Greenland hosts the second-largest ice sheet on Earth, spanning an ice thickness of approximately three kilometers. A complete meltdown would trigger a global sea-level rise of about seven meters. However, key uncertainties remain concerning the processes of meltwater accumulation, storage, and eventual release. Leading this research effort, Prof. Jianli CHEN of PolyU's Department of Land Surveying and Geo-informatics, alongside experts from Hong Kong, Mainland China, the US, the Netherlands, and Belgium, examined the hydrological processes governing Greenland's meltwater storage to enhance predictions of future ice sheet melting and sea-level impacts.
A major challenge in assessing Greenland's hydrology has been the limitations of satellite gravimetry in conducting regional studies. To address this, the research team employed NASA's Gravity Recovery and Climate Experiment (GRACE) data in conjunction with the Greenland GPS Network (GNET), which consists of multiple Global Navigation Satellite System (GNSS) stations surrounding Greenland. This network provides continuous positioning data, essential for tracking vertical bedrock subsidence due to meltwater storage.
Between 2009 and 2015, the team analyzed GNSS data from 22 stations positioned near bedrock and glacier outlets. These observations enabled them to detect seasonal meltwater storage beneath the ice sheet and measure the associated elastic deformation of the bedrock. Additionally, GNSS data were utilized to monitor large-scale mass redistribution events related to climate change, including groundwater depletion and fluctuations in lake water storage.
Findings revealed that most meltwater generated during the summer is temporarily stored within the ice sheet, reaching its peak in July before gradually diminishing. This seasonal buffering effect results in a maximum subsidence of approximately 5mm in bedrock near GNSS stations. Notably, extreme melt events in 2010 and 2012 caused bedrock subsidence to increase significantly, reaching 12mm and 14mm, respectively.
The research further indicated that meltwater retention varies regionally, with an average storage duration of about eight weeks. Some areas, such as the northeast and west, exhibit prolonged retention of around nine weeks, whereas regions in the south and southeast show shorter storage periods of approximately 4.5 weeks. As Arctic warming intensifies, predicting meltwater storage behavior becomes increasingly vital for refining sea-level rise projections. The study also suggests that current climate models may be underestimating snowmelt runoff or overestimating water retention, with necessary upward adjustments of up to 20% in particularly warm years.
"This study, which involved years of preparation, not only integrated various modern space geodetic techniques but also involved challenging expeditions to some of the most remote areas of the planet," said Prof. Chen. "The significant results underscore the importance of extensive international cooperation in addressing climate change challenges. Our research will contribute to achieving accurate model performance for warmer years, aiding in the projection of ice-sheet behavior and its impact on sea-level rise in the coming decades. This holds particular significance amidst anticipated Arctic warmings."
Related Links
The Hong Kong Polytechnic University
Beyond the Ice Age
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