Many major river deltas in the world are subsiding and consequently become increasingly vulnerable to flooding and storm surges, salinization and permanent inundation. For the Mekong Delta, annual subsidence rates up to several centimetres have been reported. Excessive groundwater extraction is suggested as the main driver. As groundwater levels drop, subsidence is induced through aquifer compaction. Over the past 25 years, groundwater exploitation has increased dramatically, transforming the delta from an almost undisturbed hydrogeological state to a situation with increasing aquifer depletion. Yet the exact contribution of groundwater exploitation to subsidence in the Mekong delta has remained unknown. In this study we deployed a delta-wide modelling approach, comprising a 3D hydrogeological model with an integrated subsidence module. This provides a quantitative spatially-explicit assessment of groundwater extraction-induced subsidence for the entire Mekong delta since the start of widespread overexploitation of the groundwater reserves. We find that subsidence related to groundwater extraction has gradually increased in the past decades with highest sinking rates at present. During the past 25 years, the delta sank on average ∼18 cm as a consequence of groundwater withdrawal. Current average subsidence rates due to groundwater extraction in our best estimate model amount to 1.1 cm yr−1, with areas subsiding over 2.5 cm yr−1, outpacing global sea level rise almost by an order of magnitude. Given the increasing trends in groundwater demand in the delta, the current rates are likely to increase in the near future.

A large proportion of the world's population lives on low-elevation (<10 m) land near the sea 1,2 , much of which is subject to subsidence due to natural and anthropogenic processes 3 . As of 2005, ~40 million people and assets worth 5% of global gross domestic product were exposed to a 1-in-100-year coastal flooding hazard 4 . By 2070, the exposed population is expected to grow more than threefold, and the value of property exposed is expected to increase to ~9% of the projected gross domestic product, with the USA, Japan and the Netherlands having the most exposure 4 . However, these estimates often rely only on projections of global average sea-level rise and do not account for vertical land motion (VLM), in terms of subsidence (downward VLM) or uplift (upward VLM) of the land surface. A different estimate of exposure could result when VLM is taken into account, particularly considering recent findings that the elevation of many coastal lowlands has, to date, been considerably overestimated 5 .The recent increase in global mean sea level (GMSL) has led to a present-day rate of rise of ~3.35 mm per year (ref. 6 ); GMSL rise since 1900 is mostly attributed to accelerated ice-mass loss of glaciers and ice sheets, plus the thermal expansion of ocean water 7 . However, the relative sea level (RSL), defined here as the elevation difference between the sea surface and the solid Earth 8 , excluding the dynamic sediment surface 9 , is of particular relevance for assessing the effects of sea-level change at any given location. RSL change is defined as the sum of geocentric sea-level change plus VLM 8 . Note that the sediment-accretion rate, which has sometimes been invoked as a term in the RSL equation 10 , merely affects local water depth, not RSL. VLM is driven by natural processes, such as glacial isostatic adjustment (GIA) [11][12][13] , tectonics and earthquakes 14,15 , and sediment consolidation, including natural compaction owing to sediment deposition (loading) [16][17][18][19] , as well as anthropogenic effects caused by peat oxidation following drainage [20][21][22][23][24] and the compaction of aquifer systems and hydrocarbon reservoirs accompanying the extraction of subsurface fluids 20,25,26 (fig. 1).These drivers can be divided into shallow processes affecting depths of less than ~25 m (for example, compaction of Holocene sediments) and deep processes (such as tectonics and compaction of pre-Holocene strata) 27 . VLM can be much greater than nearby geocentric sea-level rise alone and, in turn, GMSL rise, which is estimated, in part, based on tide-gauge records. Thus, knowing how much, where and why coastal land subsides and how its rate varies over time is essential to evaluating hazards associated with sea-level rise and estimating GMSL rise.

Deltas are low-relief landforms that are extremely vulnerable to sea-level rise. Impact assessments of relative sea-level rise in deltas primarily depend on elevation data accuracy and how well the vertical datum matches local sea level. Unfortunately, many major deltas are located in data-sparse regions, forcing researchers and policy makers to use low-resolution, global elevation data obtained from satellite platforms. Using a new, high-accuracy elevation model of the Vietnamese Mekong delta, we show that quality of global elevation data is insufficient and underscore the cruciality to convert to local tidal datum, which is often neglected. The novel elevation model shows that the Mekong delta has an extremely low mean elevation of ~0.8 m above sea level, dramatically lower than the earlier assumed ~2.6 m. Our results imply major uncertainties in sea-level rise impact assessments for the Mekong delta and deltas worldwide, with errors potentially larger than a century of sea-level rise.