Coastal river deltas threatened by more than climate change
Worldwide, coastal river deltas are home to more than half a billion people, supporting fisheries, agriculture, cities, and fertile ecosystems. In a unique study covering 49 deltas globally, researchers from Lund University and Utrecht University have identified the most critical risks to deltas in the future. The research shows that deltas face multiple risks, and that population growth and poor environmental governance might pose bigger threats than climate change to the sustainability of Asian and African deltas, in particular.
“We can clearly show that many risks are not linked to climate. While climate change is a global problem, other important risk factors like land subsidence, population density and ineffective governance are local problems. Risks to deltas will only increase over time, so now is the time for governments to take action”, says Murray Scown, associate senior lecturer, Lund University Centre for Sustainability Studies, and lead author.
Collapse of delta environments could have huge consequences for global sustainable development. In the worst-case scenario, deltas could be lost to the sea; other consequences are flooding, salinization of water, which affects agriculture, coastal squeeze, and loss of ecosystems.
The study, published in Global Environmental Change, looked at five different IPCC scenarios for global development in 49 deltas all over the world, including famous deltas such as the Nile, Mekong, and Mississippi, but also more understudied deltas such as the Volta, Zambezi and Irrawaddy deltas. The research identifies possible risks to deltas stretching 80 years into the future. The researchers based their analysis on 13 well-known factors affecting risk in deltas and drew upon unique models to identify which of these risks are most likely to endanger different deltas in the future. Risk factors include increasing population density, urban development, irrigated agriculture, changes to river discharge, land subsidence and relative sea-level rise, limited economic capacity, poor government effectiveness, and low adaptation readiness.
Population density, land subsidence and ineffective governance are high risk factors
The analysis shows that there are some risks that are more critical to deltas than others – in all of the five future scenarios. These include land subsidence and relative sea-level rise, population density, ineffective governance, economic capacity, and crop land use.
For some deltas, physical risks are especially pronounced. Land subsidence is, for example, the highest risk factor for the Mekong delta in Vietnam. Extreme sea levels are among the most concerning risk factors for deltas in China, on the Korean peninsula, and in the Colorado (Mexico) and Rhine (Netherlands) deltas.
In the Nile (Egypt), Niger (Nigeria), and the Ganges (Bangladesh) deltas, it is increasing population density that is of most concern under certain scenarios. For other deltas, it is the lack of economic capacity and government effectiveness to manage risks, for example in the Irrawaddy (Myanmar) and Congo (Angola and Democratic Republic of the Congo) deltas.
“Analysed all together, we can see that the Asian mega-deltas are at greatest risk, with potentially devastating consequences for millions of people, and for the environment. They are under pressure from population growth, intense agricultural land use, relative sea-level rise, and lagging adaptation readiness”, says Murray Scown.
Local and global approaches and a mixture of hard and soft adaptation can mitigate risks
“Instead of sitting back, governments need to think long-term, and put plans in place to reduce or mitigate risks. In the Mekong delta, for example, the Vietnamese government are making strong efforts to restrict future groundwater extraction in the delta to reduce land subsidence and salinization”, says Philip Minderhoud, assistant professor at Wageningen University and Research.
The researchers highlight that a mixture of hard (“grey”) and soft (“green”) adaptation approaches will be required to manage and mitigate delta risks. They include both hard infrastructures, like sea walls to stop the sea inundating the delta, and soft approaches using nature-based solutions. One example is the Dutch experience of creating room for the river in the Rhine delta, by lowering floodplains, relocating levees, and using spaces that are allowed to flood for grazing. Initiatives to build up delta surfaces by allowing rivers to flood and deposit sediment on the delta to maintain elevation above sea level are also promising, notes Frances Dunn, assistant professor at Utrecht University.
“By looking at the deltas together, like we have in this study, we want to highlight what can happen on a global scale if we do not address delta risk both on a local and global level. The study can also complement studies on individual deltas, and identify efforts needed connected to less studied deltas such as the Saõ Francisco or Volta delta”, says Maria Santos, professor at the University of Zurich.
JOURNAL
Global Environmental Change
ARTICLE TITLE
Global change scenarios in coastal river deltas and their sustainable development implications
Predicting the fate of shallow coastal ecosystems for the year 2100
Projections vary for seagrass meadows, macroalgal beds, tidal marshes, mangroves, and coral habitats
Peer-Reviewed PublicationIMAGE:
EXAMPLES OF SHALLOW-WATER ECOSYSTEMS
view moreCREDIT: MOKI ET AL., 2023, PLOS CLIMATE, CC-BY 4.0 (HTTPS://CREATIVECOMMONS.ORG/LICENSES/BY/4.0/)
A new study of shallow-water ecosystems estimates that, by 2100, climate change and coastal land usage could result in significant shrinkage of coral habitats, tidal marshes, and mangroves, while macroalgal beds remain stable and seagrass meadows potentially expand. Hirotada Moki of the Port and Airport Research Institute, Japan, and colleagues present their findings in the open-access journal PLOS Climate.
Shallow-water ecosystems absorb a significant amount of carbon dioxide and are therefore expected to help mitigate climate change. Meanwhile, these ecosystems themselves will likely be affected by climate change, including warming seawater. However, it has been unclear exactly how climate change might impact the future size of shallow-water ecosystems.
To help clarify their fate, Moki and colleagues estimated future changes in the total area occupied by the five shallow-water ecosystems thought to be most important: seagrass meadows, macroalgal beds, tidal marshes, mangroves, and coral habitats.
They combined data on the ecosystems’ current sizes and distributions with topographic data, and applied a global climate model to calculate potential changes through 2100. They considered two standardized hypothetical scenarios, one representing the lowest predictions for future greenhouse gas emissions (RCP2.6) and the other the highest (RCP8.5).
The analysis estimates that, by 2100, global coral habitat could shrink by up to 74 percent. Meanwhile, seagrass meadows could expand by up to 11 percent because of a predicted increase in the depth to which photosynthesis-powering sunlight can penetrate these habitats. For macroalgal beds, that depth is not predicted to vary greatly, resulting in an estimated maintenance of present area through 2100.
The estimates also suggest that tidal marshes and mangroves will retain their present size because shrinkage caused by rising sea level will be offset by expansion to new areas. However, after accounting for coastal development and land use, the analysis estimates that tidal marshes may shrink by 91.9 percent and mangroves by 74.3 percent.
On the basis of these findings, the researchers suggest employing an optimal mix of shallow-water ecosystems and man-made infrastructure to counteract coral-habitat shrinkage. Meanwhile, appropriate coastal management could harness the climate change-mitigating effects of the other four ecosystems.
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In your coverage please use this URL to provide access to the freely available article in PLOS Climate: https://journals.plos.org/climate/article?id=10.1371/journal.pclm.0000298
Citation: Moki H, Yanagita K, Kondo K, Kuwae T (2023) Projections of changes in the global distribution of shallow water ecosystems through 2100 due to climate change. PLOS Clim 2(11): e0000298. https://doi.org/10.1371/journal.pclm.0000298
Author Countries: Japan
Funding: This work was partially supported by the Environmental Research and Technology Development Fund (S-14: JPMEERF15S11408 to TK) of the Environmental Restoration and Conservation Agency of Japan, and Grants-in-Aid for Scientific Research (KAKENHI) from the Japan Society for the Promotion of Science (nos. 18H04156 and 26630251 to TK).
JOURNAL
PLOS Climate
METHOD OF RESEARCH
Computational simulation/modeling
SUBJECT OF RESEARCH
Not applicable
ARTICLE PUBLICATION DATE
22-Nov-2023
COI STATEMENT
Competing Interests: Authors Keigo Yanagita and Keiichi Kondo were employed by the company Science and Technology Co., LTD. All other authors declare no competing interests.
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