• Ashley Riane Booth

Sea Level Rise and Coastal Wetlands: Predicting the Future

This post was originally published on Envirobites.

What is happening to coastal wetlands around the world?

Imagine being told that the land your home now sits on will be part of the ocean in 50 years. Or that within the next 20 years, the area where you live will experience so much flooding that you’ll have to move further inland. This is a reality for many people around the world as coastal wetland loss and sea level rise impact their homes.

Coastal wetlands around the world are disappearing as a result of changes to water flow (hydrology), subsidence or sinking of existing land, erosion, and development. In addition to existing threats, sea level rise poses a new challenge for coastal wetlands. With rising sea levels come increased flooding and salinity, both of which will influence the plants, animals, and humans that call these ecosystems home.

Images of land loss in coastal Louisiana between 1976 and 1999.
Satellite imagery showing changes in coastal wetland area between 1976 (bottom) and 1999 (top). Photo Credit: NASA Earth Observatory | Marine Photo Bank.

Why do we care about how sea level rise will impact coastal wetlands?

Coastal wetlands serve as a buffer to protect inland areas from storm surge and erosion. However, as sea level rises, coastal wetlands are expected to decrease in area. Less coastal wetlands will translate to greater flood risk for more inland communities. These impacts can already be seen in places like coastal Louisiana where subsidence, erosion, and sea level rise have led to significant wetland loss and frequent flooding throughout the state.

Aerial image of a flooded coastal city following landfall of Hurricane Ike.
Flooding in the coastal community of New Iberia, Louisiana following Hurricane Ike. Photo courtesy of Coast Guard Jayhawk 6031.

How do we predict what the coast will look like in the future?

To better understand how sea level rise will impact coastal wetlands (and people who live on the coast), scientists develop models to predict how ecosystems will change. By predicting change over time, scientists can better understand the risks posed to communities along the coast and identify regions or ecosystems in the greatest need of protection or restoration. These models also allow scientists to understand what factors might be driving change in coastal wetlands or how wetlands will respond to increased flooding or salinity.

Aerial image of coastal Louisiana and projections of how much land will be lost by the year 2100.
For example: a projection of coastal land loss in Louisiana by the year 2100, created using a scientific model. Red areas are land that will be lost by the year 2100. Image courtesy of USGS.

What are models?

Today, mathematical or computer-generated models are common in the scientific world. Scientists base their models on real data and the current understanding of how an ecosystem or process works. By including important factors that may influence a process and using a computer to generate possible relationships between factors, scientists can get a better understanding of what may be driving the process or event they see in the real world.

Why are there so many different models for how sea level rise will impact the coast?

There are countless models that demonstrate what coastal wetlands will look like in the coming century. Part of the reason there are so many is because each builds on the ones that have gone before, or includes factors that were previously thought unimportant. Because of this, many models appear to conflict. Some project greater wetland losses than others, while others show that coastal wetlands will be able to keep up with high rates of sea level rise.

For example, in their 2009 model, Blum and Roberts outlined a scenario where much of the Mississippi River Delta is lost by the year 2100 due to reduced sediment input (sedimentation) from the Mississippi River and sea level rise.

In the past, when the Mississippi River Delta was formed, sea level rise was less than 1 mm/year and sediment carried down the River was 2-2.5 times greater than what is now present in the Mississippi River. In theory, enough sediment was deposited on the surface to physically increase elevation and help the marsh keep up with sea level rise. Now, sea level rise is 9-11 mm/year along the Louisiana coast and sediment input to coastal wetlands is greatly reduced.

Blum and Roberts argue that a lack of sedimentation will be responsible for dramatic wetland loss. However, this model does not account for plant processes like organic matter accretion (the buildup of soil through plant growth) and may overestimate the risk of sea level rise to coastal wetlands.

On the other hand, Kirwan et al. (2016) suggest that models of wetland loss overestimate their vulnerability to sea level rise. They argue that, in the past, marshes have been able to keep up with sea level rise by organic matter accretion and inland migration. Therefore, today’s coastal wetlands should be able to keep up with sea level rise by employing these same techniques. Their model, however, does not consider how sediment inputs have changed over time, or how roads and levees have prevented inland migration of coastal marshes.

If the models don't agree, why keep making models to predict wetland loss?

It is difficult to predict exactly what the coast will look like in the future, as models are often conflicting. Different models focus on different aspects of how sea level rise will impact coastal wetlands.

Despite conflicting models, it is undeniable that dramatic wetland loss will occur around the world – places like coastal Louisiana are already experiencing this phenomenon. The degree of change, however, is less clear.

Continuing to perfect models of coastal wetland change due to sea level rise is important, as it allows scientists to highlight the need to take action to slow projected losses or impacts on coastal communities. In the future, by including factors that previous models found to be important, models will be better able to predict coastal wetland change over time.

Featured image: Flooding in the coastal community of New Iberia, Louisiana following Hurricane Ike. Photo courtesy of Coast Guard Jayhawk 6031.

Source Articles:

Blum, M.D. and H.H. Roberts. 2009. Drowning of the Mississippi Delta due to insufficient sediment supply and global sea-level rise. Nature Geoscience. 2:488-491.

Kirwan, M.L., S. Temmerman, E.E. Skeehan, G.R. Guntenspergen, and S. Fagherazzi. 2016. Overestimation of marsh vulnerability to sea level rise. Nature Climate Change. 6:253-260.

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