1. Tide Gauges: Tide gauges are instruments that directly measure the height of the water surface relative to a fixed reference point. They can be installed along coastlines or on offshore structures. Tide gauges provide continuous measurements of sea level variations, including high tides and low tides.
2. Satellite Altimetry: Satellite altimetry uses radar pulses from satellites to measure the distance between the satellite and the sea surface. By combining measurements from multiple satellite passes, scientists can generate global sea level maps and monitor changes in sea level over time. Satellite altimetry provides precise measurements of sea level across vast oceanic regions.
3. Global Positioning System (GPS): GPS technology can be utilized to measure sea level changes by employing tide gauges equipped with GPS receivers. The precise positioning capabilities of GPS allow for accurate measurements of the vertical position of the tide gauge relative to a global reference frame.
4. Laser Scanning: Airborne or terrestrial laser scanning systems can measure the elevation of the coastal land and sea surface. By comparing these measurements over time, changes in sea level can be determined. Laser scanning provides detailed topographic data and is particularly useful in coastal regions with complex topography.
5. Inverted Barometers: Inverted barometers, also known as bubbler gauges, measure sea level by determining the pressure exerted by the water column. A tube or pipe is submerged below the water surface, and air is pumped into the tube until the pressure equals the water pressure at that depth. The sea level is then calculated based on the air pressure and known density of water.
6. Acoustic Doppler Current Profilers (ADCPs): ADCPs are instruments that measure water currents using the Doppler effect of acoustic signals. By deploying ADCPs on the seafloor or on buoys, scientists can estimate sea level variations by measuring changes in the water pressure at a fixed depth.
7. Satellite Gravity Missions: Satellite missions such as the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) use precise measurements of Earth's gravity field to detect changes in sea level. These missions provide global estimates of sea level variations by observing changes in the Earth's gravitational field caused by the redistribution of water masses.
The choice of sea level measurement technique depends on the specific requirements of the study, such as the accuracy needed, the spatial and temporal resolution, and the environmental conditions in the area of interest. By combining data from multiple measurement techniques, scientists can obtain a comprehensive understanding of sea level changes and their implications for coastal ecosystems and human communities.
