Dive Deeper | How We Measure – Global Sea Level Change

What tools are used to measure sea level?

Measuring sea level involves a variety of instruments and methodologies. A combination of satellite and ground-based measurements is essential for accurate and comprehensive monitoring of sea level. Each instrument provides unique data that, when integrated, offer a complete view of sea level trends and the contributing factors. This multi-faceted approach enables scientists to better understand and predict the impacts of sea level rise on global and regional scales. Here’s an overview of the primary instruments used to measure sea level:

Tide Gauges1. Tide Gauges

Tide Gauge Coverage — Tide gauges provide coverage at specific locations along the global coastlines. These tide gauges make measurements every day and some return measurements of sea level every minute. Each blue dot indicates a tide gauge that has made a measurement in the past 10 years.

Description: Tide gauges are coastal instruments that measure the change in sea level relative to a fixed point on land.
Process: They typically consist of a stilling well and a float, or a pressure sensor that detects the water level. Modern tide gauges are often equipped with radar or acoustic sensors.
Data Collection: Tide gauges provide continuous measurements and are vital for both measuring short-term changes and long-term sea level records, with some datasets extending over a century.
Strengths: Provide long records with frequent sampling, and measure relative sea level change.
Limitations: There are gaps between tide gauge records, particularly in the Global South limiting effectiveness for measuring sea level globally.

Satellite Altimetry2. Satellite Altimetry

Satellite Altimetry Coverage — Satellite altimeters like the Sentinel-6A Michael Freilich launched in 2022 orbits over Earth at an altitude of over 800 miles. It repeats its ground track, shown by the blue lines in image, every 10 days. This allows the satellite to sample the sea surface uniformly, providing a consistent record of the sea surface height over time.

Description: Satellite altimeters measure the distance between the satellite and the sea surface, providing global sea level data.
Process: Satellites, such as TOPEX/Poseidon, Jason-1/2/3, and Sentinel-6, use radar altimeters to send microwave pulses to the ocean surface and measure the time it takes for the pulses to return.
Data Collection: Satellite altimetry offers high-precision sea level measurements over the open ocean, contributing to global sea level rise assessments.
Strengths: Makes measurements of the whole ocean with an increasingly long record and measures absolute sea level change.
Limitations: Record has just surpassed 30 years in length and doesn't measure day-to-day changes in sea level.

Global Navigation Satellite System3. GNSS (Global Navigation Satellite System)

GPS Coverage of Coastlines — GNSS stations provide coverage throughout the world and cover much of the world's coastlines. Shown here are the stations with at least 3 years of data. These stations provide measurements of the movement of land every day.

Description: GNSS stations, including GPS, are used to measure land movements which help in understanding relative sea level changes.
Process: These stations provide precise measurements of vertical and horizontal land movements, essential for correcting tide gauge data for land subsidence or uplift.
Data Collection: GNSS data is crucial for differentiating between absolute sea level rise and relative changes due to land movement.
Strengths: Provides measurements of an important component of relative sea level rise.
Limitations: Records are still very short - generally on the order of a decade - and can have gaps along coastlines.

GRACE and GRACE-FO4. GRACE and GRACE-FO Satellites

Coverage of GRACE-FO — The GRACE-FO pair of satellites covers the entire globe every month. GRACE-FO measures the water mass changes both in the ocean and on land. This allows us to track changes in the ice sheets, how much water is in the ocean, and the amount of water stored on land, among other things.

Description: GRACE (Gravity Recovery and Climate Experiment) and GRACE Follow-On (FO) satellites measure variations in Earth's gravity field, which can be used to infer changes in sea level due to mass redistribution.
Process: By tracking the distance between twin satellites, GRACE detects changes in gravitational pull caused by mass changes, such as melting ice sheets and changes in ocean mass.
Data Collection: GRACE data complements other sea level measurements by providing insights into the contributions of ice melt and land water storage.
Strengths: Provides measurements of an important component of sea level rise, monitors the melting ice sheets and glaciers, tracks the movement of water globally.
Limitations: Record has just extended past two decades, and data only covers relatively large regions.

Argo Floats5. Argo Floats

Coverage_of_Argo_Profiling_Floats — The Argo profile floats cover much of the ocean and each float makes a measurement of the upper layers of the ocean about once every ten days. For about nine days, the Argo floats drift along with deep ocean currents, and then come to the surface on the tenth day, taking measurements along the way. Each blue dot is a float that returned data during a 10-day period in August 2024.

Description: Argo floats are autonomous, drifting instruments that measure temperature and salinity of the upper 2000 meters of the ocean. Some floats now reach depths beyond 2000 meters.
Process: These floats periodically dive to specific depths and then return to the surface, transmitting their data via satellite.
Data Collection: Argo data helps in understanding the thermal expansion component of sea level rise, which is a major factor in global sea level change.
Strengths: Provides measurements of an important component of relative sea level rise, allows for the direct assessment of ocean warming, and has near-global coverage.
Limitations: Records have not yet reached two decades in length and most floats do not yet reach depths beyond 2000 meters.

Next: What is the future of sea level change?