Half of the world's mangrove ecosystems, with trees whose roots stretch down into brackish water, are at risk of collapse. That's according to the first assessment from the International Union for Conservation of Nature, a leading scientific authority on the status of species and ecosystems. The new report finds that sea level rise fueled by climate change is the biggest risk.
"The results were quite shocking," says Marcos Valderrabano, program manager for the International Union for Conservation of Nature based in Switzerland. "Fifty percent of the mangroves worldwide are at risk of collapse, and that's much more than what we expected."
Mangroves can be found along coastlines and estuaries, including in Florida and Louisiana, and have evolved to thrive in stressful conditions, getting flooded by tides and hit by waves.
Mostly straddling the equator and subtropical regions, mangrove forests act as a buffer against storms and cyclones. Valderrabano says for millions of people, mangroves act as "protection against that strength of the sea."
Mangrove ecosystems provide homes for animals like birds, crabs, and fish. Many fish spend part of their life cycle in mangroves, says Dan Friess, professor at Tulane University in New Orleans and one of more than 250 experts who contributed to the assessment. Some young fish go into mangroves "when they're really small," Friess says, "and they can hide from predators within the roots of the mangroves."
Mangroves are also important for economic activity — particularly fishing— and sometimes have deep cultural significance.
While mangroves are used to surviving in harsh conditions, the new report finds climate change threatens these ecosystems. "Undoubtedly the main threat we have found is exactly climate change and sea level rise projection," Valderrabano says.
Mangrove trees can handle the salinity of seawater some of the time, but some forests can't handle being flooded with it all the time. "There's a kind of a bit of a misconception that 'oh these mangroves are by the sea, so they must love salt water, right?'" Friess says, "No. It's toxic to them just like it would be to any other plant, but they are able to tolerate it."
Extended periods of being inundated with high salinity water make it harder for mangroves to germinate and make new trees.
Though mangroves protect coasts against storms, climate change is increasing the frequency and severity of storms which can damage the trees. "They need some time to recover," Valderrabano says.
One solution is mangrove restoration projects that work "with the communities, with the right techniques," says Fernanda Adame, a professor at Griffith University in Australia who was not involved in the assessment.
Scientists agree a key solution for mangrove longevity is to reduce the greenhouse gas emissions that drive global warming, including switching from fossil fuels to renewable energy plus large batteries.
Valderrabano says that for centuries mangroves have protected coastal communities, providing food and shelter. "Now it's our turn to help the mangrove ecosystems so that they will be able to adapt," he says.
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