A new paper in Nature, published on March 4, 2026, gives an important warning: in many places, the sea along the coast was already higher than researchers assumed. The authors, Katharina Seeger and Philip Minderhoud, reviewed 385 peer-reviewed studies on sea-level rise and coastal hazards from 2009 to 2025. They found that more than 90% of those studies used a gravity-based reference model, called a geoid, as if it were the same as real local sea level. But actual coastal sea level is also shaped by ocean currents, water temperature, winds, tides, and local measurements. (nature.com)
This difference may sound technical, but the effect is large. The study found that many assessments underestimated coastal sea level by about 0.24 to 0.27 meters on average. In parts of Southeast Asia and the Indo-Pacific, the gap can be more than 1 meter. The researchers also found that less than 1% of the studies properly matched land-elevation data with measured sea-level data. Importantly, the paper does not say that global sea-level rise itself was miscalculated. Instead, it says that many impact studies started from the wrong local baseline. (nature.com)
Why does this matter for coastal cities? Because if your starting sea level is too low, future flood risk will also look too small. In the authors’ test case, using assumed sea level instead of measured sea level could underestimate the number of people living below sea level after 1 meter of relative sea-level rise by up to 68%. That is serious news when almost 900 million people already live in low-lying coastal zones worldwide, and the IPCC says this number is expected to rise above 1 billion by 2050. The IPCC also projects global mean sea level rise of 0.28 to 0.55 meters by 2100 under a very low emissions pathway, and 0.63 to 1.01 meters under a very high emissions pathway. (scimex.org)
The big lesson is simple: the future of coastal cities depends not only on how fast the ocean rises, but also on where we place today’s “zero” line. NASA also notes that local sea level can be higher or lower than the global average because land can rise or sink and ocean conditions differ by region. So this new study is a reminder that better local data can change how cities plan walls, roads, homes, and evacuation maps. Sometimes, the most important scientific discovery is not that the future changed, but that we understood the present more clearly. (science.nasa.gov)
