How Sea Level Rise Will Affect Salt Marshes

Mark Altaweel


Rising sea levels pose a significant threat to various ecosystems, with salt marshes among those most at risk. This shift has the potential to cause widespread harm to both natural habitats and human communities in the forthcoming years. Efforts to mitigate these impacts are essential to reduce the impending damage, which is becoming increasingly apparent. Despite these efforts, there remains a real risk of substantial loss to ecosystems in the near future.

90% of salt marshes by 2100 will be impacted by rising sea levels

Salt marshes are some of the most biological diverse and ecologically productive areas in the world. These areas play a critical role in protecting coastlines, nurturing biodiversity, and a source of blue carbon storage. Researchers have calculated that about 90 percent of salt marshes are under threat from rising sea levels predicted to occur by the year 2100.

Research conducted over a 50 year period by the University of Chicago’s Marine Biology Lab (MBL) Ecosystems Center shows rapid changes are happening to salt marshes. What started off as research on nitrogen levels in salt marsh areas transformed into a long-term study of rising sea levels based on long-term observations at the Great Sippewissett Marsh in Massachusetts.

Rising sea levels affect nitrogen

Already, species are changing as they adapt to changing sea levels, indicating that changes are now rapidly occurring.[1]  Changes to sea levels affect nitrogen levels that then affects vegetation. The research showed that increasing nitrogen from sea level change helped to enable higher levels of vegetation which then led to accretion of marsh surface.

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In low marshes, cordgrass thrives as nitrogen increases. On the other hand, marsh hay decreased as sea levels rose affecting high marsh species. In fact, low marsh species will likely be completely replaced by high marsh species as seas rise. In another example, saltgrass increased, which helped raise marsh levels. However, this does not allow marshes to keep up with the pace of increasing sea levels.

By the end of this century, roughly 90 percent of the world’s salt marshes will be submerged within shallow seas even if some elevation is accrued due to changing species in salt marshes. Low marsh species will, therefore, also be lost.

A view of a wetland with marshes on either side of a body of water.
Alviso Slough is where the freshwaters of the Guadalupe River meet the saline waters of the San Francisco Bay. Photo: Caitlin Dempsey.

Building sea walls to combat sea level rise along coastal areas adversely affects salt marshes

One problem is that mitigation efforts to fight rising coastal sea level rise is contributing to the loss of salt marshes. Building sea walls, for instance, can lead to the destruction of salt marshes. This will have the negative effect of preventing natural progression of salt marshes, which is needed to migrate and naturally change the coastline over time. Salt marshes could potentially migrate effectively to higher ground, but this may not be easily possible considering there might be a change in habitat.

Human development is converting salt marsh to other land uses

Overall, salt marshes face threats from coastal development as well as sea level rise.[2] The results also echo earlier work that shows global salt marshes are in major decline, potentially exacerbating climate change due to carbon storage loss.

Between 2000 and 2019, Landsat data have showed that 1,453 square kilometers of salt marshes have been lost over the 19 year period, which is twice the area of Singapore. This is equivalent of losing about 0.28% of salt marshes per year. Climate change and rising sea levels, along with human development, are causing the transformation of coastal areas into urban and other land uses, significantly contributing to the destruction of salt marshes.

Intensification of hurricanes is accelerating salt marsh loss

Additionally, major storm surges from hurricanes have exacerbated salt marsh degradation. The increasing frequency of hurricanes, a consequence of climate change, not only results in further loss of these marshes but also contributes to a significant release of carbon into the atmosphere—approximately 16.3 million metric tons annually, equivalent to the emissions of 3.5 million cars.

The United States and Russia have seen the most significant losses of salt marshes. Notably, the period between 2005 and 2009 witnessed a spike in hurricane activity, leading to unprecedented salt marsh losses in North America and even affecting regions like the Arctic. In places like Oceania, the encroachment of mangroves might be another factor behind salt marsh reduction, though its impact remains uncertain.[3]

The loss of salt marshes will have severe consequences for us, particularly coastal communities that will be most impacted. To mitigate salt marsh loss, schemes that first preserve them may be needed. For instance, not building on salt marshes should be a key first step. However, even this may only slow the loss as sea levels rise. Thus, the only way to realistically change the loss of salt marshes is to severely limit our greenhouse emissions. Even if successful, this will take some time to have a positive impact on global climate, but currently that appears to be the best option.


[1]    For more on how salt marshes are likely to be affected by changing sea levels, see:  Valiela I, Chenoweth K, Lloret J, et al. (2023) Salt marsh vegetation change during a half-century of experimental nutrient addition and climate-driven controls in Great Sippewissett Marsh. Science of The Total Environment 867: 161546. DOI: 10.1016/j.scitotenv.2023.161546.

[2]    A recent popular article about this new research on changing sea levels affecting salt marshes can be found here:

[3]    For a study on global salt marsh loss over a 19 year period, see: Campbell AD, Fatoyinbo L, Goldberg L, et al. (2022) Global hotspots of salt marsh change and carbon emissions. Nature 612(7941): 701–706. DOI: 10.1038/s41586-022-05355-z.

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About the author
Mark Altaweel
Mark Altaweel is a Reader in Near Eastern Archaeology at the Institute of Archaeology, University College London, having held previous appointments and joint appointments at the University of Chicago, University of Alaska, and Argonne National Laboratory. Mark has an undergraduate degree in Anthropology and Masters and PhD degrees from the University of Chicago’s Department of Near Eastern Languages and Civilizations.