Tracking the rise of the oceans is vital to coastal communities where roughly 44% of the world’s people live. But to accurately estimate changes in sea level, scientists need to collect regular measurements across the globe for many decades, and the existing data are uneven.

Long-term information exists for parts of Europe and North America, where sea levels have been monitored for more than 100 years. But sea-rise records aren’t as complete in Africa, South America and parts of Asia. It makes estimating past and future increases...