What is mean sea level
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Understanding Mean Sea Level: Definitions, Measurements, and Implications
What is Mean Sea Level?
Mean sea level (MSL) is a critical concept in both geographic and geologic literature, serving as the universal datum from which land heights and sea depths are measured. It is defined as the average height of the ocean's surface, measured over a long period, and is used as a reference point for elevations on land and depths in the sea .
Definitions and Interpretations of Mean Sea Level
Geodetic Mean Sea Level
Geodetic mean sea level is a hypothetical surface that represents the ocean's surface if it were undisturbed by external forces such as tides, winds, and atmospheric pressure variations. This definition is particularly useful for geodesists who study the Earth's shape and use this level to spread precise level nets over land. However, this idealized concept does not account for the persistent effects of air movements and non-uniform barometric pressure, which can introduce significant variations.
Contemporary Mean Sea Level
Contemporary mean sea level can be interpreted in two ways: as a period mean at a specific location, which is relatively easy to measure but may not be constant due to long-term cyclic trends, or as a physical surface that would exist if all disturbing forces were absent. This dual interpretation highlights the complexity and variability inherent in measuring and understanding mean sea level.
Measuring Mean Sea Level
Tide Gauges and Satellite Altimetry
Historically, tide gauges have been the primary tools for measuring sea level. These instruments track local sea level changes, which can then be averaged to estimate global mean sea level (GMSL). However, tide gauge records are often unevenly distributed and temporally limited, leading to significant uncertainties in early GMSL estimates .
With the advent of satellite altimetry in the early 1990s, more accurate and consistent measurements of GMSL have become possible. Satellite altimetry provides a global perspective, allowing for the detection of sea level changes with high precision. Recent studies using satellite data have reported an average GMSL rise of approximately 3.1 mm per year since 1993, with an acceleration of 0.1 mm per year .
Factors Influencing Mean Sea Level
Several factors contribute to changes in mean sea level, including:
- Thermal Expansion: As ocean water warms, it expands, contributing to sea level rise .
- Melting of Glaciers and Ice Sheets: The melting of mountain glaciers and polar ice sheets adds water to the oceans, further raising sea levels .
- Land Water Storage: Changes in terrestrial water storage, such as groundwater extraction and reservoir storage, can also impact sea levels, though this component remains highly uncertain .
Implications of Mean Sea Level Changes
Climate Change Indicator
Global mean sea level is a sensitive indicator of climate change. Rising sea levels are primarily driven by global warming, which causes thermal expansion of seawater and melting of ice masses. These changes have significant implications for coastal regions, including increased flooding, erosion, and habitat loss .
Regional Variability
While GMSL provides a useful global metric, local sea level changes can vary significantly due to factors such as vertical land movements, ocean currents, and regional climate patterns. For instance, some regions may experience higher rates of sea level rise due to land subsidence or specific oceanographic conditions.
Conclusion
Mean sea level is a fundamental concept in understanding our planet's geography and the impacts of climate change. While the definition of mean sea level may seem straightforward, its accurate measurement and interpretation involve complex considerations of various physical factors. Advances in satellite altimetry and improved understanding of contributing factors have enhanced our ability to monitor and predict sea level changes, providing crucial insights for climate research and coastal management.
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