Prof. Min-Hui Lo and Mr. Ren-Jie Wu of the Department of Atmospheric Sciences participated in an international study called “Fate of water pumped from underground and contributions to sea-level rise” that was published in the journal Nature Climate Change in May.

The rising sea level is a threat to coastal regions and small islands.  Hence, accurately projecting rates of sea level rise is important, especially under the influence of rapid climate change.  However, recent assessments also indicate that groundwater depletion (GWD) may become the most important positive terrestrial contribution to ground subsidence.  Future projections of man’s increasing reliance on groundwater also suggests that GWD will become the most important singular terrestrial contribution to sea level rise over the next 50 years, likely equal in magnitude to the current contributions from glaciers and ice caps.

However, a critical assumption, including the estimates used in the International Panel on Climate Change’s Fifth Assessment Report, is that nearly 100% of groundwater extracted from aquifers ultimately ends up in the ocean.  Due to limited knowledge of the pathways and mechanisms governing the ultimate fate of pumped groundwater, the relative fraction of global GWD that contributes to sea level rise remains unknown.

In the study, Prof. Lo and Mr. Wu joined their international counterparts in presenting a coupled climate-hydrological model simulation that tracked the fate of water pumped from underground so as to calculate exactly how much GWD contributes to sea level changes.  Their results show that when considering the land-atmosphere interactions from groundwater withdrawal, the fraction of GWD that ends up in the ocean is only 80%.  This indicates that previous studies have substantially overestimated the contribution of GWD to global sea level rise by a cumulative amount of at least 10mm during the 20th century and early 21st century.

The new study improves on previous estimates by accounting for feedbacks between the land, ocean, and atmosphere.  The missing 20% of water unaccounted for also means that the gap between modeled and observed sea level rise is even wider, suggesting that other sources are contributing more water than previously estimated.