The rise in sea levels is primarily driven by two key factors: the thermal expansion of seawater due to warming and the influx of freshwater from melting land ice. Understanding the long-term changes in ocean mass is crucial for grasping the current dynamics of sea level rise.
First Direct 30-Year Record of Ocean Mass Change
A research team led by Prof. Jianli Chen from the PolyU Department of Land Surveying and Geo-Informatics has made significant strides in this area. Their study provides the first direct estimates of global ocean mass changes from 1993 to 2022, utilizing advanced time-variable gravity field data obtained through satellite laser ranging (SLR).
Historically, sea level rise projections relied heavily on satellite altimetry, which measures ocean surface height. However, detailed records based on satellite gravimetry became available only after the launch of the Gravity Recovery and Climate Experiment in 2002. SLR, a well-established method, measures distances with high precision by firing laser beams between satellites and ground stations. Despite its potential, its application in ocean mass studies faced limitations due to technical challenges like a limited number of satellites and tracking stations, high orbital altitudes, and lower resolution gravitational measurements.
Innovative Modeling Unlocks Satellite Potential
To address these challenges, the research team developed a forward modeling technique that enhances spatial resolution by integrating detailed information about ocean and land boundaries. This innovative approach allows for more effective use of SLR-based gravity data in calculating long-term ocean mass changes.
The findings reveal that global sea levels rose approximately 90 mm between 1993 and 2022, with around 60% of this rise attributed to the increasing mass of the oceans. Since 2005, the primary driver of this increase has been accelerated melting of land ice, particularly from Greenland. Over the entire study period, melting from polar ice sheets and mountain glaciers accounted for over 80% of the total increase in ocean mass.
Implications for Climate Models and Future Projections
Prof. Chen emphasized that the accelerated loss of land ice due to climate warming has increasingly influenced global sea-level rise. This research enables direct quantification of ocean mass increase and offers a comprehensive view of its long-term effects on sea-level dynamics. Such data is vital for validating climate models that forecast future sea-level rise scenarios.
Dr. Yufeng Nie noted that the ocean mass changes derived from SLR analysis closely align with total sea level changes observed by satellite altimeters, once thermal expansion effects are considered. This indicates that traditional SLR techniques can now be a powerful tool for long-term climate change research.
