The global picture
Changing pressures on deltas
By developing and using scenarios of biophysical change and socio-economic development, researchers at the Water, Climate and Future Deltas hub improve the understanding of delta systems and their drivers to identify which impact these drivers may have on deltas and their adaptation strategies in the future, e.g. as land area changes and limits management options.
Drivers
Deltas are currently facing multiple challenges. These are driven by temperature and precipitation change, population growth and urbanisation, sea-level rise, land subsidence, changing river discharge and sediment delivery.
Temperature change
Figure: Global surface temperature change until 2100, relative to 1850–1900, under all SSP-RCP scenarios. Source: IPCC (2021)
Global temperature is projected to increase by 1.5-4.5°C by the end of this century. As a result, deltas will increasingly experience extreme heat and changes in precipitation.
Precipitation change
Figure: Annual mean precipitation change at 1.5°C (left), 2°C (middle) and 4°C global warming. Source: adapted from IPCC (2021)
Precipitation amount and variability are projected to change regionally with a global increase in extreme weather events with high rainfall intensity but also extended and more intense periods of drought. Deltas are therefore increasingly exposed to drought and heavy rains.
Population density change
Figure: Average population development across 49 of the world's major deltas until 2100. Source: delta population calculations based on Jones and O’Neill (2016)
Averaged over 49 of the world's major deltas, scenarios indicate that delta population densities are likely to continue to grow at least until 2050. For SSP1, SSP4 and SSP5, the trend decreases after 2050, whereas for SSP3 average population continues to increase. Population densities in individual deltas can be ten times higher than this average (Project: Global drivers, future deltas).
Dr. Murray Scown
Urbanisation
Figure: Average urban development across 49 of the world's major deltas until 2100. Source: delta calculations based on Gao and O’Neill (2020)
Averaged over 49 of the world's major deltas, urbanisation increases until 2100 under all scenarios. This means that increasingly large parts of deltas will be covered by impervious surfaces typical of urban areas and likely with associated hard flood defences, with the highest urban land fraction for SSP5 (Project: Global drivers, future deltas).
Dr. Murray Scown
Sea-level rise
Figure: Global mean sea-level rise until 2100, relative to 1900, under all SSP-RCP scenarios. Source: IPCC (2021)
Global mean sea level is rising at an accelerating rate and will continue to rise in the future, threatening deltas worldwide. Recent sea-level projections of the IPCC show a possible rise of 0.5-1.0 meter by 2100, with high-end scenarios indicating a possible rise of more than 1.5 meters in 2100.
Regional sea-level rise
Figure: Dominant components of the regional acceleration of sea-level rise under the RCP8.5 scenario. Source: adapted from Karabil et al. (2021), in preparation
Deltas are projected to experience different rates in change of sea level, and with different dominant drivers. While deltas in the Southern Hemisphere will be more affected by the loss of glaciers and changes in the surface mass of Greenland ice sheet, those in the Northern Hemisphere will be more affected by altered ice-flow from Antarctic ice sheet to ocean. Additionally, changes in land water storage have an underestimated potential in driving sea-level change for different deltas. On the other hand, the contribution of glacial mass loss is currently overestimated in SLR projections.
Dr. Sitar Karabil
River discharge change
Figure: Multi-model ensemble average percentage change (%) in maximum, mean and minimum annual discharge for the period 2081–2100 relative to 1971–1990 using the IPCCs emission scenario A1B. Source: Sperna Weiland et al. (2012)
Global projections show that semi-arid areas will experience lower river discharge, whereas in humid areas river discharge will increase. This implies that deltas may experience more river flooding during wet seasons, but also increasingly will suffer from seasonal drought.
Dr. Rens van Beek
Sediment delivery change
Figure: Projected change in mean annual fluvial sediment flux between 1990–2019 and 2070–2099. Source: Dunn et al. (2019), reprinted with permission
On average, mean annual sediment flux to the world's major deltas is projected to decrease by 38% by the end of this century as a result of anthropogenic activities, such as dam construction and land-use change.
Dr. Frances Dunn
Land subsidence
Figure: Potential global subsidence due to groundwater depletion in six selected deltas: Mississippi, Rhine-Meuse, Nile, Ganges-Brahmaputra-Meghna, Mekong, Yangtze (left-right); VL-VH: very low to very high. Source: adapted from Herrera-García et al. (2021)
Potential subsidence areas are concentrated in deltas, as these are densely populated areas with high groundwater demands. Increasing rates of subsidence in deltas raises serious concerns for their populations, facing inundation, salinization and infrastructural damage.
Dr. Gilles Erkens
Impact
As a result of these biophysical drivers, delta land area is projected to change in the future, and through the current socio-economic developments deltas may reach a locked-in state.
Delta area change
Figure: Land-area change for the world's major deltas, based on (a) observations and (b, c, d) projections of relative sea-level rise under RCP2.6, 4.5 and 8.5. Source: adapted from Nienhuis and Van de Wal (2021)
Over the past 30 years, deltas globally have experienced net land-area gain. However, these land gains are unlikely to be sustained in the coming century as sea-level rise, tides and waves become more important in shaping delta coastlines. Under RCP8.5, deltas are expected to lose 35,000 km-2 of land.
Dr. Jaap Nienhuis
Locked-in state
Figure: State of the world’s major deltas as living or locked-in. Source: adapted from Santos and Dekker (2020)
Due to natural infrastructure development (cropland and irrigation area) and/or social infrastructure development (urban areas) over recent decades, many of the deltas worldwide have reached a locked-in state, i.e. they are too costly to restore or transform back to a living delta in which it is still possible to meet local population resource needs and well-being by the services and productivity of local ecosystems.
Prof. Stefan Dekker