2030s to the 2060s onwards, requiring the relocation of island inhabitants or significant financial investments in new infrastructure. ‘The overwash events generally result in salty ocean water seeping into the ground and contaminating the freshwater aquifer. Rainfall later in the year is not enough to flush out the saltwater and refresh the island’s water supply before the next year’s storms arrive, repeating the overwash events,’ explained Stephen Gingerich, USGS hydrologist and co-author of the new report.

Full report

The full report, “Most Atolls will be uninhabitable by the mid-21st century due to sea-level rise exacerbating wave-driven flooding,” in “Science Advances” is available online.