Sunday, March 3, 2024

We’re Sinking and Sea Level is Rising

This article is excerpted from the article cited below,  the Virginia Tech news release, the NOAA 2022 update to the Sea Level Rise Technical Report and a previous blog post. 

Leonard O Ohenhen, Manoochehr Shirzaei, Patrick L Barnard, Slowly but surely: Exposure of communities and infrastructure to subsidence on the US east coast, PNAS Nexus, Volume 3, Issue 1, January 2024, pgad426,

In 2022 NOAA Released an update to the Sea Level Rise Technical Report. The report project sea level along the U.S. coastline to rise, on average, 10 - 12 inches (0.25 - 0.30 meters) in the next 30 years (2020 - 2050), matching the rise measured over the last 100 years (1920 - 2020). Sea level rise will vary along U.S. coasts because of changes in both land and ocean height.

The east coast is expected to be the relative sea level hot spot over the next three decades projected to rise on average: 10 - 14 inches (0.25 - 0.35 meters). This hot spot along the east coast extends from Cape Hatteras, North Carolina to Boston, Massachusetts with the Southern Chesapeake Bay region will experiencing the most significant rise.

In the last century this area experienced the highest rate of sea level rise in the nation and is forecast to continue to have the highest sea level rise in the next 30 years due to glacial rebound, land subsidence and the rising sea levels. In the most recent study Ohenhen et al, looked at the contribution of land subsidence on the relative sea level rise. They found that the major cities on the U.S. Atlantic coast are sinking, in some cases as much as 5 millimeters per year – a decline at the ocean’s edge that well outpaces global sea level rise. The land subsidence is due to compaction from groundwater pumping. When you withdraw the groundwater from fine-grained compressible confining beds of sediments which are typical of the coastal regions (and other areas)  and do not replace it, the land subsides. 

To examine the phenomena the scientist used space-based radar satellites to build digital terrain maps that show exactly where sinking landscapes present risks to the health of vital infrastructure within 62 miles of the coastline. Using the publicly available satellite imagery, Ohenhen et al measured millions of occurrences of land subsidence spanning multiple years. They then created some of the world's first high resolution depictions of the land subsidence.

The scientists found that New York City, Long Island, Baltimore, Virginia Beach and Norfolk are seeing areas of rapid “subsidence,” or sinking land, alongside more slowly sinking or relatively stable ground. This differential subsidence  increase the risk of damage and failure to roadways, bridges runways, building foundations, rail lines, and pipelines,

In Virginia our local land subsidence is due to glacial rebound after the Laurentide ice sheet melted, excessive groundwater extraction from the coastal aquifers, as well as the effects of the meteor impact near Cape Charles, Virginia (about 35.5 million years ago). Combined, they are all causing the relative sea level rise that is the highest along the coastline. The Aquifer-system compaction from non-sustainable groundwater extraction accounts for more than 50% of the land subsidence observed in the coastal region. 

Land subsidence barely registers as an issue of concern in public policy. However, this slow, gradual, and unapparent land sinking motion magnifies the exposure of coastal residents to the increases in sea levels due to climate change.  Subsidence increases the threat to coastal communities from sea level rise and may even triple estimates of potential flooding areas over the next few decades. Even if current climate measures successfully curb rising sea levels, continuous land subsidence may result in irreversible inundation, more frequent flooding and damage to infrastructure in these coastal regions.

Subsidence of more than a few millimeters per year are a cause for concern, particularly in densely populated areas because subsidence can undermine building foundations; damage roads, gas, and water lines; cause building and bridge collapse. Differential subsidence is most damaging especially in areas with essential facilities like hospitals, schools, or transportation hubs.

These groundbreaking new maps generated by Ohenhen et al show that a large area of the East Coast is sinking at least 2 mm per year, with several areas along the mid-Atlantic coast (Virginia) of up to 1,400 square miles, sinking more than 5 mm per year. This is more than the current 4 mm per year global rate of sea level rise. These coastal regions, where most large cities are located  are on the front lines of climate change impacts and associated uncertainties due to the combined effect of subsidence and sea level rise.  

Over the past century population migrated to the low-elevation coastal areas.  Continued accelerating sea-level rise and land subsidence will increase the future vulnerability of coastal communities worldwide. The impact of sea level rise-amplified hazards on coastal communities, such as flooding and erosion, dominates discussion and planning  in global climate change discussions, with land subsidence (due to unstainable groundwater use) relegated to the background. Land subsidence, however, is a pernicious and growing problem on a global scale with more immediate hazards to coastal areas and often presents more pressing and localized challenges. Policy changes to better manage groundwater withdrawal could slow relative sea level rise.

The lead author of this study is Leonard Ohenhen, a graduate student working with Associate Professor Manoochehr Shirzaei at Virginia Tech’s Earth Observation and Innovation Lab. This work provides important quantitative data for coastal disaster resilience planning.


Leonard Ohenhen

Manoochehr Shirzaei

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