Oceans last year reached their highest levels in three decades — with the rate of global sea level rise increasing around 35 percent higher than expected, according to a NASA-led analysis published Thursday. The unexpected rise in global levels is concerning, particularly as an indicator for what to expect around coastal cities, many of which are already experiencing more damaging flooding.

“Because the oceans cover more than two-thirds of the planet, global sea level rise is an important yardstick for the Earth’s overall climate,” said Josh Willis, a sea-level researcher at NASA’s Jet Propulsion Laboratory.

Last year’s rate of average global sea level rise was 0.23 inches (0.59 centimeters) per year, higher than the expected 0.17 inches (0.43 centimeters) per year, NASA said in a news release.

The rate of global sea level rise follows a trend of rapidly increasing rates over the past 30 years. From 1993 to 2023, the rate of global sea level rise doubled, increasing from 0.08 inches per year to 0.18 inches, another NASA-led study showed. Overall, the global sea level has climbed by 4 inches (10 centimeters) since 1993.

As rates are predicted to increase, Willis said the “expected” rate is based on this long-term rise and how fast it is speeding up. But, in any given year, he and his team see a little bit more or less because of natural cycles — like last year. The natural cycles allow extra water to reach the ocean or extra heat to be absorbed.

Global sea level is influenced by certain factors. In recent years, melting ice sheets and glaciers contributed to about two-thirds of sea level rise. Another third came from ocean warming and taking up more volume.