The Impact of Groundwater Depletion on Spatial Variations in Sea Level Change During the Past Century

The loss of continental groundwater to the oceans during the past century has elevated sea level by ~25(±5) mm, and has caused ~0.7mm/yr of sea level rise since 2005. The mass unloading associated with this groundwater depletion induces elastic uplift of Earth's solid surface and depresses the gravitational equipotential surface that defines sea level. Together, these deflections should cause slower relative sea level rise near areas of continental groundwater loss. We estimated these variations in sea level change using a model of the solid Earth's response to estimates of groundwater depletion during the past century. We find large negative deviations in relative sea level near California, Western India, the western Yellow Sea and the eastern Mediterranean Sea. Relative sea level measured by tide gauges in these areas show slower sea level rise rates compared to global averages. For example, on the western coast of India (e.g., Karachi), groundwater-induced deviations from global average sea level rise can exceed -40 mm, and our model predicts ~1 mm/yr of sea level drop since 2005. Correcting tide gauge records for groundwater depletion using our model improves their fit to the global trend estimated by Church & White (2011), and further reduces the variation of rise rates observed among regional groups of stations. We reconstructed Global Mean Sea Level (GMSL) between 1930 and 2009 taking in account groundwater depletion corrections determined from our model. We found that including groundwater depletion increases our estimate of the global rate of change of GMSL from 1.81 to 1.88 mm/yr during this time period because the observed rise at some key stations is slowed by nearby groundwater depletion. For the past 20 years, including groundwater depletion increases GMSL from 3.32mm/yr to 3.46mm/yr. Quantifying the spatial variability of sea level associated with groundwater depletion is important for understanding the variety of factors that affect sea level, and could contribute to more accurate regional forecasts of future sea level rise.