Global mean sea levels have risen about 25 centimeters (roughly 10 inches) since 1880. What makes that number alarming isn’t just the total, but the acceleration: the rate of rise has more than doubled in the past three decades, and 2024 brought an unexpectedly sharp jump.
The Numbers Over Time
Between 1880 and the early 1990s, sea levels climbed about 20 centimeters (8 inches). In the roughly 30 years since, they’ve added another 10 centimeters (4 inches). That means the ocean rose nearly as much in the last three decades as it did in the previous century.
The yearly rate tells the story more clearly. Through most of the 20th century, the ocean was rising at about 1.5 millimeters per year. By the early 1990s, that had increased to about 2.5 mm per year. Over the past decade, NASA puts the rate at 3.9 mm (0.15 inches) per year. And in 2024 specifically, a NASA-led analysis found the rate spiked to 5.9 mm (0.23 inches) per year, well above the expected 4.3 mm. Scientists are still working to understand what drove that anomaly, but the broader trend of acceleration is unmistakable.
Why the Ocean Is Rising
Two main forces push sea levels up: warmer water takes up more space, and melting ice adds new water to the ocean. Thermal expansion, the swelling of seawater as it absorbs heat, accounts for about 56% of recent sea level rise. The ocean has absorbed the vast majority of the extra heat trapped by greenhouse gases, and warmer water simply occupies more volume.
The rest comes largely from ice loss on land. The Greenland Ice Sheet has lost an average of 119 billion tons of ice per year between 1972 and 2023, contributing a total of about 17 millimeters to sea level rise over that period. Antarctica has shed an average of 107 billion tons per year since 1979, adding roughly 13 millimeters. To put those numbers in perspective, every 360 billion tons of ice lost raises the global ocean by about 1 millimeter.
The pace of ice loss has accelerated dramatically. Greenland was losing about 60 billion tons per year in the 1980s. By the 2010s, that figure had quadrupled to 246 billion tons per year. Antarctica followed a similar trajectory, going from 48 billion tons per year in the 1980s to 202 billion tons by the 2010s. Mountain glaciers around the world also contribute, though their total volume is much smaller than the two great ice sheets.
Why Some Coasts See More Rise Than Others
Sea level rise is not uniform. What matters for any given stretch of coastline is “relative” sea level: the combination of the ocean rising and the land itself sinking or lifting. In many coastal areas, the land is subsiding at a few millimeters per year, which may sound trivial but can effectively double the local rate of rise.
Some of this sinking is geological. After the massive ice sheets of the last ice age retreated about 15,000 years ago, the ground beneath them began rebounding upward, while surrounding regions slowly sank in response. The Chesapeake Bay area, for example, is subsiding at about 2 mm per year from this process alone, which is a major reason the mid-Atlantic coast experiences some of the highest rates of relative sea level rise in the U.S. Human activity adds to the problem. Groundwater pumping compresses underground rock layers, causing the land above to sink. Parts of California’s Central Valley have dropped measurably in recent decades from this effect, and coastal cities built on river deltas face similar challenges.
Ocean currents, gravitational effects from shrinking ice sheets, and regional wind patterns also redistribute water unevenly. Some stretches of ocean are rising two or three times faster than the global average, while a few areas see little change at all.
How Scientists Track Sea Level
Before satellites, the only way to measure sea level was with tide gauges: instruments bolted to piers and coastlines that record the height of the water. Some have been running for over 75 years, providing a long and valuable record. The catch is that they only measure specific points, and because they’re attached to land, they capture land sinking or rising along with any ocean changes.
Since the early 1990s, satellite altimeters have measured ocean height from space with millimeter-level accuracy when averaged globally. Satellites can’t yet match the long historical record of tide gauges, but they provide nearly global coverage, making it possible to see planet-wide trends rather than relying on a patchwork of coastal stations. The two systems complement each other, with tide gauges helping to calibrate satellite data and fill in gaps.
What Rising Seas Mean on the Ground
The most immediate consequence for most people isn’t a dramatic inundation but a steady increase in “nuisance” or high-tide flooding. The annual frequency of high-tide flooding in the U.S. has more than doubled since 2000. These are events where water overtops streets, clogs storm drains, and damages property even without a storm. They’re projected to more than triple again by 2050.
NOAA projects that U.S. coastlines will see an additional 10 to 12 inches of sea level rise by 2050, with the exact amount varying by region due to differences in land height changes. That means the country is on track to experience as much rise by 2050 as it saw over the entire previous century. By that point, moderate coastal flooding, the kind that causes real disruption and damage today, is expected to occur more than 10 times as often as it does now, even on calm, sunny days with no storms or heavy rain.
For coastal communities, this shift changes the math on everything from infrastructure planning to home insurance. Roads, sewage systems, and buildings designed for historical flood patterns will face conditions they were never built to handle, not in some distant future but within the next 25 years.