Sea level rise is the overall increase in the height of the world’s oceans. It is caused by a combination of factors, including thermal expansion (water expands as it warms) and melting of land-based ice such as glaciers and ice sheets. The ocean is absorbing more than 90 percent of the extra atmospheric heat added to our planet by human activities.
This extra heat is causing the global average sea level to rise faster than it did historically. This means that coastal flooding is expected to occur more frequently in many parts of the United States and around the world.
Rising sea levels threaten the homes and communities of millions of people, especially those in low-lying areas where flooding will be most severe. Unless action is taken, they will lose their livelihoods and the ability to live close to shore. They will also risk losing vital food sources, and their drinking water may become too salty to use.
The local rate of sea level rise can vary a lot. This is partly because the earth isn’t flat, and the pull of gravity acts differently in different places. Ocean currents and tidal fluctuations also affect how quickly the sea level changes. Scientists who track sea level change—using tide gauges, coastal buoys and satellite laser altimeters—try to account for these variations in order to hone in on long-term trends.
Temporary ups and downs in the sea level also happen a lot, and these come from things like daily tides and natural year-to-year weather changes such as El Nino. These wiggles are added on top of the global sea level rise and they can make it hard to see longer-term trends.