Four thousand years ago, rising seas decimated huge swaths of mangroves in Florida Bay.
Today, seas rising at a far greater rate, combined with increasing storms and drought, could lead to another catastrophic loss of mangroves that help keep the state from sliding into the sea, according to a new study published by the U.S. Geological Survey in the journal Nature Communications.
"This was surprising because mangroves are thought to be relatively resilient to sea level rise," said Miriam Jone, a USGS geologist and lead author for the study.
While previous studies revealed mangroves have disappeared amid rising seas in the past, this study is the first to show just how quickly that happened.
For the study, researchers looked at core samples from around Florida Bay. The samples showed that when sea levels were lower, much of the bay was part of the freshwater Everglades. But as saltwater rose, mangroves retreated inland. That coincided with an increase in storms and drought during times of climate variability, Jones said.
Scientists were surprised to discover just how quickly that happened, at time when seas were rising four to six times slower than today, she said.
That's because the sea rise coincided with more droughts and storms during a time of climate upheaval — like now — that erode the mucky peat where mangroves grow. When the peat starts to collapse, the mangroves grow at a lower elevation.
"They're unable to keep pace with the rate of sea level rise," Jones said.
Mangroves serve as a critical link between the land and sea, helping to stabilize the coastline while protecting it against violent storm surges generated by hurricanes. Mangrove forests also suck up tons of carbon from the atmosphere, providing a carbon sink for the state valued at between $2 billion and $3.4 billion.
But scientists fear they are coming under increasing threat, from both development and impacts from climate change. About half the planet's mangroves have disappeared. After Hurricane Irma, NASA researchers during aerial surveys discovered about 40 percent of mangroves in Everglades National Park had been damaged. Mangroves have evolved to withstand hurricanes, so they expected them to bounce back. But when they returned three months later, they were suprised to find how little of the forest had recovered.
In the Everglades in particular, scientists believe prolonged drought conditions created by flood control that cut off the historic flow of water south through the marshes have made the forest more vulnerable.
If that flow can be restored, Jones said it could give mangroves a better chance of surviving other conditions triggered by climate change. In the Caribbean, mangroves have fared better where ecoystems were healthier, she said.
"We know that mangroves can be resilient under high rates of sea level rise, but it also depends on how healthy the ecosystem is," Jones said.
The Everglades is now in the midst of massive $16.4 billion restoration launched in 2000. It was expected to take 30 years, but continued delays and lack of funding have mired progress, prompting the National Academies of Sciences to conclude in 2017 the work could take another 100 years at the current rate. That rate could also mean that sea rises outpaces restoration efforts.
"If you're able to at least reduce of the stressors," Jones said, "you increase the capability of the systems for survival under these accelerating rates of sea level rise we're having today."