Sea levels are rising because the ocean is absorbing enormous amounts of heat and because land-based ice is melting into it. Since 1993, global mean sea level has climbed about 100 millimeters, roughly 4 inches, and the rate is accelerating. Two processes drive nearly all of that rise: warmer water expanding in volume, and ice from glaciers and polar ice sheets flowing into the sea.
Warmer Water Takes Up More Space
When water heats up, its molecules move faster and spread apart. This thermal expansion doesn’t add any new water to the ocean, but it makes the existing water occupy a larger volume. About 30% of global sea level rise is attributed to this effect alone. NASA’s current satellite measurements put the expansion component at roughly 1.3 millimeters per year.
The ocean has absorbed more than 90% of the extra heat trapped by greenhouse gases over the past several decades. That heat doesn’t stay at the surface. It penetrates hundreds of meters deep, meaning the expansion effect is baked in for centuries even if warming stopped today. Water releases stored heat very slowly, so the ocean will continue swelling long after atmospheric temperatures stabilize.
Melting Ice Sheets in Greenland and Antarctica
The two great ice sheets sitting on land, Greenland and Antarctica, hold enough frozen water to raise global sea levels by more than 60 meters if they melted entirely. They won’t disappear anytime soon, but they are losing mass at a pace that has increased sharply since the 1990s.
In 2023, the Greenland Ice Sheet lost 177 gigatonnes of ice. Averaged over the past five decades, Greenland has shed about 119 gigatonnes per year, adding a total of 17.3 millimeters to sea level since 1972. Antarctica lost 57 gigatonnes in 2023 after a brief period of mass gain the year before. Over the longer record from 1979 to 2023, Antarctica has averaged about 107 gigatonnes of annual loss, contributing 13.4 millimeters to sea level.
Greenland loses ice primarily through surface melting during warmer summers and through glaciers calving icebergs into the ocean. Antarctica’s losses are more complex. West Antarctica is losing ice rapidly as warm ocean currents eat away at glaciers from below, while East Antarctica has been gaining some snowfall. The net result is still a loss, and West Antarctica’s glaciers are considered especially vulnerable to runaway retreat.
Mountain Glaciers Are Shrinking Fast
Outside the polar ice sheets, roughly 200,000 smaller glaciers sit in mountain ranges from the Himalayas to the Andes to the Alps. These glaciers collectively lost 6,542 billion tonnes of ice between 2000 and 2023, contributing 18 millimeters to global sea level rise over that period. That rate of loss has been intensifying, with recent years consistently breaking records for glacier retreat. Many smaller glaciers are expected to disappear entirely within decades, which will also reduce freshwater supplies for communities that depend on seasonal meltwater.
Groundwater Pumping Adds to the Problem
A less obvious contributor is groundwater extraction. When water is pumped from underground aquifers for agriculture and drinking, it eventually flows through rivers or evaporates and falls as rain, much of it ending up in the ocean. Cumulative global groundwater depletion from 1900 to 2008 totaled about 4,500 cubic kilometers, equivalent to 12.6 millimeters of sea level rise, accounting for 6 to 7 percent of the total observed rise over that period.
Dam construction partly offset this effect for decades by trapping water on land. But the era of building large new dams is slowing, while groundwater pumping continues to accelerate, particularly in India, the Middle East, and the western United States. The net transfer of water from land to ocean is growing.
Why Some Coastlines Rise Faster Than Others
Global averages mask enormous local differences. Some cities experience sea level rise two or three times the global rate, while a few places see levels holding steady or even dropping. The main reason is that the land itself moves.
Tectonic plate movement can push land up or drag it down. Sediment along river deltas compresses and sinks over time, which is why cities like New Orleans, Jakarta, and parts of the Chesapeake Bay region face especially rapid relative sea level rise. Groundwater and oil extraction can accelerate this sinking.
A counterintuitive factor is called isostatic rebound. During the last ice age, massive ice sheets pressed down on northern landmasses. When that ice melted over 20,000 years ago, the land began slowly bouncing back, and it’s still rising in places like Scandinavia and northern Canada. But areas that had bulged outward around the edges of those ancient ice sheets are now sinking back down. This is one reason the northeastern United States is experiencing faster-than-average sea level rise: the land itself is subsiding as it adjusts to ice that vanished millennia ago.
Ocean currents and wind patterns also redistribute water unevenly, creating temporary regional highs and lows that can persist for years.
How Much Higher Will Seas Get?
Projections depend heavily on how much more greenhouse gas the world emits. Under a scenario with strong emissions cuts (roughly aligned with the Paris Agreement goals), the median projection for 2100 is about 0.44 meters (roughly 17 inches) above the 1995 to 2014 baseline, with a likely range of 0.32 to 0.61 meters. Under a high-emissions scenario where fossil fuel use continues to grow, the median jumps to 0.77 meters (about 30 inches), with a range of 0.63 to 1.01 meters.
Those ranges reflect processes scientists model with medium confidence. Low-probability but high-impact possibilities, particularly the potential for rapid ice sheet collapse in West Antarctica, could push the number significantly higher. Under the worst-case ice sheet scenarios, sea level rise by 2100 could reach 1.6 meters, over 5 feet.
Even under the best-case pathway, sea levels will keep rising well beyond 2100. The ocean’s slow absorption of heat means thermal expansion continues for centuries, and ice sheets respond to warming on timescales of hundreds to thousands of years. The choices made in the next few decades determine whether that long-term rise is measured in feet or in meters.