Water levels across the planet are shifting, and the changes are accelerating. Global sea levels are currently rising at about 3.4 millimeters per year based on nearly 30 years of satellite measurements, roughly a third of an inch annually. That number has increased from about 2.5 millimeters per year in the early 1990s, and the 20th century’s rate of sea level rise was the fastest in at least 2,700 years. But the picture is more complex than a single global number suggests. Oceans, rivers, and lakes each respond to different forces, and water levels can rise or fall dramatically depending on where you are and what’s driving the change.
Why Global Sea Levels Are Rising
Two main forces push sea levels higher. The larger contributor is meltwater from land-based ice sheets and mountain glaciers, which adds roughly 2 millimeters per year to ocean volume. The second is thermal expansion: as ocean water warms, it physically expands and takes up more space, contributing about 1 millimeter per year. Together, these account for the observed 3.4 millimeter annual rise.
This may sound small, but it compounds over decades. Without human-caused warming, 20th-century sea levels would have changed very little, somewhere between a 3-centimeter drop and a 7-centimeter rise over the entire century. Instead, the rise was more than double the upper end of that natural range. Every projection for the 21st century shows faster rise than the 20th century delivered.
Projections Through 2100
How much seas rise from here depends largely on emissions. By 2050, the range is relatively narrow regardless of scenario: about 18 to 23 centimeters (7 to 9 inches) above the 1995 to 2014 baseline. The differences become stark by 2100. Under aggressive emissions cuts, the median projection is around 38 centimeters (about 15 inches). Under a high-emissions path, it reaches 77 centimeters (roughly 2.5 feet), with an upper range that could exceed 1 meter, or just over 3 feet.
Low-confidence scenarios that account for potential rapid ice sheet collapse push the upper bound even higher, up to 1.6 meters (over 5 feet) by 2100 in the worst case. These aren’t predictions so much as boundaries of what’s physically plausible if ice sheets behave in ways that are harder to model.
Why Water Levels Vary by Location
Global averages mask enormous local differences. What matters for any given coastline is “relative” sea level, which is the height of water measured against the land beneath it. If the land itself is sinking, residents experience faster rising water even if the ocean isn’t climbing any quicker in that region.
This land movement, called vertical land motion, happens for several reasons. One is glacial rebound: Earth’s crust is still adjusting to the retreat of massive ice sheets roughly 15,000 years ago. Areas that were pressed down by ice are slowly rising, while surrounding regions that had bulged outward are now sinking. The Chesapeake Bay area, for example, sinks about 2 millimeters per year from this effect alone, nearly doubling its rate of relative sea level rise compared to the global average.
Groundwater pumping is another major driver. When water is extracted from underground aquifers, the rock and sediment compact, and the land surface drops. Tectonic activity adds yet another layer of variability. Even small seismic events can cause rapid vertical shifts. These local factors can vary so much that flooding risk differs noticeably from one neighborhood to the next along the same coastline.
How Rivers and Lakes Change
Inland water levels follow a different set of rules. Lakes naturally fluctuate based on the balance between water coming in (rain, snowmelt, groundwater, streams) and water going out (evaporation, outflow, groundwater seepage). Human management adds a powerful overlay. Dams alter both the size and timing of water level swings, and irrigation and urban development divert water away from natural systems.
The effects of human water management vary by region. In the American West, water management activities are associated with larger drops in lake levels, with their impact on water loss more than twice as large as the effect of drought alone. In the Midwest, the opposite pattern holds: managed lakes tend to stay fuller and more stable, even during dry periods. Shallow lakes and those with less surface inflow lose a higher proportion of their water to evaporation, making them especially vulnerable during hot, dry stretches.
Rivers face a compounding problem from urbanization and climate change working together. Paved surfaces and buildings prevent rain from soaking into the ground, increasing runoff. Engineered drainage systems can create bottlenecks that trap floodwater. A study of Philadelphia’s Schuylkill River found that flood events once considered 1-in-50-year occurrences in the 1950s now happen statistically every 3 years. During Hurricane Ida in 2021, the river’s discharge hit nearly 100 times its average flow. The research identified a tipping point: when river flow exceeds a certain threshold, flooded area doesn’t increase gradually but jumps in a nonlinear surge, with an additional 2 to 7 percent increase in flooding when peak discharge coincides with high tide, and up to 15 percent more under projected sea level rise by 2100.
Saltwater Intrusion Into Freshwater
Rising sea levels don’t just flood coastlines. They push saltwater into underground freshwater supplies, a process called saltwater intrusion that has already affected many coastal aquifers across the United States. Since saltwater can’t be used for drinking or irrigation, this threatens the water supply for communities that depend on wells. Excessive groundwater pumping worsens the problem by lowering the freshwater table, making it easier for seawater to move inland underground.
Drought amplifies this further. In California’s Sacramento-San Joaquin Delta, reduced freshwater flowing from rivers and reservoirs allows more ocean saltwater to creep inland, affecting drinking water, agriculture, wildlife habitat, and the broader ecosystem. Managing these aquifers carefully, limiting over-pumping and maintaining sustainable withdrawal rates, is one of the few ways to slow the intrusion.
How Scientists Track Water Levels
Two primary tools measure sea level changes: tide gauges and satellite altimeters. Tide gauges are instruments anchored to coastlines that directly measure the water surface at specific points. Some have records spanning 75 years or more, providing the long historical baseline scientists rely on to identify trends. Their limitation is coverage: they only measure where they’re installed, mostly along developed coastlines.
Satellites solve the coverage problem. Since 1993, satellite altimeters have measured ocean height from space with accuracy down to the centimeter or millimeter level when averaged globally. They can map changes across virtually the entire ocean surface, revealing patterns that scattered coastal gauges would miss. The tradeoff is a shorter record, just under 30 years so far. The two methods complement each other: tide gauges help calibrate satellite data, and both independently confirm that sea level rise is accelerating.