Sea level rise is stressing coastal ecosystems and contaminating freshwater aquifers that support municipal and agricultural water supplies, natural ecosystems, and recreational activities. It also puts a burden on people living in low-lying areas by making storm surges more severe.
There are two main reasons sea levels are rising: melting ice and thermal expansion of ocean water. Warmer temperatures cause ice on land like glaciers and ice sheets to melt. Then, the melting ice flows into the ocean and raises sea level. Warmer water also expands more than cool water does, which adds to sea level.
The rate of sea level rise varies around the world, and is influenced by factors such as natural cycles and local conditions. But overall, the trend is upward.
In addition to tide gauge measurements, scientists also use satellite laser altimeters (to measure sea surface height) and direct measurement of glacier and ice sheet changes. They combine these data with mathematical models of how the Earth’s gravity affects sea level, and how it is influenced by temperature, ocean currents, tides, and storms.
Scientists use this knowledge to estimate future sea level changes based on various business as usual emissions scenarios. These estimates are called “scenarios.” The Intergovernmental Panel on Climate Change’s Fourth Assessment Report (AR4) summarized the range of possible future scenarios for global mean sea level change. For the B1 scenario with the lowest temperature increase, it projects a 18-38 cm sea level rise by 2100.