Concerns about data linking delta land gain to human action

Abstract

River deltas are diverse, economically and ecologically important ecosystems that are increasingly vulnerable to environmental change. A recent study reported that global-scale human impact on delta morphology has led to net land area gain (Nienhuis et al. 1 ). However, we argue that an unconventional definition of deltas, miscellaneous coastal features unduly characterized as deltas and misidentified delta area changes led to spurious statistics in the study by Nienhuis et al. 1 and that a recently published correction 2 does not address our concerns. We call for the rigorous verification and analysis of large environmental datasets. Delta identificationNienhuis et al. 1 defines marine river mouths with fluvial water and sediment discharge thresholds (respectively >1 m 3 /s and > 0.01 kg/s) as deltas. This definition runs counter to standard delta 3-6 geoscience definitions, but nevertheless uses morphological terms that are specific to deltas 3,4 as distinct from estuaries, such as shoreline protrusion and triangular shape. Simplified assumptions on river-mouth sediment flux redistribution by river, waves and tides used by Nienhuis et al. 1 neglect the sediment-dispersal role of these agents that render most river mouths devoid of deltas 3,5,6 . These assumptions also neglect sedimentation pathways interlinking the connected upbuilding and outbuilding components of deltas (subaerial delta-plain, subaqueous delta-front, pro-delta) that differentiate them from simple estuaries. Nienhuis et al. 1 uses automatically-generated 'buffer' areas to identify land change from global spatial data 7 , arguing for the exclusion of areas distant from channel banks and shorelines. We find that this automatic identification of buffers, without verification of their accuracy, leads to thousands of features wrongly identified as "deltas" by Nienhuis et al. 1 , including estuaries, built-

Causes and consequences of tipping points in river delta social–ecological systems

“…Whilst soil erosion in the river basins has provided the sediments to the deltas (McNeill et al 2017 ; Lenard et al 2020 ), sediment eroded due to human actions on the slopes in recent times (Nienhuis et al 2020 ) is compensated by sediment retention due to dams (Darby et al 2015 ; Dunn et al 2019 ). Consequently, many river deltas across the globe are sediment-starved and have seen a reduction in their size (Hoitink et al 2020 ; Scown et al 2023 ; Zăinescu et al 2023 ).…”

Role of atmospheric indices in describing shoreline variability along the Atlantic coast of Europe

“…Global studies of shoreline change based on satellite-derived shorelines (SDS) are becoming increasingly common (Almar et al, 2023;Ghanavati et al, 2023;Luijendijk et al, 2018;Mentaschi et al, 2018;Vousdoukas et al, 2020); however, concerns have been raised regarding satellite-derived global applications (Cooper et al, 2020;Zăinescu et al, 2023). To explore links between shoreline response and modes of climate variability, robust methodologies for deriving shorelines involving wave and/or tide corrections (Castelle et al, 2021;Vos et al, 2023) or time-and spatial-averaging techniques (Castelle et al, 2022;Warrick et al, 2023) must be applied.…”

Role of Atmospheric Indices in Describing Shoreline Variability Along the Atlantic Coast of Europe

“…Global studies of shoreline change based on satellite‐derived shorelines (SDS) are becoming increasingly common (Almar et al., 2023; Ghanavati et al., 2023; Luijendijk et al., 2018; Mentaschi et al., 2018; Vousdoukas et al., 2020); however, concerns have been raised regarding satellite‐derived global applications (Cooper et al., 2020; Zăinescu et al., 2023). To explore links between shoreline response and modes of climate variability, robust methodologies for deriving shorelines involving wave and/or tide corrections (Castelle et al., 2021; Vos et al., 2023) or time‐ and spatial‐averaging techniques (Castelle et al., 2022; Warrick et al., 2023) must be applied.…”