Tidal wetlands are critical, yet vulnerable ecosystems. Tidal marshes, mangrove forests, and tidal flats support biodiversity, protect against flooding and storm surges, sequester carbon, and improve water quality.
Due to human development and climate change, tidal wetland areas have been shrinking globally. A new study using 40 years of satellite data shows that this loss has been accelerating in the U.S. and that this acceleration is being increasingly driven by extreme weather events.
This work was led by Xiucheng Yang, a former UConn postdoctoral researcher and current senior research fellow at the University of Victoria, and Zhe Zhu, an associate professor and director of Global Environmental Remote Sensing (GERS) Laboratory in the Department of Natural Resources and the Environment in the College of Agriculture, Health and Natural Resources ( CAHNR ). The study was published in the journal Nature Communications.
"What we provide here is that we are continuously and consistently monitoring tidal wetland change," Yang says. "This way we can link the change to specific events like hurricanes or storms."
The predominant hypothesis about what is driving tidal wetland loss has long held sea level rise responsible. However, Yang and Zhu's research proves there's more to the story.
This paper marks the very first time scientists have successfully linked tidal wetland loss to specific storm events. This is especially significant given that storms are becoming more intense and frequent due to climate change.
"By separating the abrupt changes (e.g., extreme weather events) and gradual changes (e.g., sea level rise), we are able to quantify their contribution to the tidal wetland loss," Zhu says. "The total area loss is still dominated by sea level rise. But, if you're looking at the acceleration of this loss, it's actually dominated by extreme weather events (1.4 times that of the chronic stressors)."
Since 1985, the U.S. has lost more than 7.5%, or 1,600 square kilometers, of its tidal wetlands. This loss is accelerating by approximately 0.73 square kilometers per year.
Studying tidal wetlands poses a unique challenge for researchers because the landscape changes with the tides. To compensate for this influence, Yang and Zhu developed a time series model called DECODE (DEtection and Characterization of cOastal tiDal wEtlands).
While existing algorithms can determine what kind of land is shown in an image, researchers historically struggled to attribute the mapped losses to specific historical extreme weather events to determine when and why the tidal wetland area was decreasing over time.
"We don't have comprehensive driver maps," Zhu says. "So, we are using the high-frequent tidal wetland loss map to create a random sample and manually interpret, looking at the historical data, what is a major driver of this tidal wetland loss."
Tidal wetland loss is not happening consistently across the country. The Gulf Coast is losing tidal wetlands at the highest rate due to high relative sea level rise and increasing extreme weather in the region. In San Francisco Bay, however, tidal wetland area is actually increasing. This is thanks to ongoing efforts to restore and protect tidal wetlands and the natural lack of hurricanes.
One type of tidal wetland, the mangrove forest, is expanding its reach. Mangrove forests are replacing tidal marshes in some parts of Florida, Louisiana, and Texas because they are better adapted to handle sea level rise and extreme weather events.
The researchers emphasize that their findings point to a critical need for more adaptive restoration strategies.
"[Normally] after a storm, tidal wetlands can recover by themselves," Yang says. "Due to the increased frequency and intensity of hurricanes, they are losing that recovery capacity. So that means that after a hurricane, we need to provide a more proactive management plan to help them recover to predetermined conditions."
