Protecting coasts without cooking the planet
In the 2010s, when New York City decided to fortify the East River Promenade in the Lower East Side, it turned to a typical solution: emissions-intensive concrete and steel.
Around the same time, across the river in Queens, the city installed wetlands and bioswales to help Hunters Point Park South become a buffer against coastal flooding and to protect the surrounding urban area from extreme weather, including Hurricane Sandy, which made landfall during construction.
Despite a growing body of research on the benefits of such nature-based solutions (NbS), many coastal adaptation projects in the United States are still built with conventional gray engineering approaches—think lots of reinforced concrete. Trees, wetlands, and other natural elements that sequester carbon and support biodiversity, cooling, and public health are not widely or systematically integrated.
Over the last year I led a Harvard team focusing on 12 coastal adaptation projects in the United States, from Seattle to Miami. Our goal was to understand the actual carbon and cost implications of the choices being made and to test how to mitigate those impacts without compromising on performance.
We found that lower-carbon, lower-cost resilience is achievable today—but only if design, delivery practices, and policies change.
Measuring the emissions implications
Most climate policy debates separate mitigation (cutting greenhouse-gas emissions) from adaptation (preparing for impacts). On seacoasts, the two are tightly linked. Raising roads, constructing seawalls, and armoring shorelines all rely on carbon-intensive materials and processes. Without careful choices, the very infrastructure that protects communities can add to the emissions that hasten climate change.
In the Lower East Side, for example, a nature-based approach similar to the shoreline site in Queens would have emitted 77 percent less carbon.
Much of this impact comes from familiar culprits like the carbon-intensive manufacturing processes behind concrete and steel. But our research also identified a less obvious source: lightweight fill. To raise shoreline edges without causing them to sink, projects often depend on highly processed materials such as synthetic geofoam made from hydrocarbons, which carry a significant emissions burden.
Our findings show that simple changes, such as using lower-carbon concrete and sourcing materials more locally, could improve carbon impacts (the net combination of emissions reduction and sequestration increase) by an average of 64 percent. These easy wins offer substantial carbon benefits and modest cost savings, with little to no visual, programmatic, or structural implications on the project.
The larger gains come from fully embracing nature-based solutions. That might mean replacing a tall vertical seawall with a stepped edge of planted terraces, or reprioritizing a modest amount of space—an average of 12 feet—to planting instead of paving. These shifts require intention, coordination, and a willingness to work differently.
The payoff is significant: The study found potential cost savings up to 30 percent and an improved carbon impact of 91 percent. If applied globally on the projects needed to protect the 173 million residents of coastal urban and semi-urban developments who are expected to be impacted by sea level rise by 2050, 2 gigatons of emissions could also be avoided. That is comparable to 40 years of New York City's emissions.
So how do we get there? The study identified six key approaches:
- Lightweight fill: Using lower-carbon fill alternatives like expanded clay aggregates instead of traditional high-density foam or cellular concrete.
- Hyperlocal sourcing: Adaptation projects often require large volumes of heavy materials. Sourcing closer to the site reduces emissions from transportation.
- Recycled materials: On-site materials such as concrete and aggregates can be reused in new asphalt or shoreline armor, reducing emissions from sourcing virgin materials.
- Supplementary cementitious materials (SCMs): Options for replacing high-emitting cement in concrete include slag, fly ash, glass pozzolan, and many other emerging alternatives.
- Plant more: Increasing site vegetation increases carbon sequestration and has lower emissions compared with hardscaping.
- Use less: Modifying the design to reduce the overall amount of material directly reduces emissions.
Designers, engineers, and contractors can integrate these approaches by collaborating from project inception—selecting low-carbon, nature-positive materials with manufacturers, coordinating with contractors, and tracking and communicating performance over time.
The insights from this study can guide further research and help municipalities set carbon limits for site infrastructure, updating codes and standards so NbS become the default rather than the exception.
When implemented at scale, NbS can shift coastal adaptation from contributing to climate change to delivering net positive outcomes that sequester carbon, avoid future emissions, and restore ecosystems.
More information:
salatainstitute.harvard.edu/wp … -FINAL-7-31-2025.pdf
Provided by Salata Institute at Harvard University
