Melting of the Greenland ice sheet is currently the primary driver of sea-level rise. For the first time, scientists from The Hong Kong Polytechnic University (PolyU), together with a team of global researchers, have integrated various modern space geodetic techniques, particularly satellite positioning data, to monitor the subsidence of vertical bedrock and successfully quantify summer water storage in the Greenland ice sheet. This achievement offers new insights and evidence regarding the contribution of ice sheets to global sea-level rise. Findings from their research have been published in the international journal Nature.
The Greenland ice sheet is second in size only to Antarctica, with an ice thickness of about 3 kilometres. If it were to fully melt, the average global sea level would rise by seven metres. However, many questions remain unanswered regarding the processes of englacial water accumulation, storage, and ultimate release. Prof. Jianli CHEN, Professor of the PolyU Department of Land Surveying and Geo-informatics, and core member of Research Institute for Land and Space, collaborated with international experts from Hong Kong, Mainland China, the US, the Netherlands, and Belgium to explore the hydrological processes in Greenland. Their research focuses on the evolution of meltwater storage to better understand ice sheet melting behaviour and its impact on sea-level rise.
The increase in water storage caused by the melting of ice sheets is one of the key factors leading to bedrock subsidence. However, the satellite gravimetry commonly used is not optimal for conducting regional assessment of Greenland hydrology. To tackle this challenge, the research team has pioneered the use of the Greenland GPS Network (GNET) and satellite gravity measurements provided by NASA's Gravity Recovery and Climate Experiment. GNET comprises numerous Global Navigation Satellite System (GNSS) stations around Greenland to provide continuous positioning data.
The team analysed data from 22 GNSS stations close to bedrocks and glacier outlets over the period from 2009 to 2015 to detect the regional melting water storage beneath the ice sheets and quantify the elastic deformation and subsidence of vertical bedrock, thereby better understanding the spatiotemporal behaviour of meltwater. GNSS was also used to help monitor large scale mass redistribution in the climate system, such as groundwater depletion and change in lake water storage.
The study revealed that during the summer melt season, most of the meltwater is temporarily stored within the ice sheet, peaking in July and then gradually decreasing. The buffered meltwater leads to a maximum subsidence of up to 5mm in the bedrock near the GNSS stations in Greenland. In 2010 and 2012, extreme melting events led to the bedrock subsiding by as much as 12 and 14 millimeters, respectively.
The study also suggested that the duration of meltwater storage in the ice sheet at most GNSS sites is about 8 weeks but with regional variation. It is above average at about 9 weeks in the northeast and west, while at around 4.5 weeks in the south and southeast. As recent extremely high summer temperatures may become the norm in the foreseeable future, accurate prediction of meltwater storage in those years is crucial for assessing sea-level rise. Since meltwater runoff modelled from regional climate models could have overestimated water retention or underestimated snowmelt runoff, the team suggested that upward adjustments of up to 20% may be necessary for the warmest years.
Prof. Chen said, "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. 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 behaviour and its impact on sea-level rise in the coming decades. This holds particular significance amidst anticipated Arctic warmings."
