In Carbon Brief, 25 April 2018
Low-lying coral islands across the tropical oceans could become “uninhabitable” in the coming decades because of the combined impacts of sea level rise and large waves, a new study suggests.
Earlier research, based on sea level rise alone, suggested this wouldn’t happen until at least the end of this century.
But regular inundation from coastal flooding could push islands beyond a “tipping point” where groundwater resources cannot recover from infiltration by salty seawater, leaving residents with no drinkable water.
The study focuses on one atoll of the Marshall Islands, but the warning also applies to other populated atolls in the Pacific and Indian oceans, the researchers say – including those in the Cook Islands, Maldives, Seychelles and Hawai’i.
However, the study may be giving an overly-pessimistic outlook, other scientists tell Carbon Brief. This is partly down to the sea level rise scenarios used, but also because many other atolls still support residents without usable groundwater.
For the world’s small island nations, the threat of rising seas looms large. Their long-term survival hinges on how effectively climate change is mitigated and sea level rise is curbed. It was no surprise, therefore, that small island states were determined to enshrine the 1.5C warming limit in the Paris Agreement.
But as the new Science Advances study explores, rising sea levels carry another menace – they allow high waves to reach further up – and potentially over – shorelines.
At particular risk are low-lying atolls – tiny tropical islands formed from coral reefs – which can be just a couple of metres above sea level.
The diagrams below illustrate how sea level rise and large waves together increase the risk of flooding, for current (top) and future (bottom) sea levels.
Higher sea levels allow larger waves to travel across the reef towards the shore. Larger waves means a bigger wave “run-up” – i.e. how high the swash of a wave reaches up the beach – and a greater risk of “overwash” of water over the top of the beach.
Lead author Dr Curt Storlazzi, a research geologist and oceanographer at the US Geological Survey, sums the process up in a pithy sentence:
“If sea level rises, there will be larger waves, greater run-up, more overwash, and coastal flooding.”
While coastal flooding might bring to mind destruction of homes, infrastructure and livelihoods, it also threatens freshwater resources, the study says. Salty seawater can infiltrate an island’s groundwater source, known as a “lens”, rendering the water undrinkable.
The study focuses on Roi-Namur island on the Kwajalein Atoll, which is one of the 1,100 low-lying islands and 29 atolls of the Marshall Islands in the Pacific Ocean.
The island makes an ideal case study for two reasons: first, it has recently been flooded; and second, it has a network of wells that allows scientists to monitor its groundwater resources.
During the flooding event in March 2014, waves of more than six metres coincided with spring high tides, causing seawater to overtop the beach in northern parts of the island and wash inland. Groundwater levels subsequently rose by almost one metre as salty water soaked into the ground, contaminating the freshwater.
The researchers used the data they collected from the event to validate their coastal and groundwater models. Confident that their models could accurately model current events, they then ran simulations for the future.
The study considers three pathways of future climate change: an intermediate scenario (“RCP4.5”), a business-as-usual scenario (“RCP8.5”), and a more extreme version of RCP8.5 that incorporates larger contribution of meltwater from the Antarctic ice sheet. This last scenario is a “high-end, low probably – but obviously high impact” scenario, says Storlazzi.
For the mid-century (2055), sea levels under the three scenarios are projected to rise by 0.3m, 0.8m and 0.9m, respectively. For the end of the century (2105), this increases to 0.7m, 2.0m and 2.9m, respectively.
The results suggest that sea level rise alone would not affect most of the island until the end of the century. However, when the impact of large waves is included, the island is likely to “experience more severe annual wave-driven flooding events”, the paper says.
The chart below illustrates this more clearly. It shows the percentage of Roi-Namur flooded every year as a result of sea level rise alone (solid lines) and the combined impact of rising seas and large waves (dashed lines) under the three climate change scenarios.
The findings suggest sea level rise and large waves will cause regular flooding by mid-century. The subsequent damage could hit a “tipping point” whereby freshwater supplies become too salty to drink and flooding occurs too frequently for it to recover. This would make the island uninhabitable “within the lifespan of current residents”, the study says – around 2030-40 under RCP8.5 and 2055-65 under RCP4.5.
Thus, the mix of rising seas and large waves means “the timeline for sustainable human habitation on these low-lying atoll islands shortens considerably”, the paper says.
By assessing the combined impact of sea level rise and waves, the study shows that “one plus one doesn’t equal two”, says Dr Kris Karnauskas from the University of Colorado at Boulder, who was not involved in the research, but led a recent study on how small islands are likely to become increasingly arid over this century. He tells Carbon Brief:
“Sea level rise and changes in wave characteristics play off each other such that annual inundation – and, therefore, when islands become uninhabitable – occur earlier than expected by those factors individually.”
However, as Prof Bob Kopp – director of the Institute of Earth, Ocean & Atmospheric Sciences at Rutgers University – points out, the sea level rise scenarios in the study weren’t originally intended to be used as forecasts.
The scenarios were developed for the Department of Defense by the US government’s Coastal Assessment Regional Scenario Working Group (CARSWG), Kopp explains in a response he posted online:
“These scenarios were intended as tools for risk management, not as forecasts of what would be likely under different emissions scenarios.”
As a result, the conclusion that the island could become uninhabitable in the 2030s is “overly pessimistic”, Kopp says. Nonetheless, even under 1.5C of warming, groundwater supplies on Roi-Namur and other islands in the region “will be rendered irrevocably contaminated within the lifetime of a 8-year-old child alive in the Marshall Islands today”, he warns.
Roi-Namur is one of the many thousands of atolls dotted across the world’s oceans, the researchers say – in fact, it is probably at a higher elevation than most.
In addition to the Marshall Islands, atolls in the Caroline Islands, Cook Islands, Kiribati, Line Islands, Maldives, Seychelles, Society Islands, Spratly Islands, northwestern Hawai’i could all face similar risks. Storlazzi explains to Carbon Brief:
“Many of these atoll islands may be impacted by wave-driven flooding within the next few decades, such that they will have their infrastructure, agriculture and habitats negatively impacted and will not have potable [drinkable] groundwater.”
The exact scale of these risks for other atolls would need further research, notes Karnauskas, but the initial findings are a cause for concern:
“I don’t see any reason why leaders and inhabitants of other, far-away island nations shouldn’t be looking at this result with some concern right now.”
However, Prof Paul Kench at the University of Auckland argues that Roi-Namur is less characteristic of other atolls. Kench, who was not involved in the study, is a coastal geomorphologist who recently published a study on how rising seas can allow large waves to add sediment to atoll shorelines and increase their land area.
He notes that Roi-Namur has been substantially modified. The Japanese constructed a causeway between the two parts of the island during their second world war occupation and today it is a test site for a US ballistic missile defence system.
Consequently, “it is representative of a very limited number of atoll islands,” he tells Carbon Brief.
And although “an increase in frequency and magnitude of flooding in coastal systems is now widely expected with increased sea level”, there are many reasons that affect whether an atoll becomes uninhabitable, Kench says:
“Many atolls islands do not have potable groundwater lenses or such lenses are polluted. However, they still host populations. Malé in the Maldives is a classic example. Potable water can be harnessed via rainwater harvesting. Consequently, island communities have different ways to provision themselves with freshwater – which allows islands to remain inhabited.”
This is a point picked up in the study, too. For example, islands that do not have to rely on groundwater exclusively – because of rainfall capture, desalination or water imports – will be more resilient to flooding, it says.
Similarly, islands that are not low-lying will likely not see their freshwater resources threatened, the paper notes. This is because these islands receive enough rainfall in their mountains that would be unaffected by coastal flooding. That said, infrastructure on these islands is still vulnerable to damage from inundation.
Update on 25/04/2018: This article was updated to add comments from Bob Kopp
Low-lying atolls around the world will be overtaken by sea-level rises within a few decades, according to a new study. By Matthew Taylor in The Guardian, Wed 25 Apr 2018 14.00 EDT
Hundreds of thousands of people will be forced from their homes on low-lying islands in the next few decades by sea-level rises and the contamination of fresh drinking water sources, scientists have warned.
A study by researchers at the US Geological Survey (USGS), the Deltares Institutein the Netherlands and Hawaii University has found that many small islands in the Pacific and Indian Oceans will be uninhabitable for humans by the middle of this century. That is much earlier than previously thought.
Experts say the findings underline the looming climate change driven migration crisis that is predicted to see hundreds of millions of people forced from their homes in the coming years.
More than half a million people around the world live on atoll islands, often extraordinary and beautiful structures based on coral reefs. Their closeness to sea level makes them particularly vulnerable to climate change.
The increase in urgency comes because, according to the authors of the report, previous studies had been based only on predicted sea-level rises. Today’s studyalso examined the impact of wave driven flooding on the availability of fresh water.
Saltwater flooding, leading to the contamination of water sources, will see most atolls become uninhabitable sometime between 2030 and 2060, said Curt Storlazzi, USGS geologist and lead author of the new report. “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.”
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, according to the study. But as atoll islands are regularly swamped by salt water this freshwater will be contaminated making “human habitation difficult in most locations beginning between the 2030s to 2060s, 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 event,” explained Stephen Gingerich, USGS hydrologist and co-author of the new report.
The authors say although the study was focussed on Roi-Namur Island on Kwajalein atoll in the Marshall Islands, the results will apply to people living in atolls around the world – including the Caroline Islands, Cook Islands, Gilbert Islands, Line Islands, Society Islands, Spratly Islands, Maldives, Seychelles, and north-western Hawaiian Islands.
But the fate of people living on atolls will be just one part of the much wider climate migration crisis, said Dina Ionesco, head of migration, environment and climate change at the International Organisation for Migration. “Millions of people are going to be at risk from extreme heat, extreme water shortages and flooding as well as sea level rises… we are talking about something that is going to play a huge role in the years ahead in terms of forcing people to leave their homes.”