How is sea level determined
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Determining Sea Level: Methods and Challenges
Introduction to Sea Level Determination
Sea level is a critical measure in understanding climate change, coastal dynamics, and oceanography. It is defined as the local height of the ocean's surface, typically measured relative to a tidal datum that averages out high-frequency motions such as wind waves and periodic tidal changes . This article explores the various methods and challenges in determining sea level, drawing insights from recent research.
Sea Level Indicators and Relative Sea Level
Biological and Geomorphological Indicators
Sea-level indicators are essential for determining relative sea-level changes at specific locations. These indicators include biological, geomorphological, sedimentological, and chemical features found on coral reefs and other coastal environments. By locating these indicators and determining their age and elevation relative to modern counterparts, scientists can establish historical sea-level changes .
Tidal Datum and Local Fluctuations
The local sea level fluctuates due to tides, wind, currents, water temperatures, salinities, and atmospheric pressure. Relative sea level is the elevation of the sea surface relative to the land at a given location, influenced by both global and regional factors such as ice cap melting, thermal expansion, and tectonic activities .
Numerical Models and Statistical Analysis
Extreme Sea Levels and Tide-Surge Interactions
Numerical models are used to simulate tides and surges, providing data for statistical analysis. For instance, extreme sea levels along the Iberian Atlantic coast were determined using a numerical model, which revealed the importance of local effects and the increase of extreme sea levels from south to north . These models help in understanding the interactions between tides and surges, although such interactions were found to be minimal in the study region .
Empirical Orthogonal Functions (EOFs)
Another approach involves using Empirical Orthogonal Functions (EOFs) derived from global data, such as sea-surface heights from satellite altimetry or numerical ocean models. This method helps reconstruct historical sea levels, although it is sensitive to the adopted set of EOFs, especially for sparse tide-gauge networks .
Satellite Altimetry and GPS Measurements
Radar Altimetry in the Arctic
Satellite altimetry has been crucial in measuring sea level, particularly in challenging regions like the Arctic. The European Space Agency's Climate Change Initiative has provided a sea level anomaly record based on radar satellite altimetry data, showing a sea level rise of 1.54 mm/year from 1991 to 2018 . This method involves updated geophysical corrections and a combination of altimeter data to ensure accuracy .
GPS and Geoid Measurements
GPS measurements combined with high-precision geoid models offer another method for determining mean sea level. This approach helps resolve discrepancies between geodetic and oceanographic methods, providing an independent determination of geoidal height differences .
Meteorological and Geological Influences
Atmospheric Centers of Action (COAs)
Meteorological factors, such as the North Atlantic Oscillation (NAO), significantly influence sea level variability and trends. Dimensional indices of the position and intensity of COAs correlate with sea level changes, affecting winds, pressure, and sea-surface temperatures .
CO2 and Long-Term Sea Level
On geological timescales, the volume of ice stored on land, influenced by atmospheric CO2 levels, largely determines global sea level. A well-defined relationship between CO2 and sea level suggests that long-term sea-level rise can be predicted based on CO2 concentrations, with significant implications for future sea-level projections .
Conclusion
Determining sea level involves a combination of biological indicators, numerical models, satellite altimetry, GPS measurements, and an understanding of meteorological and geological influences. Each method has its challenges and limitations, but together they provide a comprehensive picture of sea-level changes and trends. Understanding these methods is crucial for predicting future sea levels and mitigating the impacts of climate change.
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