As sea levels rise under the influence of climate change, global coastlines—and the bustling cities they host—are at risk of being swallowed whole. If greenhouse gas emissions continue at or exceed current rates, land now occupied by 480 million people could be vulnerable to annual flooding by the end of the century, according to a study published Tuesday in the journal Nature Communications. Most at-risk shoreline residents live in Asia, the researchers say, where hundreds of millions make their home near expanding oceans.
The projections didn’t account for future population growth or coastal erosion. But the estimates, which draw on present demographic data, are the highest ever reported, thanks to a new, more accurate method for calculating land elevation.
Older techniques for assessing coastal vulnerability, which rely on satellite data, are easily flummoxed by tall buildings and trees, leading to overestimates in altitude—and misleadingly low risk scores for heavily forested areas or cities packed with skyscrapers.
To root out and correct these errors, the new model—the brainchild of Climate Central researchers Scott Kulp and Benjamin Strauss—used artificial intelligence that re-analyzed old data. The topographical maps it generated suggest that there are many more people living at vulnerable elevations than once thought. While previous estimates showed that about 65 million people currently reside in areas expected to flood at least once a year, the new numbers bump this figure up to 250 million, more than tripling known global risk.
The number of people in vulnerable coastal zones will continue to swell with the seas if global warming progresses unchecked, Kulp and Strauss found. In the most dire climate change scenario tested, which would raise global temperatures nearly 8 degrees Fahrenheit over pre-industrial levels, almost half a billion people could be affected by 2100. Residents of Asia would be hardest hit, accounting for more than 70% of those imperiled. Within the next few decades, the results suggest, an entire swath of southern Vietnam could be underwater, including Ho Chi Minh City.
“An incredible, disproportionate amount of human development is on flat, low-lying land near the sea,” Strauss told Jonathan Watts at The Guardian. “We are really set up to suffer.”
Projections aren’t guarantees, however. The estimates do diminish somewhat if temperature rises are minimized, the authors note. And protective measures like sea walls have made it possible for millions of people around the world to live comfortably in areas below the high tide line, Strauss told Denise Lu and Christopher Flavelle at The New York Times.
But human-made barriers can only do so much. Countries should start preparing to relocate their citizens inland, Dina Ionesco of the International Organization for Migration, an intergovernmental group that coordinates action on migrants and development, told Lu and Flavelle. The notion of mass migrations, which have never occurred on this scale, is controversial, and will require similarly unprecedented efforts, noted Ionesco, who wasn’t involved in the study. “We’ve been trying to ring the alarm bells,” she said. “We know that it’s coming.”
As more data is collected and models continue to be refined, the exact numbers could change, US Geological Survey researcher Dean Gesch, who wasn’t involved in the study, told Adam Vaughan at New Scientist.
But the big jump from previous estimates should still highlight the urgency of our situation, Strauss told Vaughan. “To us, it’s a staggering difference,” he said. “It’s a completely new perspective on the scale of this threat.”