(CNN) Low-lying atolls in the Pacific Ocean have long been considered some of the most vulnerable areas to climate change, as rising sea levels threaten to submerge them.
But over the past decade, scientists have noted a puzzling phenomenon: some islands are getting bigger.
A new study released last week examined the evolution of Jeh Island, a sparsely populated atoll that's part of the Marshall Islands, a Pacific nation made up of a remote chain of coral atolls and volcanic islands between the Philippines and Hawaii.
Researchers found the island's land area has increased by 13% since 1943 due to a buildup of sediments from the existing coral reef. Healthy coral reefs naturally produce sediment -- in fact, that's what atolls are made of.
"You can still see an island grow at a time when most people and most models would suggest they should be eroding," said the study's co-author Murray Ford, an expert in Pacific reef island systems from Auckland University.
Ford and Paul Kench, from Simon Fraser University in Canada, compared the island's land mass in aerial photos from 1943 and 2015. They also radiocarbon-dated sediment deposited on the island to find out when the coral remnants were alive, and discovered that sediment on parts of the island had been deposited after 1950, suggesting the island's growth is relatively new.
At the same time, global sea levels have been rising. Satellite data shows waters around the Marshall Islands have risen about 7 millimeters (0.3 inches) each year since 1993, according to a Pacific Climate Change Science country report. That's more than the global average of 2.8 to 3.6 millimeters (0.11 to 0.14 inches) per year.
The new study is significant because it shows islands can continue growing, even when sea levels are rising.
"We have found islands are resilient in the face of rising seas and that sediment supply to some atolls is out-pacing sea level rise," Ford said. "What we don't know is how that will play out in coming decades."
Atolls are often only around two meters (6.6 feet) above sea level, but sea levels could rise by more than that by the end of this century, according to scientists. Sea levels are rising because greenhouse gas emissions are warming the oceans and planet, melting polar ice caps and glaciers.
A 2018 US Geological Survey study found that many low-lying atolls will be uninhabitable by the middle of this century.
And last year, the Asian Development Bank president Takehiko Nakao said the four atoll nations -- the Marshall Islands, Tuvalu, Kiribati, and the Maldives, which are together home to more than half a million people -- were the most vulnerable on the planet to climate change.
"For the atoll nations, climate change is not a distant threat for a future generation to face but an immediate emergency, with tropical storms and rising seas taking their toll on human lives, livelihoods, and infrastructure," Nakao said.
But over the past decade, a number of studies have found that some atolls are actually getting bigger. A 2010 study showed that some Pacific islands hadn't eroded, and a 2018 study of 30 Pacific and Indian Ocean atolls, including 709 islands, found that no atoll had lost land area. More than 88% of islands were either stable or increased in area, according to the study.
"That started a bit of a goldrush in terms of studies," Ford said. "The signal was kind of consistent -- there's no widespread chronic erosion of atoll islands in the Pacific."
Ford says his study is the first to not only show an island increasing in size, but also to conclusively establish why that might be happening naturally over time.
"This is the first time we can see the islands form, and we can say the stuff making that island is modern ... so it must be coming from the reef around the island," said Ford. "It's entirely the skeletons of the reef and the organisms that live on it."
The fact that some islands aren't "sinking" doesn't mean climate change isn't an issue -- it is still very much a concern.
Research released in September by scientists from the University of Hawai'i at Manoa shows that atoll islands face a variety of threats from rising sea levels. As sea levels get higher, islands may experience more frequent flooding that could deteriorate freshwater reserves and make atolls uninhabitable. Extreme tides could also cause coastal erosion.
And scientists don't know for sure whether atolls will continue to produce sediment at a rate to stay above sea level, but the University of Hawai'i researchers estimated that atolls would be impacted differently, depending on their elevation.
"Even within one nation, there are differences in how islands will respond," said Haunani Kane, a graduate student in geology and geophysics. And that can be difficult to predict.
Atolls are formed from coral reef sediment, but reefs may not continue to produce the same levels of sediment in the future if the ecology of the reef suffers.
Research shows climate change is already having a huge impact on coral reefs -- a study presented earlier this year estimated that about 70-90% of all existing coral reefs are expected to disappear in the next 20 years due to warming oceans, acidic water and pollution.
"What happens to the ecology of the reef in the future is a big driver in what happens to the ecology of the island in the future," said Ford. "If you turn off that engine room of sediment generation, then you potentially will see that effect the island."
Ford said that scientists don't yet understand the time scales -- if a reef died today, for instance, it's not clear whether it would take a year or a decade or more for it to have an effect on the levels of sediment produced.
But for people living on the islands, the fact the land mass is growing may not alleviate their fears of losing their home to climate change.
According to Ford, most people who live on atolls live on parts of the islands that have been modified or developed by people. Manmade structures aren't likely to experience a natural buildup of sediment, said Ford, partly due to their construction and residents' desire to maintain them.
"They're not going to tolerate sand washing onto the surface of the island and interrupting their activities," he said.
But residents can learn from the natural process, he added.
"In some ways, nature is kind of providing a template for how to adapt to sea level rises and that's to build your islands higher and to build your islands through acquiring sediment," he said. "It's good news from that perspective, but it sure as heck doesn't help the ... people on the populated islands."