Past and future contribution of global groundwater depletion to sea‐level rise

Recent studies suggest the increasing contribution of groundwater depletion to global sea‐level rise. Groundwater depletion has more than doubled during the last decades, primarily due to increase in water demand, while the increase in water impoundments behind dams has been tapering off since the 1990s. As a result, the contribution of groundwater depletion to sea‐level rise is likely to dominate over those of other terrestrial water sources in the coming decades. Yet, no projections into the 21st century are available. Here we present a reconstruction of past groundwater depletion and its contribution to global sea‐level variation, as well as 21st century projections based on three combined socio‐economic and climate scenarios (SRES) with transient climate forcing from three General Circulation Models (GCMs). We validate and correct estimated groundwater depletion with independent local and regional assessments, and place our results in context of other terrestrial water contributions to sea‐level variation. Our results show that the contribution of groundwater depletion to sea‐level increased from 0.035 (±0.009) mm yr−1 in 1900 to 0.57 (±0.09) mm yr−1 in 2000, and is projected to increase to 0.82 (±0.13) mm yr−1 by the year 2050. We estimate the net contribution of terrestrial sources to be negative of order −0.15 (±0.09) mm yr−1 over 1970–1990 as a result of dam impoundment. However, we estimate this to become positive of order +0.25 (±0.09) mm yr−1 over 1990–2000 due to increased groundwater depletion and decreased dam building. We project the net terrestrial contribution to increase to +0.87 (±0.14) mm yr−1 by 2050. As a result, the cumulative contribution will become positive by 2015, offsetting dam impoundment (maximum −31 ± 3.1 mm in 2010), and resulting in a total rise of +31 (±11) mm by 2050.