Coastal communities around the globe depend on tidal marshes and mangroves for the diverse ecological, economic and flood-mitigating services they provide. These relatively flat wetland systems (Fig. 1) commonly reside just above mean sea level, making them one of the ecosystems most at risk of being drowned by rising sea levels. But tidal wetlands will not disappear without a fight. Writing in Nature, Schuerch et al.1 present global-scale modelling that suggests that tidal wetlands are less vulnerable to sea-level rise than was thought. However, the scale of future wetland loss or gain depends greatly on the degree to which coastal communities accept or prevent the landward advances of these living coastal systems into newly inundated areas.
Tidal wetlands are dynamic, living systems that have the ability to accelerate their growth as the sea level rises, using biophysical feedbacks — complex interactions between plant growth, water flow and sediment trapping. Furthermore, the flooding of low-lying areas by rising seas provides new locations that can accommodate tidal wetlands or allow them to grow. Are these adaptive mechanisms enough to save global wetlands from drowning in the future? Schuerch and colleagues’ modelling study was framed to answer exactly this question.
The authors consider not only the adaptation potential of tidal wetlands to future sea-level rise in the vertical direction (upward growth) and horizontally (landward advance), but also the human response to their landward migration. Their modelling work was no small task, requiring the integration of large global data sets of shoreline topography, wetland distributions, concentrations of suspended sediments in wetland waters, tidal ranges (the vertical difference between high and low tides), and projected changes in sea level and coastal populations.
Schuerch et al. also incorporated a key feedback into their model to address the fact that the rate at which a wetland grows vertically to keep pace with sea-level rise is dictated not only by the plant matter it can produce, but also by how efficiently it traps sediment. So, in areas that have a sufficient sediment supply and tidal range, the model accounts for the acceleration of wetland growth that is due to the increased amount of sediment carried into and deposited on the wetland over a tidal cycle as sea levels rise.
As noted earlier, tidal wetlands can colonize new low-lying areas as they become flooded by sea-level rise, but little is known about the mechanisms that govern this inland migration. As a first cut, Schuerch and colleagues postulated that all newly inundated areas behind current wetlands and below a certain human population density will be converted to tidal wetlands — the second criterion is relevant because the size and extent of artificial barriers that impede inland wetland migration, such as roads, flood protection and other coastal infrastructure, are assumed to scale with population.
The authors tested what happens when the population threshold above which no wetland is created was set at either 5 or 20 people per square kilometre. These are considered to be the lower and upper bounds, respectively, of the population densities for which wetlands will migrate landward without any action being taken by humans to support or prevent the migration — that is, in ‘business as usual’ (BAU) scenarios. The authors also tested higher thresholds of 150 and 300 people per square kilometre, which they respectively describe as moderate and extreme nature-based adaptation scenarios, because they would require the replacement of current flood protection and infrastructure with alternatives that would allow wetlands to migrate into more-populated regions.
The authors’ results suggest that global tidal wetland loss will be 0–30% by 2100 for the BAU scenarios in which wetlands migrate only into sparsely populated regions. These losses are well below those estimated in past studies2–4, and highlight the need to consider adaptive feedbacks in future sustainability studies of tidal wetlands. Just as importantly, the nature-based adaptation scenarios suggest that wetland gains as high as 60% could be made when measures are taken that allow wetlands to migrate into more-populated areas.
Some of the findings are not too surprising. The study assumes an overly simple mechanism for inland wetland conversion, for which it is perhaps to be expected that the biggest increase in wetland extent will occur for the greatest sea-level rises under the most extreme nature-based adaptation scenarios. Other parts of the study are particularly interesting, however. For example, when the authors carried out simulations in which the concentration of sediments suspended in tidal wetland waters drops by 50% from present-day values, the extent of wetlands decreases by only 6%. Reductions in sediment supply caused by damming and other human activities have been a primary concern as contributors to wetland loss5, but Schuerch and co-workers’ results indicate that the degree to which wetlands are allowed to migrate into newly flooded lowland areas could have a much greater impact on wetland sustainability.
One notable caveat to the current findings is that the analysis assumes new wetlands are initiated at a fairly high tidal level. Such super-elevated wetlands will eventually drown if they lack a sufficient sediment supply to sustain them. However, the timescale for this drowning in the simulations was relatively long, and did not occur by 2100, the end of the modelled period.
Schuerch and colleagues’ results highlight major gaps in our knowledge of wetland sustainability. Most of the modelled wetland gains were for mangrove systems, which currently represent about 70% of tidal wetlands1. However, our understanding of adaptive feedbacks in mangroves is poor compared with our understanding of tidal marshes. Data describing the inland migration of wetlands are also extremely limited, and environmental factors such as pre-existing soil and vegetative conditions could restrict migration to much lower extents than those projected by Schuerch and colleagues. However, the authors’ study is a crucial step towards realistic assessments of future wetland changes, and highlights the key roles of both sea-level rise and nature-based adaptation strategies in providing new spaces where lowlands can sustainably accommodate the growth of tidal wetlands.
Nature 561, 183-185 (2018)