Mangrove Forests Won't Survive Expected Sea-Level Rise by 2050 If Greenhouse Gas Emissions Aren't Reduced

The Earth's valuable mangrove forests likely won't survive expected sea-level rise by 2050 if humanity does not reduce its greenhouse gas emissions, a study has suggested.

Around the world, around 80 different species of mangrove trees have been identified, all of which grow in warm, shallow, coastal waters located in tropical and subtropical regions of the planet, such as the U.S. Gulf Coast and the Indian subcontinent.

Mangrove forests are rooted in salty sediments that are often submerged by water. However, the upper trunk, branches and leaves of mangrove trees usually lie above the waterline. Most of these forests get flooded at least twice a day as the coastal tides rise and fall.

Mangrove forests provide vital ecosystem services to humans, plants and animals all over the globe. For example, they help to stabilize the coastline, providing protection from storms, waves and tidal erosion. The complex roots systems of these forests also provide a habitat for fish and other animals.

Furthermore, they also store large amounts of carbon, with studies suggesting that these ecosystems can sequester four times more of the substance than rainforests.

However, coastal mangrove forests are threatened by rapid sea-level rise, with some scientists warning that they are at risk of gradually drowning.

Despite their ecological importance and the existential threats that they face, scientists know relatively little about how mangrove trees respond to rising waters due to a lack of long-term observational data.

In order to shed light on this issue, an international team of scientists led by Neil Saintilan from Macquarie University in Australia examined sediment core samples from 78 tropical and subtropical lies in order to work out how mangroves responded to past changes in the rate of sea-level rise, which went from more 10 millimeters (0.39 inches) per year nearly 10,000 years ago to almost stable around 4,000 years later.

Saintilan said he decided to investigate the issue after reading several studies suggesting that tidal wetlands could survive quite high rates of sea-level rise, such as more than 10 millimeters per year—around three times the current rate.

"While these were encouraging findings, I had my doubts because they were based on very short-term observations," Saintilan told Newsweek. "We decided that a better approach might be to look at how mangroves across the world responded to rapid sea-level rise the last time this occurred—nearly 10,000 years ago at the end of the last glacial period."

Using computer models, the researchers then used these findings to estimate the chances of mangrove forests surviving under different future sea-level rise projections, which are based on varying greenhouse gas emission scenarios.

The scientists found that when the rate of sea-level rise exceeded 6-7 millimeters per year—which is what climate models predict will occur by 2050 under current high-emissions scenarios—the mangrove trees would likely not be able to survive the rising waters, according to a study published in the journal Science.

"There was good news and bad news. The good news was that mangroves were clearly capable of surviving much higher rates of sea-level rise than we have around the world at present," Saintilan said. "There were many examples where mangroves were able to keep pace with sea-level rise of 5 millimeters per year; the current rate is just over 3 millimeters per year."

"However, there was little evidence that mangroves could keep pace with sea-level rise of over 7 millimeters per year, and this threshold was lower for mangroves on coral reef settings, which failed to keep pace with sea-level rise above 5 millimeters per year. If the rate of sea-level rise doubles, mangroves are in serious trouble," he said.

Mangrove forest
Mangrove forest, Ten Thousand Islands, Florida Everglades. Tim Graham/Getty Images

"If we keep to the Paris Agreement and control emissions, it is not too late to save the world's mangrove forests—because the lower emissions scenarios do not suggest sea-level rise to exceed 5 millimeters per year. However, under the mid- to high-emissions scenarios, we expect these thresholds to be exceeded."

According to Saintilan, the evidence suggests that if the rate of sea-level rise doubles to about 6-7 millimeters per year, existing mangrove forests will slowly drown.

"This may take a few decades in some places, or more than a century in others, but it is inevitable. Mangroves at this point will seek to respond how they have responded in the geological past when sea-level rise exceeds these rates: they will try to move landward with the rising sea, forming fringes along the strandline," Saintilan said.

"However, unlike in the past, there are now impediments to their landward movement in many parts of the world, due to the construction of coastal defenses. We call this the 'coastal squeeze.'"

The loss of mangrove ecosystems could lead to an increase in carbon dioxide in the atmosphere, exacerbating climate change. Furthermore, humans, plants and animals would no longer benefit from the important ecosystem services that they provide.

"We have the power to determine whether the world's mangrove forests survive or fail. Like much else, it hinges on how well we control greenhouse gas emissions. The encouraging news is that a low emissions future is also a future with mangroves," Saintilan said.