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In Order to Understand the Danger Posed by Sea-Level Rise, We Need to Look to the Past

Evidence of rapid sea-level rises around Southeast Asia thousands of years ago suggests that natural fluctuations in sea level could exacerbate the effects of climate change.

By Kate Wheeling


An aerial view of Belitung, Indonesia. (Photo: Oscar Siagian/Getty Images)

Residents of island nations and coastal cities around the world have known about sea-level rise for some time. Climate scientists predict that, by the end of the century, rising temperatures could melt enough ice from the Arctic and Antarctica to raise sea levels by two to four feet, depending on how much more carbon dioxide and other greenhouse gases humans continue to pump into the atmosphere. But our oceans are subject to natural fluctuations as well; everyone knows about daily tides, but scientists also study what they call “interannual sea level variations”—tidal cycles that can span years or decades, which govern just how far inland the oceans reach.

A new study finds that, thousands of years ago, the waters around Southeast Asia—specifically, the Java Sea—temporarily rose nearly two feet. Should such a rise naturally occur again, coupled with climate change-induced sea level increases, it could spell disaster for the tens of millions of people who live near sea level in the region.

Clearly, understanding how sea levels have fluctuated in the past can provide critical insights into what will happen along coasts in the future, but observational records only go back so far. To estimate sea levels going back thousands of years, the researchers looked at coral microatolls—small, circular coral colonies. Microatolls grow outward in concentric rings, and the top layer grows or dies down over time, based on the height of the water. Much as scientists can use a tree’s rings to measure its age and growing conditions, researchers can approximate historical sea levels from analyses the coral structures.

The team looked at two microatoll sites on opposite sides of Belitung Island in Indonesia. They found that, at least twice between 6,850 and 6,500 years ago, the sea level in the region rose and fell by roughly two feet. Such a rapid, natural fluctuation is likely to happen again, according to Benjamin Horton, a professor at Rutgers University and a lead author on the study. “If it happened in the past, it will happen again in the future,” Horton says, and it could either moderate sea-level rise caused by human activities, or greatly exacerbate it.

Horton’s finding is the latest addition to a growing body of research that shows just how rapidly sea level can rise. “In Antarctica right now, there are these big cracks appearing in the ice shelves, and that starts to indicate that our climate system is responding rapidly,” Horton says.

While it’s becoming ever more clear that the Earth responds rapidly to climate change, exactly what caused the rapid rise of the ocean in Southeast Asia over 6,000 years ago is still a mystery. “One of the things that my community—sea level geologists—try to do is go back into the past to try and work out when sea levels changed, how fast they changed, and what the mechanisms are,” Horton says. “And in this case we’ve solved the first two. Now our community is trying to understand the processes.”

But we know already that human activities are driving sea level rise, and we can still take action. “That’s one of the key things about climate change: We still have the ability to change out future,” Horton says.