Sea levels are rising much faster than previously thought. According to a new study, 40% of the world’s population living in coastal regions is at high risk. The findings reaffirm the importance of tackling greenhouse gas emissions to avoid catastrophic climate damage such as floods or tidal surges.

The Intergovernmental Panel on Climate Change (IPCC), the world’s leading group of climate experts, said in its most recent assessment that the sea level was unlikely to rise beyond 1.1 meters by 2100. But they were too optimistic. According to a group of researchers at the University of Copenhagen’s Niels Bohr Institute, things are way worse.

Using historical data on the sea-level rise to validate models relied on by the IPCC to make its assessment, they found that sea levels could rise as much as 1.35 meters by 2100 –– 25 centimeters more than the IPCC’s most dire prediction. It might not seem like a lot, but for cities living right near the sea and small island states, it could actually spell disaster, displacing millions of people from their coastal cities and villages.

“It’s not great news that we believe the former predictions are too low,” Aslak Grinsted, a co-author of the study, told The Guardian. “The models used to base predictions of sea-level rise on presently are not sensitive enough. They don’t hit the mark when we compare them to the rate of sea-level rise we see when comparing future scenarios with observations going back in time.”

Grinsted and his team used a linear regression model to connect the rise in average temperatures to sea level rise, focusing on measuring their results against preexisting estimates. They analyzed historical data to factor in how the dynamic nature of the environment can accelerate or decelerate rising sea levels.

The results from their regression model showed that the projected global sea-level rise by the end of the 21st century by the IPCC is “at best conservative.” The world would need to emit 200 gigatons to 300 gigatons less carbon dioxide, and cool the Earth by about 0.6° Celsius, for sea-level rise to correlate with previous models, they added.

“The scenarios we see before us now regarding sea-level rise are too conservative – the sea looks, using our method, to raise more than what is believed using the present method,” Grinsted told Bloomberg. Following their results, the researchers are already in touch with the IPCC about incorporating its results in next year’s assessment.

Benjamin Horton, a professor at Singapore’s Nanyang Technological University, who wasn’t involved in the study, told Mongabay that the research is important since computer models aren’t always accurate at predicting the future. “The methodology suggests that there’s a real risk of very substantial sea-level rise,” he said.

Rising sea levels are one of the major results of the climate crisis, causing flooding and threatening coastal habitats. They are caused by melting of land-based ice such as glaciers and thermal expansion from ocean warming. Oceans absorb more than 90% of the increased atmospheric heat linked to emissions from human activity.

Just in the United States, almost 40% of the population lives in densely-populated coastal areas, where sea level plays a role in flooding, shoreline erosion, and hazards from storms, according to US National Ocean Service. On a global scale, eight of the 10 largest cities are near a coast, according to the UN Atlas of the Oceans.

The study was published in the journal Ocean Science.