R.J. Nicholls’s research while affiliated with University of East Anglia and other places

Asian megadeltas, specifically the Ganges‐Brahmaputra‐Meghna, Irrawaddy, Chao Phraya, Mekong, and Red River deltas host half of the world's deltaic population and are vital for Asian countries' ecosystems and food production. These deltas are extremely vulnerable to global change. Accelerating relative sea‐level rise, combined with rapid socio‐economic development intensifies these vulnerabilities and calls for a comprehensive understanding of current and future coastal flood dynamics. Here we provide a state‐of‐the‐art on the current knowledge and recent advances in quantifying and understanding the drivers of coastal flood‐related hazards in these deltas. We discuss the environmental and physical drivers, including climate influence, hydrology, oceanography, geomorphology, and geophysical processes and how they interact from short to long‐term changes, including during extreme events. We also jointly examine how human disturbances, with catchment interventions, land use changes and resource exploitations, contribute to coastal flooding in the deltas. Through a systems perspective, we characterize the current state of the deltaic systems and provide essential insights for shaping their sustainable future trajectories regarding the multifaceted challenges of coastal flooding.

River deltas ofer numerous ecosystem services and host an estimated global population of 350 million to more than 500 million inhabitants in over 100 countries. To maintain their sustainability into the future, deltas need to withstand sea-level rise from global warming, but human pressures and diminishing sediment supplies are exacerbating their vulnerability. In this Review, we show how deltas have served as environmental incubators for societal development over the past 7,000 years, and how this tightly interlocked relationship now poses challenges to deltas globally. Without climate stabilization, the sustainability of populous low-to-mid-latitude deltas will be difcult to maintain, probably terminating the delta–human relationship that we know today.

Intermittently open/closed estuaries provide important ecosystem services but are often overlooked in coastal–catchment research and management. These estuaries are highly vulnerable to human/climate disturbances due to their tendency to close off from the ocean, yet their processes/dynamics remain under-researched. This study maps the global distribution of at least 2,245 intermittent estuaries, whose catchments currently support 55 million people, with projections rising to 101 million by 2100. Assessing three decades of scholarly articles indicated that only 7% of these sites have been studied. Academic literature on intermittent estuaries accounted for 0.5% of the total literature on all estuaries, despite these systems representing 4–5% of the estimated total number of global estuaries. Significant research gaps exist in Asia, South America, and Africa, where the largest, most susceptible populations reside. 90% of the existing research on intermittent estuaries is conducted in (southern) Africa (42%), Oceania (35%), and North America (14%), predominantly through domestic efforts. From 1992 to 2023, 60% of the research focused on physio-chemical and eco-hydro-geomorphological topics, with minimal attention to ecosystem services, climatic/human disturbances, and management. Our assessment underscores the need for increased focus on intermittent estuaries and suggests strategies to promote international collaborations, including leadership from intergovernmental organisations.

The definition of decision support tools in the context of climate change and adaptation is explored, highlighting the variation in approaches to design and form of tools. Several challenges are identified that have impeded the successful development of decision support tools, including financial restrictions, time constraints and meaningful stakeholder engagement. We highlight a number of potential areas for future research, including work to address the challenges of scaling up decision support tools and stronger frameworks for guiding stakeholder engagement.

Coastal subsidence can significantly increase rates of relative sea level change over and above climate-related changes, and has been reported to be particularly large in densely populated regions. However, due to the small scale variability of vertical land motion (VLM) and the sparsity of VLM observations, there is currently still low confidence in VLM and the uncertainties this introduces in sea level change estimates. Therefore, we synergize currently available VLM observations (from GNSS, InSAR, tide gauges, and satellite altimetry) to investigate the impact of VLM on relative sea level changes and their uncertainties, as well as the implications for coastal populations. We find that the average contemporary (1995-2019) global relative sea level change experienced by coastal populations (5.14 mm/year) is more than twice as large as the average coastal relative sea level change (2.09 mm/year). This is because rates of subsidence tend to be higher in densely populated areas. A significant part of the coastline (representing 40% of the coastal population) lacks measurements or access to the direct measuring stations which are necessary to constrain VLM rates estimated with InSAR. Missing, or imprecise VLM constraints contribute to the increased population-weighted uncertainties in the relative sea level change trends of ~3 mm/year. Thus, future community efforts are needed to improve the observational database in the disproportionately exposed and densely populated coasts in order to reduce uncertainties in estimates of future relative sea-level rise and their societal implications.

Climate change and economic growth are having a profound influence on the integrity of socio-economics and ecology of coastal Bangladesh. In the extreme, there are widespread expectations of inundation and coastal abandonment. However, results from our integrated assessment model (IAM) show that over the next 30 years, development choices might have a stronger influence on livelihoods and economic wellbeing than climate driven environmental change. The IAM simulates the economic development of rural areas by coupling physical models (driven by expectations of climate change) with economic models (informed by a series of policy decisions). This is done using substantial primary, secondary and stakeholder-derived biophysical and socio-economic datasets, together with shocks such as cyclones. The study analyses the future socio-ecological sensitivity to climate change and policy decisions and finds that well managed development is as important as adaptation to mitigate risks, reduce poverty and raise aggregate well-being. This analysis enables decision makers to identify appropriate development pathways that address current social-ecological vulnerability and develop a more resilient future to 2050 and beyond. These policy actions are complementary to climate adaptation and mitigation. Our IAM framework provides a valuable evidence-based tool to support sustainable coastal development and is transferable to other vulnerable delta regions and other coastal lowlands around the world.

Sea level rise (SLR) is a long‐lasting consequence of climate change because global anthropogenic warming takes centuries to millennia to equilibrate for the deep ocean and ice sheets. SLR projections based on climate models support policy analysis, risk assessment and adaptation planning today, despite their large uncertainties. The central range of the SLR distribution is estimated by process‐based models. However, risk‐averse practitioners often require information about plausible future conditions that lie in the tails of the SLR distribution, which are poorly defined by existing models. Here, a community effort combining scientists and practitioners builds on a framework of discussing physical evidence to quantify high‐end global SLR for practitioners. The approach is complementary to the IPCC AR6 report and provides further physically plausible high‐end scenarios. High‐end estimates for the different SLR components are developed for two climate scenarios at two timescales. For global warming of +2°C in 2100 (RCP2.6/SSP1‐2.6) relative to pre‐industrial values our high‐end global SLR estimates are up to 0.9 m in 2100 and 2.5 m in 2300. Similarly, for a (RCP8.5/SSP5‐8.5), we estimate up to 1.6 m in 2100 and up to 10.4 m in 2300. The large and growing differences between the scenarios beyond 2100 emphasize the long‐term benefits of mitigation. However, even a modest 2°C warming may cause multi‐meter SLR on centennial time scales with profound consequences for coastal areas. Earlier high‐end assessments focused on instability mechanisms in Antarctica, while here we emphasize the importance of the timing of ice shelf collapse around Antarctica. This is highly uncertain due to low understanding of the driving processes. Hence both process understanding and emission scenario control high‐end SLR.