Rising global mean sea level (GMSL) is one of the clearest signs of a warming planet. Researchers at The Hong Kong Polytechnic University (PolyU) have produced the first highly precise 30-year (1993-2022) record of changes in global ocean mass, also known as barystatic sea level. Their findings show that the growing mass of the oceans has played a leading role in pushing sea levels higher. Over this period, GMSL increased at an average rate of about 3.3 mm per year, and the rate itself has been speeding up, underscoring the intensifying impact of climate change. The study was published in the Proceedings of the National Academy of Sciences.
Sea level rise is mainly driven by two processes. One is the expansion of seawater as it warms, since the oceans absorb roughly 90% of the excess heat trapped in Earth’s climate system. The other is the addition of water from melting land ice, which increases the total mass of the oceans. Tracking long-term changes in ocean mass is therefore essential for understanding why sea levels are rising today.
First Direct 30-Year Record of Ocean Mass Change
The research team was led by Prof. Jianli Chen, Chair Professor of Space Geodesy and Earth Sciences in the PolyU Department of Land Surveying and Geo-Informatics (LSGI) and a core member of the PolyU Research Institute for Land and Space. Dr. Yufeng Nie, Research Assistant Professor of LSGI, served as the lead and corresponding author. Together, they provided the first direct estimates of global ocean mass change spanning 1993 to 2022 using time-variable gravity field data collected through satellite laser ranging (SLR).
Previously, projections of sea level rise relied heavily on satellite altimetry, which measures the height of the ocean surface. Detailed records of barystatic sea level based on satellite gravimetry only became available after the launch of the Gravity Recovery and Climate Experiment in 2002. SLR, a long-established space geodetic method, works by firing laser pulses between satellites and ground stations to measure distances with high precision. However, its use in studying ocean mass has been limited because of technical challenges. These include the small number of satellites and tracking stations, the high orbital altitude of the satellites, which restricts detection to broad gravitational patterns, and the relatively low resolution of the gravitational measurements.
Innovative Modeling Unlocks Satellite Potential
To overcome these obstacles, the team developed a forward modeling approach that improves spatial resolution by incorporating detailed information about the boundaries between oceans and land. This method allows SLR-based gravity data to be used more effectively for calculating ocean mass changes over long periods.
Using this approach, the researchers determined that global sea level rose by about 90 mm between 1993 and 2022. Roughly 60% of that increase was caused by the growing mass of the oceans. Since around 2005, rising ocean mass has been the main factor driving higher sea levels. The primary source of this added water is accelerated melting of land ice, especially from Greenland. Over the full study period, melting from polar ice sheets and mountain glaciers contributed more than 80% of the total increase in global ocean mass.
Implications for Climate Models and Future Projections
Prof. Jianli Chen said, “In recent decades, climate warming has led to accelerated land ice loss, which has played an increasingly dominant role in driving global sea-level rise. Our research enables the direct quantification of global ocean mass increase and provides a comprehensive assessment of its long-term impact on sea-level budget. This offers crucial data for validating coupled climate models used to project future sea-level rise scenarios.”
Dr. Yufeng Nie added, “The research showed that the ocean mass changes derived from SLR analysis align well with the total sea level changes observed by satellite altimeters, after accounting for the effect of ocean thermal expansion. This demonstrates that the traditional SLR technique can now serve as a novel and powerful tool for long-term climate change studies.”
