One in three people are at risk from rising sea levels

More and more people around the world are packing into coastal areas, where water meets land. A new study maps coastal population levels from 2000 to 2018, showing that growth along shorelines dramatically outpaced inland population growth.

The numbers are not small. Roughly two billion people lived within about 31 miles of the shore in 2018, and about one billion lived within roughly 6 miles.

Studying coastal populations

Lead author Arthur G. Cosby of Mississippi State University (MSU) and his colleagues traced annual population counts in thin inland bands from the shoreline to about 31 miles.

They looked for clear patterns that would hold across continents and across time.

They relied on Oak Ridge National Laboratory’s LandScan Global dataset, which estimates where people actually are over a day rather than just where they sleep.

This approach, often called ambient population, captures workers, students, tourists, and others who spend time near the coast even if they live farther inland.

To place people accurately on the map, LandScan uses a dasymetric method, which blends census counts with layers like roads, land cover, and satellite images.

The team summed the people inside each 3 mile band to build a clean, year by year picture of coastal settlement.

What the numbers show

Coastal bands closest to the water had the densest populations, and each step inland held fewer.

The pattern followed a tight power law that the authors call the CoPop Curve, with an exponent near minus 0.9 and a fit near 0.99 in 2018.

From 2000 to 2018, the near coast added 463 million people, and half of that increase landed within the first roughly 6 miles.

“We found: large concentrations of population in relatively small bands and regions along the coast. Growth rates put the trend in sharper focus. “Coastal regions increased by 26.6%,” wrote Cosby.

Coastal population risks

Living near the water brings exposure to more than tides and sea breezes. Many coastal cities are also sinking, a process known as subsidence, which makes relative sea level rise faster than the global average.

A 2021 global analysis estimated that people in coastal zones typically face 7.8 to 9.9 millimeters per year of relative sea level rise when subsidence is included.

That pace pushes floods farther inland, forces expensive defenses, and raises the odds that storms will do more damage.

Infrastructure has to keep up with people. New roads, housing, water systems, and ports crowd into places where floods, erosion, and saltwater intrusion can disrupt life and work.

How this compares to older estimates

In the 1990s, global estimates used wide buffers from the coast and coarser data.

A classic Science letter put about 37 percent of the world within roughly 62 miles of the coast in 1994 and 44 percent within roughly 93 miles.

The new work keeps the focus on the bands people actually occupy near the shore, using horizontal distance and finer grids.

That shift helps separate truly low elevation coastal zone (LECZ) exposure from broader, less risky areas that happen to be within a long drive of the beach.

Where the growth is happening

Asia carried the largest share of people living near the coast and also the largest increase, adding about 268 million within the near coast between 2000 and 2018.

Africa grew fastest in percentage terms and added about 97 million near the coast over the same period.

Together, those two continents accounted for about 79 percent of all new near coast residents during the study window.

That tilt toward Asia and Africa matters because many of their coastal regions are engine rooms of economic activity and are already straining to manage water, land, and public health.

The CoPop Curve gives planners a simple, tested way to estimate how people stack up across narrow coastal belts. It is not a toy number, it is a compact description of how settlement falls off with distance from the shore.

Because the curve held across continents and across the 19 year record, it can be used to cross check local counts, stress test evacuation plans, and target limited adaptation funds.

It can also help modelers plug realistic human pressure into coastal flood and erosion models without guessing.

A quick reality check on scale

The United Nations marked the “Day of 8 Billion” in 2022, a milestone that continues to frame how tight space can feel in crowded regions.

The UN sets that benchmark and reminds us that slowing global growth does not erase local surges.

Even small percentage changes translate into huge absolute numbers when the base is so large.

A few percent shift within 31 miles of the shore can mean tens of millions of people added to places already hard to defend.

Cities and counties do not control the ocean, but they do control buildings and behavior. Zoning, elevation standards, and building codes can keep new homes and critical services out of harm’s way.

Natural buffers help too. Restored dunes, marshes, and mangroves absorb energy, slow waves, and create room for water to spread without wrecking homes and roads.

Future of coastal populations

Routine tracking is essential. Ambient counts, mobile data, and high resolution images can show seasonal pulses and long term shifts that simple headcounts miss.

Better data should also feed fair choices. Investments that reduce risk in the first miles from the coast can protect both residents and the economy that depends on them.

The study is published in Scientific Reports.

Image from Statista based on data from Scott A. Kulp and Benjamin H. Strauss, Nature Communications.

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