Global mean sea level is rising mainly because ocean water expands as it warms and because glaciers and ice sheets on land are losing mass. Water stored on land and changes in ocean circulation add smaller contributions. The rise is not uniform: local sea level also depends on whether the land is sinking or rising, currents, winds and gravity.
Thermal expansion
As seawater warms, its density decreases and the same mass occupies more volume. The ocean has absorbed most of the excess heat trapped by rising greenhouse gases, so thermal expansion has been a major contributor to sea-level rise.
Melting land ice versus sea ice
Glaciers and the Greenland and Antarctic ice sheets sit on land. When they melt or flow into the sea, they add water to the ocean. Floating sea ice already displaces water, so its melting has little direct effect on sea level—similar to ice melting in a full glass. Sea-ice loss still has major climate and ecosystem effects.
Global and local sea level
Global mean sea level averages the ocean as a whole. Relative sea level is what a coast experiences compared with the adjacent land. Groundwater extraction, sediment compaction or tectonics can make land sink, increasing local rise. Uplift after the loss of ancient ice sheets can offset it in other places.
Changes in currents, wind and atmospheric pressure redistribute water. Ice loss also changes gravity and Earth’s rotation, creating regional patterns. A global average is therefore not a local forecast.
How sea level is measured
Tide gauges measure water relative to a fixed point on land and provide long local records. Satellite altimeters measure sea-surface height across most of the global ocean. GPS and other geodetic tools show how the land itself moves. Combining them separates ocean change from vertical land motion.
What are the effects?
Higher baseline water allows storm surge and high tides to reach farther inland. Consequences include more frequent coastal flooding, erosion, saltwater intrusion into aquifers and soils, wetland change and damage to infrastructure and cultural sites. Risk depends on elevation, waves, exposure, ecosystems and social vulnerability.
Adaptation and mitigation
Cutting greenhouse-gas emissions limits long-term rise. Communities can pair updated elevation and flood maps with early warning, raised or relocated infrastructure, risk-informed building, restored wetlands and dunes, and managed retreat where protection is not sustainable.
No single seawall solves every problem; hard structures can shift erosion or squeeze habitats. Planning should use local scenarios and involve affected communities. The restored melting glaciers and sea-level experiment offers an accessible starting point for learners.
Frequently asked questions
Will sea level rise the same amount everywhere?
No. Land motion, circulation, winds and the geography of ice loss produce regional differences.
Does melting an iceberg raise sea level?
Only minimally through salinity and density effects because floating ice already displaces nearly its mass in water. Melting land ice is the major direct contributor.