New research has found that scientists have been using methods that underestimate the potential toll of sea-level rise. One result is that hundreds of millions more people worldwide might be at risk from rising waters than previously estimated by western experts.
Philip Minderhoud was standing on the shores of the Mekong delta in Southern Vietnam, 10 years ago, when he spotted something odd.
The water wasn’t where it was supposed to be. As a graduate student researching sinking coastal land, he was familiar with maps in the region, but the sea level was higher than they indicated — “much higher,” he recalled.
That observation by Dr. Minderhoud was the first indication of a “widespread mismatch,” he said, between scientific understanding and reality in the region.
And now a new study, published Wednesday in the journal Nature, has found that the vast majority of scientific studies that calculate coastal sea level have underestimated how high the water already is.
Coastal sea levels are, on average, eight inches to a foot higher than these scientific studies and maps indicate worldwide, the research found.
The discrepancies are much bigger in certain regions, like Southeast Asia and Pacific nations, where ocean dynamics are more complex. There, coastal sea levels are up to several meters higher than commonly estimated.
It’s hard to imagine not being able to trust a map in an age where GPS and satellite imagery is so prevalent. But the new study has identified a far-reaching problem in the method that researchers use to understand coastlines and how they might change in a warming climate.
This doesn’t mean that these studies are wrong in their other conclusions about the rate of sea-level rise as the planet heats up. But it does mean that they’re working from the wrong starting point.
That matters as governments and policymakers turn to science to understand how much land — and how many people — may be affected as the world warms and oceans rise, said Katharina Seeger, a postdoctoral researcher at the University of Padova, who led the study while working toward her Ph.D. at the University of Cologne. “I didn’t expect the discrepancy to be so immense,” she said.
In their study, Ms. Seeger and Dr. Minderhoud, now an associate professor who studies land subsidence and sea level rise at Wageningen University & Research and Deltares, a scientific institute in the Netherlands, checked 385 other peer-reviewed papers to see how their calculations compared. They found that less than 1 percent had correctly assessed where sea levels are today.
The problem starts with a decades-old method that compares satellite-based measurements of land elevation to something scientists know as a “geoid model,” which is a technique for estimating mean sea level based on Earth’s gravitational field. This method was once considered “state-of-the-art,” Dr. Minderhoud said, and was commonly taught in graduate school.
But other satellites and instruments can measure real sea level and reveal local differences from factors like currents, winds and tides, which can also influence sea levels but are not included in the gravitational-field model. Scientists can most accurately estimate sea level when both pieces of the puzzle are combined correctly.
But largely, the new study found, that hasn’t been the case. Some 90 percent of the studies that Dr. Minderhoud and Ms. Seeger checked relied only on the older and commonly-used method of mapping sea levels with Earth’s gravitational field. Another 9 percent of studies, most of which are relatively recent, did use both kinds of data, but seemingly failed to combine them correctly.
Robert Kopp, a climate and sea level scientist at Rutgers University who was not involved with the study, said the work addresses a technical issue that will matter far more to scientists than to decision makers at local levels. “In general, people who are exposed to high-tide flooding know where the ocean is,” Dr. Kopp said. Scientists have long said sea level rise will affect many people, and the new study doesn’t change that, he said.
However, from a global perspective, the findings indicate that hundreds of millions more people — particularly in Vietnam, the Philippines, Indonesia, the Maldives and other Southeast Asian and Pacific nations — are living closer to sea level than widely assumed by Western experts and policymakers.
Climate Central, a climate science and communication organization, has a publicly available sea level rise map that is among the 1 percent of models that got the methodology right. Ben Strauss, its president and chief scientist, said the paper was an important contribution to a challenging problem.
“Modern geoids are sophisticated,” Dr. Strauss said. “But the wind still blows, there are ocean currents, and there are things that pile up to make the water higher or lower than what the geoid would predict.”
Torbjorn Tornqvist, a geology professor at Tulane University in Louisiana who was not involved with the study, said the findings were a real “wake up call,” with wide implications. “We finally have a really full blown, robust effort to sort this out and the result, it’s somewhat dramatic,” Dr. Tornqvist said. “It turns out we’ve been comparing apples to oranges nearly all the time.”
Sachi Kitajima Mulkey covers climate and the environment for The Times.
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