The rise in global mean sea level (GMSL) is a critical indicator of climate change. The Hong Kong Polytechnic University (PolyU) researchers have utilised advanced space geodetic technologies to deliver the first precise 30-year (1993-2022) record of global ocean mass change (also known as barystatic sea level), revealing its dominant role in driving GMSL rise. Their research further indicates that GMSL has been increasing at an average rate of approximately 3.3 mm per year with a notable acceleration observed, highlighting the growing severity of climate change. The research findings have been published in the Proceedings of the National Academy of Sciences.
GMSL is primarily driven by two factors: the thermal expansion of seawater — as the oceans absorb around 90% of the excess heat in the Earth's climate system — and the increase in global ocean mass, which is mainly caused by the influx of freshwater from melting land ice. Therefore, long-term monitoring of global ocean mass change is essential for understanding present-day GMSL rise.
A research team led by Prof. Jianli CHEN, Chair Professor of Space Geodesy and Earth Sciences of the PolyU Department of Land Surveying and Geo-Informatics (LSGI) and a core member of the PolyU Research Institute for Land and Space, together with Dr Yufeng NIE, Research Assistant Professor of LSGI and the lead and corresponding author of the research, has, for the first time, provided direct observations of global ocean mass estimates between 1993 and 2022 by utilising time-variable gravity field data derived from satellite laser ranging (SLR).
In the past, scientists have relied on long-term observations from satellite altimetry to project sea-level rise. Barystatic sea level records based on satellite gravimetry only became available with the launch of the Gravity Recovery and Climate Experiment in 2002. SLR is a traditional space geodetic technique used to accurately measure the distance between satellites and ground stations via laser ranging. However, fundamental constraints of SLR, such as the limited number of satellites and ground stations, the high altitude of the satellites (which means SLR-derived gravitational changes capture only the longest wavelengths) and the low-degree gravitational measurements, have restricted its direct application in estimating ocean mass change.
To effectively utilise SLR-derived gravitational fields for accurate estimates of ocean mass change, the research team implemented an innovative forward modelling technique that tackles spatial resolution limitations by incorporating detailed geographic information of ocean-land boundaries. This approach enables long-term monitoring of global ocean mass changes.
The research revealed that an increased rate of GMSL resulted in a global average sea-level rise of approximately 90 mm between 1993 and 2022, with about 60% of this rise attributable to ocean mass increase. Since around 2005, the rise in GMSL has been primarily driven by the rapid increase in global ocean mass. This overall increase is largely driven by the accelerated melting of land ice, particularly in Greenland. Throughout the entire study period, land ice melt from polar ice sheets and mountain glaciers accounted for over 80% of the total increase in global ocean mass.
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 said, "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."
