The increasing frequency of once-in-a-decade droughts across the globe has prompted urgent scientific inquiry into hydrological changes and their broader ecological impacts. A pioneering study led by the Department of Land Surveying and Geo-Informatics at The Hong Kong Polytechnic University (PolyU), in collaboration with international researchers, has unveiled a worrying trend: rapid global soil moisture depletion is significantly contributing to rising sea levels.
Published in the prestigious journal Science, the study marks a milestone in hydrological research by using space geodetic observation technology and advanced global hydrological change data to track land water storage changes over the past 40 years. The research provides fresh insights into the large-scale redistribution of water within the Earth system and the drivers of terrestrial water storage loss.
New Techniques, Alarming Findings
Led by Professor Jianli Chen, a core member of the PolyU Research Institute for Land and Space, the research team utilized data from satellite gravity and altimetry missions, including the Gravity Recovery and Climate Experiment (GRACE) and its successor GRACE Follow-On. By integrating these with global mean sea level measurements and polar motion data, the team was able to observe terrestrial water storage variations at continental scales—something previously limited due to difficulties in measuring groundwater and root zone soil moisture.
One of the study’s key innovations is a novel method for estimating global soil moisture, significantly improving the accuracy of continental and global-scale hydrological models. The data reveals a stark picture: a substantial and ongoing loss of land-based water storage that has had a measurable impact on sea level rise.
Between 2000 and 2002, the Earth lost approximately 1,614 billion tons of water from terrestrial reservoirs to the oceans, resulting in a 4.5mm rise in sea levels. This is more than double the contribution from the current melting of Greenland’s ice sheet, which adds about 0.8mm annually. Since then, terrestrial water depletion has continued at a slower but steady pace, with no signs of reversal.
A Planet Losing Its Moisture
Compared to the period from 1979 to 1999, global average soil moisture dropped significantly between 2003 and 2021. This loss has affected not only agricultural productivity but also the Earth’s physical balance. From 2003 to 2011, the Earth’s rotational pole shifted approximately 58cm toward 93° East Longitude—an indicator of mass redistribution driven largely by water loss from land to ocean.
The researchers identified several contributing factors to this abrupt hydrological shift, including prolonged precipitation deficits, stable but high evapotranspiration rates, shifting rainfall patterns, and increasing ocean temperatures—all consequences of global warming. Analysis using ERA5-Land soil moisture data from the European Centre for Medium-Range Weather Forecasts further confirms large-scale water losses in Africa, Asia, Europe, and South America.
In Asia and Europe, particularly, regions impacted by severe soil moisture loss have expanded beyond their initial zones to encompass wider areas of East and Central Asia and Central Europe.
Agricultural Pressures and the Need for Better Models
With intensified agricultural irrigation in regions such as northeast China and the western United States, and a general trend toward global greening, soil moisture is likely to decline further—especially in semi-arid regions with high agricultural activity.
The team emphasizes the need to refine land surface models by incorporating such factors as irrigation and greening patterns to enhance predictions of long-term terrestrial water storage changes under evolving climate conditions.
A Tool for the Future
“Sea level change and Earth rotation serve as indicators of large-scale mass changes in the Earth system,” said Prof. Jianli Chen. “Accurately measured sea level changes and variations in Earth rotation provide a unique tool for monitoring global water cycle changes. By integrating multiple space geodetic observations, we can more effectively identify and understand the driving forces behind these changes.”
The findings serve not only as a wake-up call but also as a crucial resource for climate scientists and policymakers. The study offers critical data to support improved water resource management and the development of effective climate change mitigation strategies, as humanity faces growing challenges from both extreme drought and rising seas.
