Despite consistently rising sea levels, most of the island’s coastline has not modified since the Nineteen Sixties.
With global sea levels projected to rise by 44 centimeters (17 inches) by the end of the century, atolls resembling Aldabra – United Nations Educational, Scientific and Cultural Organization (UNESCO) inscribed on the UNESCO World Heritage List in the Seychelles and residential to the world’s largest population of giant tortoises – could also be in danger of drowning in the ocean.
New testnonetheless, it shows that despite consistently rising sea levels, most of Aldabra’s coastline has not modified since the Nineteen Sixties.
Despite climate change, the coastline of Aldabra Atoll is (mostly) stable
An atoll is formed when corals attach themselves to the shores of a volcanic island or platform in the ocean. Over time, the volcano erodes and falls into the sea, leaving a ring-shaped reef. Winds and waves deposit crushed coral from the surrounding reefs on top of the ring, eventually creating islands that rise above sea level. The central lagoon of Aldabra is surrounded by 4 large islands: Grand Terre, Malabar, Picard and Polymnie.
Most of the world’s coral atolls are positioned in the Pacific and Indian Oceans. Famous atolls include the Maldives, a preferred tourist destination, and Bikini, which the United States used as a nuclear testing site until the Fifties. Aldabra has the distinction of being one of the highest atolls, with a mean height of 8 meters (26 ft). For comparison, in the Maldives the average height is 1 meter (3.3 ft).
Their low elevation makes the atolls especially special sensitive to rising sea levels.
“Atolls are dynamic and have adapted over hundreds or even thousands of years to grow vertically with changing sea levels,” he added. Annabella Constanceconservation and space ecologist at the Seychelles Islands Foundation and first writer of the study, wrote in an email to . “However, this balance depends on the availability of loose sediment from nearby coral reefs.”
As reefs erode in increasingly hotter and more acidic oceans, sediment becomes scarcer, making it harder for atolls to maintain up with rising sea levels.
In a brand new study, scientists from the University of Zurich and the Seychelles Islands Foundation tracked changes in Aldabra’s coastlines using aerial and satellite imagery from 1960 to 2011. The results were surprising.
Constance noted that “61% of the coastline remained unchanged, while the areas that changed changed by an average of about 25 centimeters per year, which is less than the global average for atolls.” This rate of change includes each erosion and accretion: sections of the coastline that were eroded lost a mean of 25 centimeters (10 in) per yr, while accretionary areas gained land at the same rate.
The authors also noticed local differences. Aldabra’s outer coastline, bordering the open ocean, has modified by a mean of 15 centimeters (6 in) per yr. The inner shore, surrounding the atoll’s central lagoon, was changing at a much greater rate of 32 centimeters (about 13 inches) per yr.
Support against rising sea levels
It has been explained that Aldabra’s extraordinary resilience is probably going because of its geological history Paweł Kencha coastal geomorphologist from the National University of Singapore who was not involved in the study. “The key difference is that much of the coastline has been above sea level for the last 125,000 years and has become lithified (cemented),” Kench wrote in an email to .
This historical cementation explains why the inner and outer shorelines of the atoll are changing at different rates: much of the ocean-facing outer shore consists of solid limestone ridges, while the inner shore comprises more loose sediments which have not had time to lithify . “This makes the ocean coastline more resilient to the effects of sea level change,” Kench wrote.
“We must protect these unique ecosystems to ensure their continued resilience.”
The lack of human habitation is probably going one other factor affecting Aldabra’s ability to remain above water. Mangrove trees on the atoll were harvested until Aldabra became a protected site in the Nineteen Seventies. Since then, the coastline of one of the atoll’s islands, Picard, has grown to 161 meters (528 feet) as thriving mangrove forests trap and retain sediment. The study authors suggest that reducing destructive activities resembling logging and construction could help other atolls deal with rising sea levels.
“Our research shows that Aldabra’s resilience to sea level rise is likely linked to its high conservation status. This is an important lesson, especially now that significant tourism development has begun in the Aldabra group of islands,” Constance wrote, referring to the planned construction of a luxury complex escape on Assumption Island, about 30 kilometers (19 miles) south of Aldabra. “We must protect these unique ecosystems to ensure their continued resilience.”
—Caroline Hasler (@carbonbasedcary), science author