Environmental Change, Earth Deformation, Coastal Resilience

Rising sea levels pose significant challenges to coastal communities and ecosystems (IPCC, 2021). The global sea level rose by 0.20 m between 1901 and 2018, with an average rate of 1.3 mm/year between 1901 and 1971, increasing to 1.9 mm/year between 1971-2006, and further increasing to 3.7 mm/year between 2006 and 2018 with high confidence (IPCC, 2021). This acceleration will likely continue throughout the 21st century.  However, on a local scale, the rate of regional sea-level change could be larger than the average global sea-level change due to processes that affect vertical land motions such as groundwater pumping/recharge, sediment compaction, hydrologic loading, long-term tectonics, Glacial Isostatic Adjustment (GIA), dynamic topography, and sediment loading alongside changing climate. To quantify the impacts of relative Sea-Level Rise (RSLR) on coastal flooding, socioeconomic exposures, and risks, it is crucial to obtain accurate Vertical Land Motion (VLM) rates at management-relevant resolution (Tens of meter square (m2)). Relative sea-level change refers to sea surface changes over time concerning a local land elevation in response to changing climate and non-climatic processes. 

As a part of my doctoral degree, I applied an advanced multitemporal algorithm to combine the entire archive of Sentinel-1 and ALOS SAR satellites and GNSS observations to span more than a decade of land elevation change in the region and obtain spatially and temporally high-resolution long-term VLM maps and improve their uncertainties. The availability of these datasets enables the first comprehensive evaluation of future flooding and inundation hazards for the Chesapeake Bay by combining VLM rates with the LiDAR topographic dataset, SLR projections under different Shared Socioeconomic Pathways (SSPs) scenarios, and storm surge height estimates from Hurricane Sandy and Isabel for the period 2030-2100 (See figures below). High-resolution VLM rates with mm-level accuracy, improved forecasts of relative SLR rates, and updated maps of inundation hazards are essential for informing policymakers and authorities and developing flood resiliency plans to compact severe consequences of climate change in the region.

 This approach is also being applied in other global projects, and I have had the opportunity to collaborate on projects in Africa (manuscript in prep. see details of AGU Abstracts below) and the US Coasts (Nature paper published in 2024). 

I am currently expanding my local-scale research to a global scale. The manuscript relating to this work is submitted (see details below).

Left: Projected inundation area from both subsidence and SLR under SSP 1–1.9 in years 2030, 2050, and 2100 (top panels, panels (a)–(c), respectively). The bottom panel (d)–(f) highlights zoomed-in inundation from sea level and subsidence at 2100.

Right : Inundated area from subsidence and SLR () (in orange), and only SLR (in blue in ) at SSPs 1–1.9, 1–2.6, 2–4.5, 3–7.0, and 5–8.5 (panels a-e), respectively, with likely upper (83%) and lower (17%) ranges. Inundation area (km2) from storm surge based on (f) October 2012 Hurricane Sandy data sets and (g) Hurricane Isabel recorded on 18–19 September 2003 from NOAA and SLR scenario of highest emission SSP 5–8.5 in km2 (medium confidence).

Related Publications

1. Ohenhen L. O., Shirzaei M., Ojha, C., Sherpa S.F., Nicholls R.  (Nature). Disappearing Cities in US Coasts (https://doi.org/10.1038/s41586-024-07038-3 ).

2. Sherpa, S. F., Shirzaei, M., & Ojha, C. (2022). Disruptive Role of Vertical Land Motion in Future Assessments of Climate Change-Driven Sea Level Rise and Coastal Flooding Hazards in the Chesapeake Bay, Journal of Geophysical Research. https://doi.org/10.1029/2022JB025993

Manuscript Under Revision and In Prep. 

3. Sherpa, S.F., & Shirzaei, M. (Science Advances, Revision). Global Climate Change and Local Land Subsidence Accelerating 21st-Century Relative Sea Level. 

4. Dasho O. A., Ohenhen L. O., Sherpa S.F., Almar R., Shirzaei M. (In prep.). Future Flood Hazards in Africa’s Coastal Cities Due to Climate-Driven Sea Level Rise and Local Vertical Land Motion. 

Conference Abstract (#Talk)

1. Dasho O. A., Ohenhen L. O., Sherpa S.F., Almar R., Shirzaei M. (2024). Future Flood Hazards in Africa’s Coastal Cities Due to Climate-Driven Sea Level Rise and Local Vertical Land Motion. In the American Geophysical (AGU) conference, San Francisco (Scheduled) 

2. #Sherpa, S.F., Shirzaei M.,Ojha C.(2021). Sea-Level Rise and Flooding Hazard Assessment in the Chesapeake Bay of the United States: Analysis of Subsidence and Sea-Level Rise Scenarios using Radar Remote Sensing. In the American Geophysical (AGU) conference, Application of Equitable Solutions to Future Sea Level Rise Session. December 13, New Orleans, USA.