Saturday, July 20, 2024

Coastal wetlands can’t keep pace with sea-level rise, and infrastructure is leaving them nowhere to go

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Wetlands have flourished along the world’s coastlines for thousands of years, playing valuable roles in the lives of people and wildlife. They protect the land from storm surge, stop seawater from contaminating drinking water supplies, and create habitat for birds, fish and threatened species.

Much of that may be gone in a matter of decades.

As the planet warms, sea level rises at an ever-faster rate. Wetlands have generally kept pace by building upward and creeping inland a few meters per year. But raised roadbeds, cities, farms and increasing land elevation can leave wetlands with nowhere to go. Sea-level rise projections for midcentury suggest the waterline will be shifting 15 to 100 times faster than wetland migration has been clocked.

Many wetlands, like these on Cape Cod, are now bordered by human infrastructure, including towns and roads.
Lance Cheung/USDA

I have been studying coastal geology and wetlands for more than 40 years. The rates of sea-level rise that we’re seeing now mean portions of today’s coastal wetland ecosystems will be lost to the ocean in the years and decades to come at a tempo never seen before.

Wetland plants were able to keep pace in the past

The presence of expansive wetlands along the coasts is in large part because sea level has been relatively stable for millennia.

Wetland plants could easily adapt to small changes in the ocean level because those changes were typically less than 1 millimeter per year. The plants naturally create or trap sediment, building elevation to keep up with rising seas. Wetland ecosystems are also adept at migrating horizontally and therefore could colonize areas of the coastline that were slowly flooded over time.

However, the world’s climate began to change about a century and a half ago. Fossil fuel combustion in factories and vehicles sent increasing amounts of carbon dioxide into the atmosphere, raising global temperatures. This also warmed the oceans, causing them to expand, and accelerated the melting of glaciers and ice sheets. The combination of thermal expansion of seawater and melting land ice has added volume to the ocean, causing sea level to rise at ever-faster rates.

Three charts show thermal expansion as the highest contributor to sea level rise so far

Contributors to sea-level rise.
Climate.gov

Water level data collected by the National Oceanic and Atmospheric Administration at gauges distributed along the U.S. coast indicates the average rate of sea-level rise is now about 10 millimeters per year – much faster than in the past – and it is expected to continue accelerating. During the second half of the 21st century, scientists expect sea-level rise to average about 30 millimeters per year. That’s 30 times faster than it was rising before the Industrial Revolution.

Sea level is rising faster than wetlands can adapt

The rates of sea-level rise along the mid-Atlantic and southeastern U.S. coasts are now faster than most coastal wetland plant communities can build elevation.

Within decades, it is very likely that much of these crucial ecosystems will drown. Areas once occupied by a lush, expansive and contiguous coastal wetland ecosystem will, over time, become riddled with small, isolated ponds that enlarge, interconnect and eventually transform to open seawater.

A view across a marsh with tall grasses, open expanses of water and trees in the background, include many dead trees at the edge of the forest.
Trees are becoming a ghost forest in parts of the Blackwater National Wildlife Refuge, where sea-level rise and land subsidence are raising water levels.
Ataraxy22 via Wikimedia, CC BY-SA

Illustrations paired with satellite images show the most wetland migration where elevation and urban landscapes don't block the way and the least in urban areas.

Wetlands before and after sea-level rise: Illustrations paired with satellite images and models of those examples with sea-level rise show how wetlands can migrate (a-d) or be blocked by elevation (e-h) or human infrastructure (i-l). Dark pink areas are current wetlands. Light pink indicates future wetlands. Black indicates urban areas that prevent migration. The examples are from Apalachicola River Delta, Fla.; Laguna Madre, Texas; and Tampa Bay, Fla.
Nicholas M. Enwright, Kereen T. Griffith, Michael J. Osland/U.S. Geological Survey

A regional analysis I recently conducted of published horizontal rates of wetland migration along the U.S. mid-Atlantic and eastern Gulf of Mexico coastal plains suggests wetlands can move inland at an annual pace measured in meters to tens of meters. In the same areas, however, theoretical rates at which the shoreline will move inland – calculated using NOAA sea-level projections in the year 2050 – range between 100 to 150 meters per year. That’s 15 to 100 times faster than wetlands can move.

The inundation models that scientists typically use to emulate coastal wetland resilience to rising seas assume wetlands will migrate inland at the same rate as the shoreline. As a result of the work I recently completed, it appears the models have overestimated coastal wetland resilience and that wetland losses this century will be higher than predicted.

Even under the most ambitious scenarios for reducing carbon dioxide emissions, sea-level rise will likely continue to accelerate this century. That means the loss of wetland ecosystems as they currently exist along our coastlines will continue for generations as the climate continues to warm and the oceans rise.

What you need to know about salt marshes. The News & Observer.

3 ways to protect these valuable ecosystems

So, what can be done to save our coastal wetlands? What actions can be undertaken to minimize the loss of these ecologically and economically important areas?

First, land managers could implement programs to acquire or create conservation corridors adjacent to areas where coastal wetlands currently exist. These undeveloped or natural landscapes will provide a pathway through which the wetlands can move inland as sea level rises.

A large white bird wanders through water at the edge of grasses.
Wetlands are valuable nurseries for fish and provide habitat for other wildlife, including many coastal birds species.
Rachel C via Unsplash, CC BY-ND

Second, it seems ever more urgent to remove human-built structures – roads, dikes, levees – that will prevent the landward advance of coastal wetlands.

A project in Oregon did this by removing an earthen dike to allow the wetlands to migrate. In South Florida, engineers elevated miles of U.S. Highway 41, locally known as the Tamiami Trail, between Miami and Naples to allow the resumption of water flows that had been cut off.

An elevated roadway under construction over a far-reaching marsh area.
Elevating the Tamiami Trail allowed fresh water that had been blocked by the road to flow into the wetlands, reviving the ecosystems.
National Park Service

Finally, in areas historically drained for agriculture and flood control, surface water management projects can restore freshwater flow toward the coastal zone. In South Florida, for example, rainwater in the Everglades prevented salt water from advancing underground – until extensive canals were built to drain the region for development and agriculture. Several large water management projects are now redirecting rainwater back into the Everglades to slow the advancing underground saltwater plume, which can be lethal to freshwater wetland and upland plant communities.

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