Global sea level rose higher than expected last year. Here’s why.

Oceans last year reached their highest levels in three decades – with the rate of global sea level rise increasing around 35% 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 per year, higher than the expected 0.17 inches 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 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. But 2024 was different. The main driving force, the team said, was an unusual amount of ocean warming that led to more expansion.

High ocean temperatures have surprised scientists in recent years. The warmth of the world’s ocean has been consistently on the rise since the late 1980s, but rates skyrocketed to new levels in the past two years. An unprecedented streak of ocean heat since 2023 led some scientists to think that this part of Earth’s system has fundamentally changed in a way that it cannot be reversed on a human timescale.

The warm ocean temperatures can be detrimental to marine ecosystems, including disrupting our planet’s coral reef populations. It also helps rapidly intensify hurricanes, like Hurricane Milton in 2024.

The extreme ocean warmth comes as global temperatures on Earth reached their highest on record in 2023 and then again in 2024. In 2024, temperatures were about 2.3 degrees Fahrenheit warmer than the 20th century average.

Global sea level rise rates, Willis said, tells us how much total water or heat is added to the oceans. He said that “the rate of global rise is telling us something about what to expect at most coastlines around the world.”

While global sea levels are increasing quicker than expected, many coastal cities are experiencing even greater increases on a local level. Local sea level rise is influenced by other factors, such as land-sinking, ocean currents and differences in land height, for instance.

More than a dozen tide gauges from Texas to North Carolina showed sea levels are at least 6 inches higher than they were nearly 15 years ago. In some of these areas, flooding of roads and land – especially in areas where people of color live – has become more likely and dangerous.