Measuring, modelling and projecting coastal land subsidence

Abstract

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.

A global analysis of extreme coastal water levels with implications for potential coastal overtopping

“…On one hand, it is currently impossible to observe sea levels right at the coast all along the global coastline and in particular wave contributions to ECWLs. On the other hand, accurate measurements of global coastal morphological evolution and subsidence trends are still to be done despite promising local to regional emerging satellite techniques [55][56][57] . In the near future, an advanced approach such as that presented by Vos et al 58 to reconstruct foreshore slope from satellite-derived shoreline tracking and tide level could be applied globally.…”

Projections of Global Delta Land Loss From Sea‐Level Rise in the 21st Century

“… Effect of sea‐level rise on delta land change. (a) Histogram of subsidence and sea‐level rise rates for all coastal deltas (Dangendorf et al., 2019; Oppenheimer et al., 2019; Shirzaei et al., 2021). Note that subsidence extends beyond the axis range, in part because these data are not available for most deltas.…”

Global morphodynamic response of deltas to sea-level rise in the 21st century

“…We obtain measures of RSLR from various sources. For historic conditions, we use regional ( 1degree) sea-level reconstructions from 1985-2015 (Dangendorf et al, 2019) combined with local vertical land movement (uplift, subsidence) observations based on GPS stations nearest to each delta (Blewitt et al, 2018;Shirzaei et al, 2021). For future projections (2007-2100), we use IPCC SROCC scenarios for RCP2.6, 4.5, and 8.5 (Oppenheimer et al, 2019) and assume land subsidence remains unchanged from their modern rates (2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014).…”