Key takeaways
- Local sea level can rise much faster or slower than the global average.
- Sinking land, shifting ocean currents and the gravity of melting ice sheets all play a part.
- Planning decisions should be based on relative sea level at the coast in question.
No two coastlines are the same
Global sea level is an average. Along any particular coast, the sea can rise faster or slower than that average, and in a few places it is even falling. Relative sea level, the height of the sea measured against the land, is what matters for flooding, erosion and planning.
Three big reasons local sea level differs
1. The land is moving
In many places the land itself is sinking, a process called subsidence. Pumping groundwater, oil and gas compacts the sediments below cities such as Houston, New Orleans, Jakarta and Manila. River deltas sink naturally as their sediments settle. Elsewhere, land is still rising or sinking in response to the melting of the great ice sheets of the last ice age, which ended about 20,000 years ago. Parts of Alaska and Scandinavia are rising so quickly that local sea level is falling, while the Chesapeake Bay region is sinking.
2. The ocean is not flat
Winds, currents and differences in water temperature and salinity tilt the sea surface by up to a meter or more across ocean basins. When currents such as the Gulf Stream slow down or shift, water piles up against the coast. Stretches of the U.S. Gulf and Southeast coasts saw sea level rise two to three times faster than the global average in the 2010s because of changes like these.
3. Gravity and melting ice
Ice sheets are so massive that they pull ocean water toward them. As an ice sheet melts, that pull weakens and the nearby sea falls, while sea level rises more than average far away. Melting in Greenland therefore raises sea level more in the Southern Hemisphere, and melting in Antarctica raises it more along the coasts of North America.
Putting the pieces together
Scientists describe relative sea level change at a location as the sum of a few terms: the change in the height of the sea surface (from ocean warming, added water, currents and gravitational effects) minus the change in the height of the land (vertical land motion). Satellites measure the first term offshore. GPS stations and satellite radar interferometry measure the second. Tide gauges measure the difference directly.
Vertical land motion
Land motion is often the hardest piece to pin down because it can vary over short distances and change over time. A tide gauge on a pier built on fill may sink faster than a GPS antenna on bedrock a few kilometers away. Satellite radar interferometry now maps land motion across entire coastlines at high resolution, revealing hot spots of subsidence that tide gauges miss.
Ocean dynamics
Year to year and decade to decade changes in winds and currents cause regional sea level to swing above and below the long term trend. Along the U.S. West Coast, El Niño raises sea level by 10 to 30 cm for months at a time. In the western tropical Pacific, strong trade winds in the 2000s and 2010s drove regional rise of more than 10 mm per year. These swings matter for planning because they can bring decades of rise in just a few years.
Sea level fingerprints
Every source of melting ice leaves a distinctive spatial pattern in sea level change, called a fingerprint, set by changes in Earth's gravity field, its rotation and the elastic response of the solid Earth. Fingerprints let scientists identify which ice sheets are contributing to rise at a given place, and they are built into modern projections such as those in the IPCC Sixth Assessment Report.