Scientists Look to Ancient Hawaiian Reefs for Clues About Future Sea-Level Rise

Along the shores of the Hawaiian island of Molokai, dynamic summer winds swipe the surface of the sea, churning up the waters that surround the island. In the ocean, a sea turtle hastily dips back below the surface after coming up for air. On land, a team of researchers trod through sun-scorched dunes like ducklings in a row, tucking themselves between geological nooks and crannies along the shoreline—searching for answers from a distant past.

Billy D’Andrea, a paleoclimatologist for Columbia University’s Lamont-Doherty Earth Observatory, scrunches his nose as he peers up at the sun beneath his wide-brimmed hat. “Just imagine what these winds were like during a glacial period,” he says.

In such a colder time, when large ice sheets covered much of the Earth, dry, whirling winds were more than twice as strong as they are today.

D’Andrea and his colleagues have come to Hawaii to look for fossilized coral from a different span, the last interglacial period, which lasted from around 125,000 to 110,000 years ago. The planet’s warmer temperatures then matched what many scientists predict we may experience in the next 80 to 100 years, making it an ideal era of study in forecasting the effects of a changing climate on our future. The coral will help them reconstruct past sea levels and come closer to an understanding of what increasing temperatures and melting ice sheets may mean for the island’s rising seas.

Around two million years ago, basaltic volcanic domes erupted and rose from the waters here, forming the fishlike-shaped island of Molokai. As ice sheets grew and melted around the world, sea levels fell and rose along these shorelines, bringing the coral ecosystems with them to different elevations. Were you to map Molokai by observing its shape, you’d find D’Andrea and his fellow researchers by trailing the northern shoreline westward from the island’s “dorsal fin.” Just east of what resembles a fishtail, the geochemists examine the dunes on Moomomi Preserve, in search of clues that indicate the island’s previous sea levels.

The researchers have just arrived from Oahu, where they spent ten days also in search of fossilized coral from the last interglacial period. Because all coral forms in the water, with some species of coral only forming closest to the surface, fossilized coral reefs are an optimal indicator of previous sea levels. And on the Hawaiian islands, there is an abundance of such coral. “In situ” coral, meaning coral that is “on site” or in its original location of growth, is the ultimate marker, because it signals where the sea level was at a given time. Beyond fossilized coral, the researchers looked for keystone bugs, shrimp burrows, notches and sea cave deposits—other indicators of previous sea levels. The research is part of an ongoing global project looking at fossilized reefs across various islands, including the Bahamas, Australia and Bermuda.

On Molokai, the researchers carefully and thoughtfully survey the wind-scoured, sun-beaten rocks. “We look at what’s happening on the shoreline, what organisms are living in different environments, and then try to find that in the rock record,” says Blake Dyer, a geochemist at the University of Victoria in Canada who is also part of the team. “When you see this thing that is 100,000 years old, how can we connect it back to something meaningful about the climate or history of this place?”

Dyer notes that humans have been trying to answer questions regarding the histories of places for thousands of years, observing what’s around us and trying to make some sort of sense of it all. As visitors, Dyer mentions that learning and understanding stories from the people of the land is just as important as the scientific analysis—if not more important.

Haunani Kane, a geologist at the University of Hawaii, shares a Native perspective with the research team, noting that in the Hawaiian creation chant, the Kumulipo, corals are seen as sacred, ancestral beings. “Learning from corals is like learning from family,” she says. “These islands are great libraries and records for how places evolve with changes in water level, and it’s really special to be able to understand how our islands have changed in the past.”

When a fossilized reef is found, the team pinpoints coral locations using GPS, then flies a drone above the site to capture images. The researchers use the photographs to create a high-resolution 3D map of the area, which will allow them to put their finger on any coral they want to examine and see precisely what elevation and position it’s in.

Back in the scientists’ labs in upstate New York and Victoria, British Columbia, corals will be identified and dated through isotope analysis, linking the age of the creatures with sea levels that resulted from ice sheet melts within the last interglacial period.

“Measuring sea level and how it has changed in different places through time actually helps us to understand how the ice sheets must have behaved in the past,” says D’Andrea. “This is all in trying to better understand how rapidly the different ice sheets and mountain glaciers on the planet today are going to melt.”

Since 2002, NASA satellites have shown that land ice sheets on both Antarctica and Greenland have been melting rapidly. About 150 billion tons of ice mass are lost per year on Antarctica. While on the other side of the planet, Greenland loses about 270 billion tons of ice mass per year.

Lamont-Doherty geophysicist Jacky Austermann has spent years tracing the contribution of Antarctic and Greenland ice sheet activity to sea-level rise during the last interglacial period. “Reconstructing the sea level during a time when polar temperatures were warmer gives us insight into, as temperatures warm currently, how much sea level will rise both in the near and more distant future,” she says.

But what was once a steady ripple and retreat of warming and cooling between glacial and interglacial periods is not so steady now. Due to climate change, temperatures are rising fast, and scientists anticipate that by 2100, global temperatures will be 2 degrees Celsius warmer than pre-industrial levels. “In the last interglacial, temperatures were a little warmer, but they warmed very slowly over thousands of years,” says Austermann.

D’Andrea emphasizes the significance of this research in a global context, noting that rising sea levels are already affecting a vast number of communities worldwide. Looking ahead, the impact is predicted to increase. “So anything that we can do to improve our predictions and our estimates of how rapidly sea level is going to change is important,” says D’Andrea.

On Molokai, preparation for sea-level rise has taken on many forms. Locals like Malia Akutagawa, who co-founded Sustainable Molokai, are utilizing ancestral knowledge to inform and prepare residents for the changes that come with a warming planet.

“We have been deconstructing traditional Hawaiian chants, songs and stories,” says Akutagawa, who teaches law and Hawaiian studies at the University of Hawaii at Manoa. “Then we apply that knowledge to orient us to what grew here at that time and use it as a baseline goal of how to restore the land.”

Since ancient times, Hawaiians have been using Molokai’s coastal fish ponds to produce a constant supply of protein. Within the next 80 years, sea level is projected to cover not only the fish ponds but also coastal roads, a town and other residences.

Akutagawa says a retreat will become necessary as the seas continue to rise, but organizing it will be a complex feat. The island’s relatively small strip of coastal land is quickly met with its steep mountains, many of which are home to sacred cultural sites. “We have a lot of decisions to make,” says Akutagawa, “and sea-level rise is really where the impacts are.”