By Mariana Jordan Webb
Lighthouses—intentionally constructed in exposed areas close to sea level—have long been the “canary in the coal mine” when it comes to talking about historic structures being impacted by sea level rise. In recent decades, several historic lighthouses have been relocated inland and to higher ground due to encroaching waters. More recently, the threat of sea level rise has expanded to endanger entire neighborhoods in low-lying coastal areas like Annapolis, Maryland; San Francisco; Galveston, Texas; and New Orleans. I recently worked as an intern for the National Trust’s Preservation Green Lab and used geographic information system (GIS) mapping to develop new insights about the threats that sea level rise poses to historic resources in Boston, Massachusetts.
Quantifying Global Sea Level Rise
Global sea level rise is one of the most tangible consequences of a changing climate. In 2014 the National Oceanic and Atmospheric Administration’s (NOAA) National Ocean Service observed that the global sea level was 2.6 inches above the highest annual average on satellite record. Since then NOAA estimates that sea level have risen and will continue to rise at least 1/8 of an inch each year. While that may not seem like much, increased frequency of storm surges coupled with higher global sea levels threaten to flood coastal areas in the United States and around the world.
Furthermore, rising sea levels represent a clear social justice issue. Rural coastal communities, as well as areas with lower-income residents, are already finding it challenging to secure the substantial resources needed to protect their residents—and their historic resources—from sea level rise. Some communities, such as the Native American Isle de Jean Charles tribe in Louisiana, are relocating to higher, inland ground, forced to leave behind their homes and historic lands.
With significant coastal land areas projected to flood in the coming years, it is important to understand which communities will be most vulnerable in order to mitigate risks. The National Trust is invested in understanding how sea levels will potentially impact historic resources along the coasts of the United States. This work will help quantify the threat of sea level rise, delivering essential information to decision-makers who can help mitigate the effects of global warming as well as help communities proactively prepare for future flooding.
Analyzing the Impact of Sea Level Rise in Boston
The National Trust selected Boston, Massachusetts, as a prototype for this GIS mapping for two reasons: because of its high density of historically significant buildings and for its coastal location and large number of historic resources located close to sea level. The analysis used historic and demographic datasets derived from the Trust’s Atlas of ReUrbanism. Sea level rise data were based on a combination of the NOAA’s Coastal Services sea level rise inundation datasets and Federal Emergency Management Agency flood zone data. GIS software was used to overlay the datasets, creating vulnerability maps that illustrate which of Boston’s historic structures are at risk from various sea level rise projections.
A number of interesting results emerged from the GIS analysis of the historic, demographic, and sea level rise datasets:
- While 2 feet of sea level rise will only impact 0.1 percent of Boston’s population, 6 feet of sea level rise will impact or displace close to 4.5 million Boston residents—19 percent of the population.
- Six feet of sea level rise will impact 74 percent of buildings that are categorized as either national, state, or local historic landmarks or are in national or local historic districts.
- The median income in Boston parcels impacted by 2 feet of sea level rise is $260,000, compared to $110,000 for 6 feet of sea level rise and $94,000 overall. In other words, the properties first impacted by sea level rise will be higher-value real estate.
This analysis, if performed nationwide, could provide an objective and reproducible risk assessment quantifying the vulnerability of coastal historic structures to a range of projected sea level rise. Improved understanding of the potential impacts of sea level rise will help the National Trust better advocate for proactive preparedness measures to protect at-risk historic resources. It also positions the Trust to assess the national need for policies to mitigate sea level rise. The Trust, along with its regional and local partners, can use sea level risk assessments to mitigate impacts and protect vulnerable historic structures.
Mariana Jordan Webb, a junior at Dartmouth College majoring in geography, Earth sciences, and Spanish, worked as an intern with the Preservation Green Lab during winter 2017.