The increasing frequency of once-in-a-decade agricultural and ecological droughts has prompted a research team from the Department of Land Surveying and Geo-informatics of PolyU to collaborate with international experts on a comprehensive study of hydrological changes. The team analysed land water storage changes over the past 40 years using advanced space geodetic observation technology and global hydrological data. This approach revealed a rapid decline in global soil moisture, contributing to substantial water flows into the oceans and rising sea levels.

Addressing the challenges of understanding continental-scale water storage variations requires the integration of models and observations across the atmosphere, hydrosphere, and lithosphere, especially since polar motion reflects mass redistribution within the Earth system. Previous difficulties in measuring groundwater and root zone moisture have limited insights in this area. To tackle this, Professor Chen Jianli from the PolyU Department of Land Surveying and Geo-informatics and a core member of the Research Institute for Land and Space, along with his team utilised satellite altimetry and gravity missions, including the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On, to facilitate continental-scale observations of water storage changes.

Their study introduced novel methods for estimating global soil moisture, enhancing the accuracy of models that assess soil moisture variations under climate change. Notably, the melting of Greenland’s ice sheet is recognised as the largest contributor to rising sea levels, adding about 0.8 mm annually. The research shows that from 2000 to 2002, global terrestrial water storage declined significantly, losing 1,614 billion tonnes – twice the amount contributed by Greenland’s melting ice – equating to a 4.5 mm rise in sea levels.

From 2003 to 2021, there was a notable decrease in global average soil moisture compared to the period between 1979 and 1999.

Moreover, global warming, changing rainfall patterns and increasing ocean temperatures are key factors for the abrupt decline in terrestrial water storage. This is supported by the ERA5-Land soil moisture data of the European Centre for Medium-Range Weather Forecasts, which reveals substantial losses in Africa, Asia, Europe, and South America. In regions with increasing agricultural irrigation, such as northeast China and the western United States, soil moisture may decline further. This highlights the urgent need for improved land surface models to achieve a comprehensive understanding of long-term changes in water storage.

Professor Chen noted, “Sea level changes and the Earth’s rotation serve as indicators of large-scale mass changes within the Earth system. Accurately measured sea level changes and variation in the Earth’s rotation provide a unique tool for monitoring large-scale mass changes in the global water cycle. By integrating multiple modern space geodetic observations, we can do a comprehensive analysis of the driving factors behind changes in terrestrial water storage and rising sea levels. This, in turn, provides reliable data for climate and Earth system science experts to further investigate drought issues, aiding authorities in formulating water resource management and climate change mitigation strategies to address new challenges posed by climate change.”