In harbors and ports around the world, tide gauges bob up and down with the sea, recording its height over time. In some places, these instruments—through various iterations—have been recording continuously since 1700. Originally installed to help fishing and merchant vessels plan when to enter and leave harbors, the data produced by these old-school gauges has been co-opted by scientists, and now forms the basis of climatologists’ understanding of long-term sea level rise. But as a new study shows, because the majority of these tide gauges were located in North Atlantic port cities, scientists have been systematically underestimating the rate of global sea level rise.

“The gauges are concentrated in the northern hemisphere, in Europe and North America, where traditionally we’ve had a lot of shipping and commerce,” says Phil Thompson, an oceanographer at the University of Hawai‘i at Manoa. “That’s contributing to why we don’t get really good spatial coverage over the globe.”

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