Rising sea levels occur because of thermal expansion (warming oceans) and glacial melt. These two factors have combined to result in about 8 inches of sea level rise since 1880, half of which occurred in the past 30 years.
Sea levels have risen by 8 inches since 1880
Since 1880, sea levels have already risen by 8 inches because of climate change. Although eight inches may sound small, it is a large amount when considering the entire world's oceans.
The rate of sea level rise is increasing
Half of all the sea level rise since 1880 occurred in the past 30 years, showing scientists how oceans are rising even faster today. In fact, the rate is 30% greater than it was in 1992.
Scientists use satellite radar technology for extreme accuracy
Since 1993, scientists have measured sea levels with incredible precision using satellites which send a radar signal to determine the exact height of the oceans across the world.
What We Know
Global sea levels have been rising since 1900, as the graph below from NASA shows. Sea levels have risen by over 200 millimeters, or about eight inches, since 1900. Although this amount may seem small, it is taking place across the entire world's oceans, an area of over 139.7 million square miles.
Sea level rise is primarily caused by two factors: ice and glacial melt and thermal expansion.
When ice sheets and glaciers melt, they release new freshwater into the ocean. This added freshwater raises sea levels. If all glaciers and ice sheets in the world melted, it would raise global sea levels by 195 feet. While it is unlikely that all ice would melt, even just a small portion would have devastating consequences. Already, glaciers across the world and both the Antarctic and Greenland ice sheets have experienced major melting. Indeed, ice loss was the largest contributor to sea level rise in the most recent decades.
Thermal expansion is a scientific concept where water expands as it warms. Because another indicator of climate change is warming oceans, thermal expansion is happening. Approximately one-third of global sea level rise is because of thermal expansion.
You can read more about oceans warming here.
The rate of sea level rise is also increasing. Annual sea level rise is 30% higher than it was in 1992 when NASA first began studying sea level rise via satellites. However, it's important to note that sea level rise is not uniform across the world. Some areas have a significantly higher sea level rise rate than others, putting them at an even higher risk for dangerous flooding.
How We Know
NASA and the Intergovernmental Panel on Climate Change (IPCC) are the world's leading bodies on sea level rise. Scientists primarily rely on satellites using radar waves to determine sea level rise, combined with techniques to eliminate other variables, such as moisture in the air which could slow radar waves down.
To determine the causes of sea level rise, scientists use a similar method to track how much ice is lost very precisely. From that, they can calculate how much water melting glaciers and ice sheets added to the oceans. In addition to satellites, they also use airplane readings, ships, and supercomputers to assist in their data gathering.
Kulp, Scott A., and Benjamin H. Strauss. “New Elevation Data Triple Estimates of Global Vulnerability to Sea-Level Rise and Coastal Flooding.” Nature Communications 10 (October 1, 2019): 4844. https://doi.org/10.1038/s41467-019-12808-z.
Ramsayer, Kate. “Rising Waters: How NASA Is Monitoring Sea Level Rise.” National Aeronautics and Space Administration, 2020. http://nasa.gov/specials/sea-level-rise-2020.