Tidal wetland stability in the face of human impacts and sea-level rise

Sea-level rise enhances carbon accumulation in United States tidal wetlands

Twenty-first century sea-level rise could exceed IPCC projections for strong-warming futures

Climate impacts on coastal and estuarine systems take many forms and are dependent on the local conditions, including those set by humans. We use a biocomplexity framework to provide a perspective of the consequences of climate change for coastal wetland ecogeomorphology. We concentrate on three dimensions of climate change affects on ecogeomorphology: sea level rise, changes in storm frequency and intensity, and changes in freshwater, sediment, and nutrient inputs. While sea level rise, storms, sedimentation, and changing freshwater input can directly impact coastal and estuarine wetlands, biological processes can modify these physical impacts. Geomorphological changes to coastal and estuarine ecosystems can induce complex outcomes for the biota that are not themselves intuitively obvious because they are mediated by networks of biological interactions. Human impacts on wetlands occur at all scales. At the global scale, humans are altering climate at rapid rates compared to the historical and recent geological record. Climate change can disrupt ecological systems if it occurs at characteristic time scales shorter than ecological system response and causes alterations in ecological function that foster changes in structure or alter functional interactions. Many coastal wetlands can adjust to predicted climate change, but human impacts, in combination with climate change, will significantly affect coastal wetland ecosystems. Management for climate change must strike a balance between that which allows pulsing of materials and energy to the ecosystems and promotes ecosystem goods and services, while protecting human structures and activities. Science-based management depends on a multi-scale understanding of these biocomplex wetland systems. Causation is often associated with multiple factors, considerable variability, feedbacks, and interferences. The impacts of climate change can be detected through monitoring and assessment of historical or geological records. Attribution can be inferred through these in conjunction with experimentation and modeling. A significant challenge to allow wise management of coastal wetlands is to develop observing systems that act at appropriate scales to detect global climate change and its effects in the context of the various local and smaller scale effects.

Sea-level rise from a warming climate threatens to inundate coastlines around the world.1 But some of the world’s most vulnerable coasts—those fringing flat delta plains, mainly in Southeast Asia—face the far more immediate threat of sinking land.2 Induced mainly by human activities on a local rather than global scale, this phenomenon, known as land subsidence, can outpace sea-level rise substantially. Indonesia’s biggest city, Jakarta, is sinking at an average rate of 5–10 cm per year,3 much faster than the global rate of sea-level rise, which clocks in at 3.2 mm per year, according to the recent estimates.1 Should subsidence in Jakarta continue unabated, the city could sink up to 6 m by the end of the century, according to JanJaap Brinkman, a water management specialist with Deltares Research Institute in Delft, the Netherlands.

Coastal flooding and wetland loss in the 21st century: changes under the SRES climate and socio-economic scenarios

During the Holocene, North American ice sheet collapse and rapid sea-level rise reconnected the Black Sea with the global ocean. Rapid meltwater releases into the North Atlantic and associated climate change arguably slowed the pace of Neolithisation across southeastern Europe, originally hypothesized as a catastrophic flooding that fueled culturally-widespread deluge myths. However, we currently lack an independent record linking the timing of meltwater events, sea-level rise and environmental change with the timing of Neolithisation in southeastern Europe. Here, we present a sea surface salinity record from the Northern Aegean Sea indicative of two meltwater events at ~8.4 and ~7.6 kiloyears that can be directly linked to rapid declines in the establishment of Neolithic sites in southeast Europe. The meltwater events point to an increased outflow of low salinity water from the Black Sea driven by rapid sea level rise >1.4 m following freshwater outbursts from Lake Agassiz and the final decay of the Laurentide ice sheet. Our results shed new light on the link between catastrophic sea-level rise and the Neolithisation of southeastern Europe, and present a historical example of how coastal populations could have been impacted by future rapid sea-level rise.

Increasing flood risk and wetland losses due to global sea-level rise: regional and global analyses

An accelerating rate of sea level rise (SLR) is causing huge inundation pressure on coastal wetlands worldwide. Vegetation of coastal wetlands plays a key role in stabilizing the coast and accreting sediment in order to mitigate the negative impact of SLR. The ability to accrete sediment is influenced by individual species traits; however, there are insufficient information and indicators to identify differences in the adaptability of various coastal vegetations to SLR at a regional or global scale. Here, the potential adaptation of 27 plant populations in coastal wetlands subject to SLR was evaluated using a compiled global dataset and a marsh equilibrium model. Sediment accretion efficiency differed among plant populations, but most coastal marsh populations and a few mangrove populations had relatively high accretion rates; habitats with high accretion rates will have a better potential to deal with the threat of SLR. These results showed that latitude and efficiency shared a nonlinear relationship, and plant stem density and root structure were among the important factors that influenced the efficiency. Fibrous root plant populations had a greater sediment accretion efficiency than tap root plant populations, and perennial populations had a greater sediment accretion efficiency than annual plant populations. These findings can provide key parameters relating to the sediment accretion efficiency of hydrological and geomorphic models on a global scale. This study offers some novel insights into the dynamic changes in coastal wetlands following SLR that will be particularly useful in devising appropriate strategies for the protection and management of coastal wetlands.

Will worldwide sea level soon rise rapidly because of a shrinkage of the West Antarctic ice sheet (WAIS)? Here I give a personal perspective of that probability. The crucial question is not whether large changes in ice mass can occur, but how likely it is that a large, rapid change, say a several-fold increase in the 20th-century rate of about 2 mm a-1, will occur in the next century or two from a West Antarctic cause.Twenty years ago Weertman proposed that a marine ice sheet is inherently unstable. But Weertman’s analysis was based on a simple model of a marine ice sheet that did not include fast-flowing, wet-based ice streams, which are now known to dominate the grounded ice sheet. Modern analyses do not definitively determine just how ice streams affect the stability of the WAIS, but it can at least be said that there is no compelling theoretical reason to expect a rapid rise in sea level from the WAIS triggered by ice-shelf thinning.Of the three main ice-drainage systems in the WAIS, the one that flows into Pine Island Bay might be a particularly likely site for accelerated flow since there is no ice shelf to restrain the inflowing ice streams, yet measurements show that this system is not significantly out of mass balance. If the “Ross Embayment” system, which has undergone several sudden glacial reorganizations in the last thousand years, were unstable one might expect a history of large changes in the total outflow of ice into the Ross Ice Shelf, yet the total outflow in the “Ross Embayment” has remained relatively unchanged despite the large internal perturbations, a fact that, points to a stable, not an unstable, system. Study of the third major drainage from the WAIS, into the Ronne Ice Shelf, also suggests that there is no gross discordance between the present velocity vectors and flow tracers in the ice shelf, although the evidence is limited.In the light of the evidence for recent stability, it is difficult to see how climate warming (whether anthropogenic or natural) could trigger a collapse of the WAIS in the next century or two. Thus, I believe that a rapid rise in sea level in the next century or two from a West Antarctic cause could only occur if a natural (not induced) collapse of the WAIS were imminent. Based on a concept of pseudo-random collapse once per major glacial cycle, I estimate the chances of that to be on the order of one in a thousand.

Will worldwide sea level soon rise rapidly because of a shrinkage of the West Antarctic ice sheet (WAIS)? Here I give a personal perspective of that probability. The crucial question is not whether large changes in ice mass can occur, but how likely it is that a large, rapid change, say a several-fold increase in the 20th-century rate of about 2 mm a-1, will occur in the next century or two from a West Antarctic cause. Twenty years ago Weertman proposed that a marine ice sheet is inherently unstable. But Weertman’s analysis was based on a simple model of a marine ice sheet that did not include fast-flowing, wet-based ice streams, which are now known to dominate the grounded ice sheet. Modern analyses do not definitively determine just how ice streams affect the stability of the WAIS, but it can at least be said that there is no compelling theoretical reason to expect a rapid rise in sea level from the WAIS triggered by ice-shelf thinning. Of the three main ice-drainage systems in the WAIS, the one that flows into Pine Island Bay might be a particularly likely site for accelerated flow since there is no ice shelf to restrain the inflowing ice streams, yet measurements show that this system is not significantly out of mass balance. If the “Ross Embayment” system, which has undergone several sudden glacial reorganizations in the last thousand years, were unstable one might expect a history of large changes in the total outflow of ice into the Ross Ice Shelf, yet the total outflow in the “Ross Embayment” has remained relatively unchanged despite the large internal perturbations, a fact that, points to a stable, not an unstable, system. Study of the third major drainage from the WAIS, into the Ronne Ice Shelf, also suggests that there is no gross discordance between the present velocity vectors and flow tracers in the ice shelf, although the evidence is limited. In the light of the evidence for recent stability, it is difficult to see how climate warming (whether anthropogenic or natural) could trigger a collapse of the WAIS in the next century or two. Thus, I believe that a rapid rise in sea level in the next century or two from a West Antarctic cause could only occur if a natural (not induced) collapse of the WAIS were imminent. Based on a concept of pseudo-random collapse once per major glacial cycle, I estimate the chances of that to be on the order of one in a thousand.

The western part of the Netherlands has a typical Dutch landscape with fen-meadows that consist of wet pasture lands with drained peat soils alternated by natural and artificial lakes, ditches, reed swamps and quaking fens (see fig. 1). This area has been and is still being continuously drained, so as to keep the land dry and suitable for agriculture, pasture and residence. Water levels are artificially controlled in the region by local water management authorities. This drainage has resulted in subsidence of a couple of meters over the last centuries. As a result, the polders with fen-meadows now lie 1–2 m below sea level. In addition to that, we also find deep polders which used to be large lakes and were reclaimed in the 17th century for agricultural use. Presently, these polders are 2-6 m below sea level. The continuing subsidence of the surface in the polders and the rise in sea level caused that about 25% of the Netherlands is now being situated below mean sea level (up to 6.7 m). Without dikes and dunes 65% of the land would be flooded daily. This situation makes the Netherlands vulnerable to storm surges and river floods. The ’Green Heart’ (Groene Hart) is the rural center of the Dutch Randstad, surrounded by the biggest cities in the Netherlands: Amsterdam, The Hague, Rotterdam and Utrecht. The soil of the Green Heart contains mainly sand, peat and clay. The ground water level is controlled in order to avoid fast subsidence due to peat oxidation and at the same time to maintain a dry surface. Peat is composed of organic material which oxides when it is in contact with air, reducing in volume and consequently resulting in subsidence, and therefore, bringing the surface gets closer to the ground water. Thus, in order to have a dry ground suitable for agriculture, construction and recreation the land is periodically drained. Observing precise subsidence rates of peat and marsh lands using geodetic techniques is notoriously difficult, due to the difficulty of installing fixed benchmarks in this type of soil. Moreover, because of the soft soils, modern buildings have pile foundations, with pilings up to 25 m long, reaching to stable pleistocene sand layers. Consequently, while subsidence due to shallow surface compaction continues, most new buildings remain relatively stable. Figure 2 shows the subsidence rates estimated by 2 for a ground water level of -40 cm below surface. The results were obtained using boreholes. The area with the maximum deformation rates corresponds to the Krimpenerwaard, where a deformation of -5 to -11 mm/yr is expected. In this contribution we investigate the use persistent scatterer interferometry, (PSI) 1 to study shallow deformation in wetlands in The Netherlands. PSI utilizes a time series of space borne radar scenes to select scattering objects whose reflecting properties remain fairly constant over time and are therefore minimally affected by noise. The information about deformation is extracted from the interferograms, which contain the phase differences between two radar images. The PSI technique as developed at the TU Delft 3 is based on creating a first-order network of measurements, using the most coherent objects to estimate and remove atmospheric artifacts. Then a denser second order network is built from which the full deformation velocity field is derived. One of the major limitations of PSI techniques is that we cannot be certain about the object we observed. In any case, the position of the object is known with an error of about 10 m. However, PSI overcomes the limitations of traditional geodetic methods. It provides a very dense distribution of measurements (~ 100/km2 in urban areas, sensor dependent) and high observation frequency (once a month or higher, depending on satellite requirements).

Organic blue carbon sequestration in vegetated coastal wetlands: Processes and influencing factors

Every year, millions of migratory shorebirds fly through the East Asian-Australasian Flyway between their arctic breeding grounds and Australasia. This flyway includes numerous coastal wetlands in Asia and the Pacific that are used as stopover sites where birds rest and feed. Loss of a few important stopover sites through sea-level rise (SLR) could cause sudden population declines. We formulated and solved mathematically the problem of how to identify the most important stopover sites to minimize losses of bird populations across flyways by conserving land that facilitates upshore shifts of tidal flats in response to SLR. To guide conservation investment that minimizes losses of migratory bird populations during migration, we developed a spatially explicit flyway model coupled with a maximum flow algorithm. Migratory routes of 10 shorebird taxa were modeled in a graph theoretic framework by representing clusters of important wetlands as nodes and the number of birds flying between 2 nodes as edges. We also evaluated several resource allocation algorithms that required only partial information on flyway connectivity (node strategy, based on the impacts of SLR at nodes; habitat strategy, based on habitat change at sites; population strategy, based on population change at sites; and random investment). The resource allocation algorithms based on flyway information performed on average 15% better than simpler allocations based on patterns of habitat loss or local bird counts. The Yellow Sea region stood out as the most important priority for effective conservation of migratory shorebirds, but investment in this area alone will not ensure the persistence of species across the flyway. The spatial distribution of conservation investments differed enormously according to the severity of SLR and whether information about flyway connectivity was used to guide the prioritizations. With the rapid ongoing loss of coastal wetlands globally, our method provides insight into efficient conservation planning for migratory species.

Lim, C.; Park, S.-H.; Kim, D.-Y.; Woo, S.-B., and Jeong, K.-Y., 2017. Influence of steric effect on the rapid sea level rise at Jeju Island, Korea. In: Lee, J.L.; Griffiths, T.; Lotan, A.; Suh, K.-S., and Lee, J. (eds.), The 2nd International Water Safety Symposium. Journal of Coastal Research, Special Issue No. 79, pp. 189–193. Coconut Creek (Florida), ISSN 0749-0208. According to a report by the Korea Hydrographic and Oceanographic Agency (KHOA), the sea level rise in west coast (1.31 mm/yr), east coast (2.69 mm/yr) and south coast (2.05 mm/yr) of Korea are different. Especially, the seas around Jeju Island show one of the most rapid increase of sea level rise in recent years. This investigation on the data from Jeju and Seogwipo tidal station indicated that the sea level rises around Jeju and near Seogwipo were 5.63 mm/yr and 3.75 mm/yr, respectively. This shows that the northern and southern parts of the island have different temporal rates of sea level rise. The purpose of this research is to analyze the influence of the steric effect caused by density change in the water to the rate of sea level rise, by using sea temperature and salinity data measured by the National Institute of Fisheries Science (NIFS). It is found that the trend of seawater density variation near Jeju Island changed by the time of around 2006 and so does the sea level rise, indicating that there is a meaningful correlation between sea level rise and density variation. The rate of steric height rise by the density were calculated, resulting in a 15.8% and 23.2% of sea level rise in Jeju and Seogwipo, respectively.

Saltmarshes provide important ecosystem services including habitat for wading and migratory birds, nursery grounds for commercial fish species, carbon storage, and flood defence through wave attenuation. Stimulating saltmarsh growth may improve the local level of flood protection, reducing the need for costly engineering works to sea walls and defences, whilst also enhancing the provision of other services. This is particularly important at locations where there is a need to restore and compensate for the loss of saltmarsh due to erosion caused by sea level rise, land claim, and a reduction in sediment supply.  One method of encouraging marsh growth is through the construction of sedimentation fields or polders, typically out of brushwood fencing, to reduce current velocities and wave heights with the aim of increasing sedimentation rates.  However, little is known of the impact polders have on the timing and rate of sediment delivery, or of saltmarsh response to changes in hydrodynamics. This is particularly the case for relatively exposed sites with a large tidal range, with most sedimentation fields constructed in sheltered locations with micro- to meso-tidal ranges such as the Wadden Sea.Here, we present results from a study of a macro-tidal sedimentation field at Rumney Great Wharf, Severn Estuary, Wales, which was constructed between 1999 and 2005. Field investigations, conducted during May to June and November to December 2023, involved the deployment of sediment traps and measurements of the current velocity and suspended sediment concentration to assess spatial and temporal variations in sediment delivery. Results indicate increased sediment availability in areas of lower elevation, with sediment trap data suggesting a difference of up to 3.7 g / cm2 / day due to elevation differences. Sediment cores were also collected and analysed from both inside and outside of the polders to assess the physical functioning of the marsh that has formed following polder construction.Findings provide insights into the suitability of sedimentation fields as a form of saltmarsh restoration and coastal flood defence, which are discussed in terms of mitigating against sea level rise and increased storm magnitude and frequency. This study also provides an evaluation of the potential for wider implementation of sedimentation fields as part of shoreline management strategies. It is recommended that further assessment is conducted to evaluate the influence of fence design, including length, height, and orientation, on site evolution to maximise provision of flood defence and wider ecosystem service delivery from these schemes.