The Basics | Local Sea Level Change – U.S. Sea Level Change

Key Takeaways

Relative sea level is the height of the ocean compared to the land next to it. Local sea level is generally given as this relative value.
Local sea levels depend on changes in land height, ocean currents, and other factors in addition to changes in the global sea level.
In some places, like the Mid-Atlantic and Gulf Coast regions of the U.S., sea levels are rising faster than global sea level.
Impacts from temporary changes in sea level during high tides and storms are getting worse for areas where sea level is rising.

Global vs. Local Sea LevelGlobal Versus Local Sea Level

The global sea level is the average ocean height across the entire planet. It gives an overall picture of how sea levels are changing and how the ocean is responding to global warming. Local Sea Level refers to the sea level at a specific location. It is often considered synonomous with relative sea level, which is the height of sea level compared to the land next to it. The local sea level depends on many factors. Changes in land height, ocean currents, and other natural factors all play a role. When factored in, local sea level rise can differ from the global rise by 100% or more.

Sea Level Snippets

Even though there are regional variations, sea level has risen almost everywhere across the globe in the past three decades. From satellite measurements, we know that the sea level has risen in 98% of the ocean from 1993 to the present.
So much ice is being lost from the ice sheets that it is impacting the rotation, shape and gravity of Earth. Locations that are close to where the ice is melting actually feel less of the resulting sea level rise than locations further away.

Factors Affecting Local Sea LevelLand Height, Ocean Currents, and Other Factors Affecting Local Sea Level

The illustration shows causes of local sea level change. There are three houses on the left on top of the land. The land has arrows up and down that show vertical land motion. In the middle bottom are white wavy lines to show changes in ocean currents. In the middle top there are arrows pointing up with more blue water showing global sea level rise. On the middle right is a grayed out box that shows a cloud with white lines coming down for rain to highlight temporary changes from tides, storms, and other natural changes. On the right side of the image is an illustration of a melting glacier on land with a lower water level showing the movement of water from melting glaciers to the tropics. — Global sea levels are rising due to climate change. However, the height of the ocean locally also depends on other factors such as vertical land motion and changes in ocean currents. Additionally, Earth's gravity causes impacts from melting of ice sheets and glaciers that move water away from the glaciers and toward the tropics. Tides, storms, and other natural changes (e.g. El Niño) can also alter the local sea level temporarily, but these stack on to the background sea level height at the time of the event. Credit: Allie Braun/NASA JPL.

Over years, land can move up or down. This land movement is called vertical land motion. The land moves up and down from natural causes, such as tectonic plates moving or sediment settling, and human causes, like pumping out groundwater or removing fossil fuels from the ground causing land to sink.

When the land next to the ocean sinks, it has the apparent effect of the sea level going up faster in that area. This movement of land downward is called subsidence. Conversely, when the land rises, it can cause the sea level relative to land to stay constant or go down. When land rises, it is referred to as uplift. Vertical land motion encompasses both subsidence and uplift.

Changes to ocean currents from climate change can also play a role in local sea level rise over time. Ocean currents are like rivers in the sea and help transport heat around the planet. Warmer water and added freshwater from rapidly melting ice sheets to the ocean impact the ocean currents causing them to shift their speed or location slightly. These shifts can then impact the sea levels in a particular area. These changes are part of what is called sterodynamic sea level change. This is the combined effect on sea level from changes in the ocean’s temperature, saltiness, and motion.

The map of the U.S. shows the North American continent along with the Caribbean islands and Hawaii with arrows placed over different coastal regions of the U.S. The arrows are divided into three colors, blue for sterodynamic change, tan for vertical land motion, and light blue for land ice melt. There is also a circle with a number in it next to the region label noting the change in sea level since 1990 in inches. The Southern Alaska region is dominated by vertical land motion. The Eastern and Gulf Coasts of the U.S. have more contribution from sterodynamic change, with the Western Gulf and Northeast Coast also having a large contribution from vertical land motion. The Southwest Coast and Caribbean Islands have the largest contribution from land ice melt. The Northwest Coast and Pacific Islands have fairly even contributions from all three. — Global sea level rise is not impacting regions across the U.S. the same everywhere. Rather, it depends on local factors like vertical land motion and how changes in ocean warming and currents, known as sterodynamic sea level change, are influencing sea level changes along the coastline. The total amount of sea level change for the region since 1990 is in the circle. The arrows are colored to show how much sterodynamic change (blue), vertical land motion (tan), and land ice melt (light blue) have contributed to sea level change in that region since 1993. The more of that color, the larger the contribution it has had on sea level change. The arrow’s size is smaller for regions with less change and larger for regions with more change. Credi: Allie Braun/NASA JPL.

These local and regional changes impact how fast sea levels are rising in that area. That’s because these happen in addition to global sea level rise. So if the land is sinking next to rapidly warming waters, that local sea level will often rise faster than the global sea level. In the United States, local sea levels are rising faster in the Gulf Coast and Mid-Atlantic regions in particular.

Over hours or days, tides and weather also make local sea levels rise and fall temporarily. These wiggles happen on top of the starting sea level height. For many areas, as the starting sea level rises each year due to climate change, the impacts from tides and storms are getting worse.

Dive Deeper: More about the factors that cause sea level to change locally.

Sea Level 101

What Causes Local Sea Level Change?

Global vs. Local Sea LevelGlobal Versus Local Sea Level

Factors Affecting Local Sea LevelLand Height, Ocean Currents, and Other Factors Affecting Local Sea Level

Next: How do we measure sea level change?