Some Scientists Predict These Islands Are Doomed, But That's Not the Whole Story
Low-lying, white-sand
islands lined with palm trees and perched on tropical coral atolls are the
stuff of vacation dreams. It's long been claimed that they will eventually
disappear as the sea level rises because of global warning, but when that might
happen has been unclear.
A study published Wednesday
(April 25) in the journal Science Advances suggests the islands could become
uninhabitable in as little as 40 years. However, other scientists vigorously
contest the study's conclusions.
The study is based on
an analysis of waves rolling up to a highly militarized island — that looks
nothing like a vacation fantasy — called Roi-Namur on Kwajalein Atoll, in the
Central Pacific's Marshall Islands. The research was funded largely by the U.S.
Defense Department.
Atolls are made of
tropical and subtropical coral that grew around the caldera of volcanoes as
their rim sank into the sea. Coral and marine animals with calcium skeletons,
ground up by the waves, eventually formed enough sand that waves pushing the
sand onto reefs created islands. These started appearing around 5,000 years
ago, and many were eventually colonized by Polynesians, Micronesians and
Melanesians.
Curt Storlazzi, the
paper's lead author, told Live Science that the biggest of these waves, thought
to have reached great enough heights to wash over atoll islands every two or
three decades in the past, will flood at least half of each island once a year when
the sea level has risen by about 3 feet (1 meter). This could occur by 2105,
according to some ice-melt scenarios modeled by the scientists, or as soon as
2055 under more pessimistic models involving ice-shelf collapse.
These calculations,
Storlazzi said, would apply to atoll islands across the globe, or about 25,000
islands.
"There's nothing
wrong with the waves washing over the islands per se," said Storlazzi, a
geologist who studies waves for the U.S. Geological Survey at the University of
California, Santa Cruz. "When it happens every 20 years, the communities
have the time to recover from the effects of flooding." Afterward, rain
washes away the salt that leached into the porous, sandy ground and refreshes
the lens of fresh water that lies a foot or two below the island's surface and
it floats above the seawater, he said. In other words, plants and people can
survive.
But at the rate of once
a year, Storlazzi said, the plants will die, the fresh water won't have time to
return and people won't be able to repair the flooding damage to roads and
homes — so they will simply leave.
Most atoll islands with
be fine, study’s critic says
Paul Kench, head of the
University of Auckland's School of Environment and a prolific author of studies
on atolls, said the new study's analysis of the wave dynamics at Roi-Namur
might apply to just half a dozen islands around the world — not to all of them.
"It's the waves
washing over the islands that brought them to their present
configuration," Kench, who wasn't involved in Storlazzi's research, said
in a phone interview from New Zealand. "As the sea level keeps rising, the
islands will rise too, and they will impede flooding events. So these are
unlikely to become as frequent as predicted in this paper."
The research, he added,
also ignores the responses of the atoll dwellers, who could build new
structures on stilts and harness foreign aid to acquire solar-powered
desalinators.
In February, Kench,
with Murray Ford and Susan Owen, published a paper in the journal Nature
Communications showing that the islands making up Tuvalu and their population
had fared just fine as the Central Pacific sea-level rose nearly 6 inches (15
centimeters) in the past half century and that such resilience could be
expected to continue. Another study by Kench and the same co-authors, this one
published in 2014 in the journal Geophysical Research Letters, found that Jabat
Island, in the Marshall Islands, emerged at a time when the sea was rising roughly
as fast as it is today. Overall, he said, he has studied the evolution of at
least 600 atolls, found that the vast majority have stayed the same or
naturally increased in size, and he expects most of them to stay pretty much
the same for the rest of this century.
In contrast to nearly
all other atoll islands, Roi-Namur was thoroughly bulldozed during and after
World War II for military purposes, Kench said. "The island has been so
reconfigured that it's lost the ability to receive sand and grow," he added.
Similar destruction has also compromised South Tarawa, the capital of Kiribati,
where 60,000 people are packed into 6 square miles (16 square kilometers) and
are highly vulnerable to flooding.
The study was based on
an examination of waves on Roi-Namur island (top), which hosts a U.S. military
base. But some scientists say its findings don’t apply to typical atolls, such
as the Atafu atoll in the Pacific Ocean (bottom).
The study was based on
an examination of waves on Roi-Namur island (top), which hosts a U.S. military
base. But some scientists say its findings don’t apply to typical atolls, such
as the Atafu atoll in the Pacific Ocean (bottom).
Credit: Google Maps
(top); NASA Earth Observatory (bottom)
Limits to "doomed
atoll" finding
Storlazzi, who insisted
his findings apply to atoll islands all over the world, doesn't dispute that
waves washing over a typical sand island will make it rise. But he explained
that for this study of Roi-Namur, the team assumed the island would not rise at
all.
Storlazzi explained
that the model didn't account for island rise because the margin of error for
such a prediction was too great. Plus, the growth "is only a tenth of the
overwash thickness, so there will always be more overwash during large-wave
events than the island can grow vertically to offset them," he said. It is
precisely these events that will make life impossible on these atolls, he
added.
In contrast, Kench and
other geomorphologists say the record shows that, as the sea rises, the waves
push up sand ridges on the beaches, thus preventing the rest of the island from
being flooded. In addition, the new study didn't account for the vertical
growth of the coral in the reef flats where the waves form. That means that if
the sea level rose by 3 feet, the amount of water on these flats will be that
much greater and the waves much bigger. However, coral does grow vertically in
these flats as the sea level rises. How fast it will continue to do so remains
unclear as hot-weather events kill more and more corals.
Kench added that the
study highlighted the problems of islands with human-made modifications like
seawalls, causeways and reclaimed land that have disrupted the natural
mechanisms that have allowed the lightly populated or pristine islands to
naturally adapt to sea-level rise.
Virginie Duvat, a
professor of coastal geography at the University of La Rochelle-CNRS, in
France, specializes in atolls. She agreed with Kench that all but the most
disfigured atoll islands seem to be adapting well to sea-level rise so far.
But that doesn't mean
the residents of these islands are guaranteed a bright future. "If we tip
into a world that is getting warm very fast, I think there are going to be all
kinds of combinations of phenomena that will interact in ways we can't begin to
predict," Duvat told Live Science.
"For instance, if
the corals start dying off en masseand can't recover, they might well keep
producing sand to feed the beaches for another century, but the amount of
accessible fish on the reef is going to crash, and people won't have enough to
eat," she said. "Or soil salinization might kill off the coconut
trees, which are the only source of cash for most people.
"You can't take
current processes and expect to see them continue for a century," she
added. "That's why I'm prudent."
source:
https://www.livescience.com/62423-are-atoll-islands-doomed-by-global-warming.html
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