Mean sea level
Mean sea level (MSL) refers to the average height of the ocean's surface, providing a reference for measuring elevation and depth. Unlike local sea level changes caused by tides, storms, or atmospheric pressure, MSL represents a more stable global average, typically assessed over extended periods. It is influenced by the amount of water stored in glaciers and ice sheets, which is affected by global temperatures. Rising global temperatures lead to melting ice and contribute to an increase in MSL, with significant implications for low-lying coastal regions. For instance, nations like the Maldives, which have minimal elevation above sea level, face severe risks of land loss due to rising waters. Additionally, as MSL increases, coastal areas experience heightened vulnerability to flooding, especially during storms or high tides. Recent measurements indicate that MSL has been rising at an average rate of approximately 3.1 millimeters per year, with projections warning of further acceleration, potentially endangering millions of people living in vulnerable coastal areas. Understanding MSL is crucial for assessing climate change impacts and planning for sea-level rise mitigation.
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Subject Terms
Mean sea level
Definition
Sea level is the height of the surface of the ocean at any given location. Sea level is highly variable and can undergo very rapid changes due to such events as tides, tsunamis, changes in barometric pressure, wind-generated waves, and even freshwater floods. While these events can produce changes in sea level of several meters, they are local in scale and of a very short duration, generally lasting only for hours. Mean sea level is the average, global height of the sea surface, independent of these local, short-term changes. Changes in mean sea level are on the order of a few millimeters per year.
Mean sea level at specific locations can be calculated using tide gauge records and subtracting the effects of annual changes in atmospheric pressure and long-term changes in tidal ranges, which are driven by astronomical factors. Changes in global mean sea level can be calculated using satellite-based radar altimetry, such as with the TOPEX/Poseidon satellite. The radar altimeter measures the height of the satellite above the ocean, based on the time it takes for a radio signal to travel from the satellite to the sea surface and back. Since the actual altitude of the satellite is known, any changes in the altimeter measurement reflect changes in the height of the sea surface itself.
Significance for Climate Change
On short timescales (decades to centuries), mean sea level is a function of the amount of water stored as ice in glaciers and ice sheets. As global temperature rises, less water is stored as ice, contributing to a rise in mean sea level. A rise in mean sea level in response to global warming has important societal consequences. First, such a rise contributes to a loss of land, as coastal areas are slowly inundated by water. This is a concern for certain low-lying island nations, such as the Maldives or Tuvalu. The Maldives is a nation made up of twelve hundred islands in the Indian Ocean, which has a maximum elevation of only 2.5 meters above current sea level. Thus, the Maldivian population of approximately 380,000 people is highly vulnerable to even a slow rise in mean sea level. For other nations, a rise in mean sea level is also a concern because of increased hazards from flooding during high tides—especially spring tides—and storms. A rise in mean sea level provides a higher upon which tidal fluctuations build. According to the Fourth Assessment Report released by the Intergovernmental Panel on Climate Change, from 1993 to 2003, mean sea level rose approximately 3.1 millimeters per year. Sea levels continued to rise, reaching over 10 centimeters above 1993 levels by 2024. Scientists warned that despite the world's efforts to reduce carbon emissions, global sea level rise was accelerating. If this trend continues, more than 900 million people living in low-lying coastal areas will be placed in environmental danger.
"Sea Level Rise: Everything You Need to Know." World Economic Forum, 20 Sept. 2024, www.weforum.org/stories/2024/09/rising-sea-levels-global-threat/. Accessed 20 Dec. 2024.