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.
