Sea levels rise when extra water enters the ocean or when existing ocean water expands because it’s getting warmer. Global sea levels have gone up about 4 inches (10 centimeters) since 1993, and the annual rate of rise has more than doubled over that period. Several distinct processes drive this change, some natural and some accelerated by human activity.
Warming Water Takes Up More Space
The ocean absorbs more than 90 percent of the extra heat trapped by greenhouse gases. As water warms, its molecules move faster and spread apart slightly, causing the entire volume of the ocean to expand. This process, called thermal expansion, has accounted for roughly one-third of global sea level rise observed by satellites since 2004. No new water is added to the ocean in this case. The water already there simply occupies more space.
Thermal expansion is slow and distributed across the full depth of the ocean, which makes it easy to overlook. But the ocean is enormous. Even a tiny increase in the average temperature of all that water translates into a measurable rise at the surface. And because the deep ocean takes centuries to fully absorb heat from the surface, thermal expansion will continue long after temperatures stabilize.
Melting Ice Sheets in Greenland and Antarctica
The two largest contributors of new water to the ocean are the ice sheets covering Greenland and Antarctica. Between 2002 and 2023, Greenland lost about 270 billion metric tons of ice per year, raising global sea levels by roughly 0.8 millimeters annually. Antarctica lost about 150 billion metric tons per year, adding another 0.4 millimeters. Combined, these two ice sheets raise sea levels by nearly 1.2 millimeters each year.
Greenland’s ice melts primarily from warmer air temperatures and increased surface melting, while Antarctica loses ice mainly through warming ocean water eating away at the undersides of ice shelves along the coast. When those shelves thin or break apart, the glaciers behind them flow faster into the sea. The Intergovernmental Panel on Climate Change has warned that the western part of the Antarctic Ice Sheet may have already crossed a tipping point where its disintegration becomes irreversible. A similar collapse likely occurred during a warm period roughly 125,000 years ago, when polar temperatures were only two to three degrees Celsius higher than today, a range that could become reality by 2100.
Mountain Glaciers Around the World
Outside the polar regions, thousands of smaller glaciers in the Andes, Alps, Himalayas, and other mountain ranges are also shrinking. These glaciers are highly sensitive to warming because they sit at elevations where even a small temperature increase pushes conditions above freezing. Estimates suggest mountain glaciers could contribute up to half a meter of sea level rise over the coming decades and centuries, though the uncertainty around that number remains higher than for the polar ice sheets. Individually these glaciers are small, but collectively they hold a significant volume of frozen water that flows into rivers and eventually reaches the ocean.
Groundwater Pumping and Land Water Changes
Not all sea level rise comes from climate change directly. Humans move enormous quantities of water around the planet through irrigation, dam building, and groundwater extraction. When water is pumped from deep underground aquifers for agriculture or drinking, it eventually reaches the surface, evaporates, falls as rain, and flows to the ocean. This transfer of ancient groundwater into the active water cycle adds real volume to the seas. Dam construction works in the opposite direction by trapping water on land, but the net effect of all human water management combined has been to move water from land to ocean, contributing a measurable fraction of total sea level rise.
Why Sea Levels Don’t Rise Evenly
If you picture sea level rise as water filling a bathtub, the reality is messier. Sea levels rise faster in some places and slower, or even fall, in others. Most of this unevenness comes from ocean dynamics: currents driven by wind, temperature differences, and salinity redistribute water around the globe. Regional climate cycles like El Niño and La Niña shift ocean circulation patterns, temporarily raising levels in some basins while lowering them elsewhere. The Pacific Decadal Oscillation, a longer cycle, does the same over decades.
Land movement matters just as much for what coastal communities actually experience. In river deltas and coastal plains, sediments naturally compact over time, causing the ground to sink. Tectonic plate movement can push land up or pull it down. And in regions that were once buried under ice age glaciers, the Earth’s crust is still rebounding upward now that the weight is gone, while areas at the edges of those former ice sheets are sinking as the mantle slowly adjusts. In places like parts of the U.S. Gulf Coast and Southeast Asia, this land sinking (subsidence) can make local sea level rise two or three times faster than the global average, even though the ocean itself is rising at the same rate everywhere nearby.
How Fast It’s Accelerating
Before 1990, global sea levels rose at about 1.1 millimeters per year. Since the early 1990s, satellite measurements show the rate has jumped to roughly 3.1 millimeters per year. That’s nearly a tripling. The long-term expected rate based on satellite data since the early 1990s now sits at about 4.4 millimeters per year, reflecting continued acceleration. Short-term fluctuations still happen: La Niña events, for instance, temporarily store more water on land and can slow the rise for a year or two. But the underlying trend keeps climbing.
Projected Rise by 2100
How much the seas ultimately rise depends on how much more greenhouse gas enters the atmosphere. Under a very low emissions scenario, the median projection puts global sea levels about 0.38 meters (roughly 15 inches) higher by 2100 compared to the 1995-2014 baseline. Under very high emissions, that median projection climbs to 0.77 meters (about 2.5 feet), with a plausible upper range exceeding 1 meter. Low-confidence but high-impact scenarios involving rapid Antarctic ice sheet collapse could push levels even higher, potentially reaching 1.6 meters under the worst case.
These numbers represent global averages. Many coastal cities will see more than the average because of local subsidence, ocean currents, and gravitational effects from ice loss. The difference between the low and high projections is not a matter of scientific uncertainty alone. It reflects choices about emissions that are still being made.