As islands like O’ahu very slowly drift from the magmatic hotspot that once formed it, the island’s weight is slowly causing it sink into the ocean at around 0.6 millimeters per year.
However, a new study shows that in some areas of O’ahu, that subsidence rate can be as high as 25 millimeters per year, roughly 40 times faster than expected.
Researchers believe this is largely due to some areas, such as the more industrial Mapunapuna region, experiencing compaction as it was originally built on sediment and artificial fill.
The geologic story of Hawaii has historically been one of ascension. More than a million years ago, when the Pacific Island Plate moved atop a volcanic hotspot rising through the Earth’s crust, sea-level volcanic islands formed what eventually became the U.S.’s 50th state. Now, a new study from the University of Hawai’i at Manoa reports that the island chain may be reversing course—literally.
Published in the journal Communications Earth & Environment, the study analyzes subsidence (the gradual caving in or sinking of an area) on the island of O’ahu, home of Pearl Harbor and the state capital, Honolulu. They found that in some areas of the island, located 185 miles northwest of the Big Island (or Hawai’i) which rests on top of the island chain’s hotspot, the subsidence rate was at around just 0.6 millimeters per year. However, they also recorded areas that were sinking a stunning 40 times that rate at roughly 25 millimeters per year. This rate, along with the localized nature of the subsidence, is what caused researchers to raise a few eyebrows.
“Our findings highlight that subsidence is a major, yet often overlooked, factor in assessments of future flood exposure,” UH Manoa’s Kyle Murray, lead author of the study, said in a press statement. “In rapidly subsiding areas, sea level rise impacts will be felt much sooner than previously estimated, which means that we must prepare for flooding on a shorter timeline.”
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Part of the reason for this discrepancy is that industrial areas such as the Mapunapuna area is built on sediment and artificial fill, which, according to the researchers, leads to increased compaction compared to other areas of O’ahu. This subsidence rate far outpaces the long-term rate of sea level rise, which is around 1.54 millimeters, and could cause problems for the region’s shoreline on a shorter timetable.
“In places like the Mapunapuna industrial region, subsidence could increase flood exposure area by over 50% by 2050, while compressing flood preparedness timelines by up to 50 years,” UH Manoa’s Phil Thompson, a co-author of the study, said in a press statement.
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Efforts to address climate concerns on O’ahu, such as the science-based, community-driven Climate Ready O’ahu, are preparing for increased sea level rise and increased soil erosion along with other climate change-induced events, such as wildfires and flash flooding. While the conservation of wetlands and dune ecosystems will help stabilize shorelines, the researchers note that taking into account this concerning rate of subsidence will be vital for understanding the true timeline required to implement these climate adaptation strategies.
“Our research provides critical data that can inform state and county decision-making, helping to improve flood exposure assessments, infrastructure resilience, and long-term urban planning,” UH Manoa’s Chip Fletcher, a co-author and director of Climate Resilience Collaborative, said in a press statement. “This work directly serves the people of Hawai‘i by ensuring that local adaptation strategies are based on the best available science.”
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Darren lives in Portland, has a cat, and writes/edits about sci-fi and how our world works. You can find his previous stuff at Gizmodo and Paste if you look hard enough.