Sharp depletion in soil moisture drives land water to flow into oceans, contributing to sea level rise
Since polar motion reflects mass redistribution within the Earth system, integrating models and observations across the atmosphere, hydrosphere and lithosphere is crucial. However, previous challenges in measuring terrestrial water storage, particularly groundwater and root zone soil moisture, limited understanding of hydrological depletion at continental scales. Prof. Jianli CHEN, Professor of the PolyU Department of Land Surveying and Geo-informatics and core member of the Research Institute for Land and Space and the international team employed satellite altimetry and gravity missions, including the Gravity Recovery and Satellite Experiment (GRACE), and GRACE Follow-On, to enable continental-scale observations of terrestrial water storage variations. By integrating this with global mean sea levels and polar motion data, the team has explored terrestrial water storage depletion patterns. Notably, this study introduced novel methods for estimating global soil moisture, which improves the accuracy of continental and global scale modeling to enable a more effective understanding of soil moisture variations under climate change.
The melting of Greenland's ice sheet is recognised as the largest single contributor to the rise in global sea levels, adding approximately 0.8mm annually. This study reveals that between 2000 and 2002, the global terrestrial water storage significantly declined, with a total of 1,614 billion tons of water lost to the oceans, which is twice as much as resulting from the current melting of Greenland ice, and equivalent to a 4.5mm rise in sea levels. Since then, the rapid loss of terrestrial water storage has been followed by a more gradual but continuous depletion, with no signs of recovery.
In addition, compared to the period from 1979 to 1999, a notable decline in global average soil moisture was observed from 2003 to 2021. Between 2003 and 2011, the Earth's pole shifted 58cm toward 93° East Longitude, demonstrating that the continued decline in soil moisture is leading to a reduction in terrestrial water storage.
The team also pointed out that precipitation deficits and stable evapotranspiration caused by global warming, changing rainfall patterns and increasing ocean temperatures are likely the key factors for the abrupt decline in terrestrial water storage. The ERA5-Land soil moisture data of the European Centre for Medium-Range Weather Forecasts' corroborates these findings, showing substantial terrestrial water storage losses in Africa, Asia, Europe, and South America. In Asia and Europe, the affected areas expanded from northeastern Asia and eastern Europe to broader regions across East and Central Asia, as well as Central Europe, following the sharp water storage depletion observed between 2000 and 2002. Read More Evidence of geothermal activity within icy dwarf planets
With increasing agricultural irrigation in regions such as northeast China and the western United States, and global greening, soil moisture may further diminish in semi-arid areas with intensive agriculture and high levels of greening. The team suggests the need for improved land surface models which consider these factors for a more comprehensive understanding of long- term changes in terrestrial water storage.
Prof. Jianli Chen said, 'Sea level change and Earth rotation serve as indicators of large-scale mass changes in the Earth system. Accurately measured sea level change and variation in Earth rotation provide a unique tool for monitoring large-scale mass changes in the global water cycle. By integrating multiple modern space geodetic observations, it enables comprehensive analysis of the driving factors behind changes in terrestrial water storage and sea level rise. 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.'
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