- More than half of mangroves worldwide may face high or severe risk by 2100 due to increased tropical cyclones and sea level rise, with experts predicting Southeast Asia to be hardest hit under all emissions scenarios.
- A new risk index combines multiple climate stressors — cyclones and sea level rise — with ecosystem service value, providing a novel, globally scalable tool for risk assessment and conservation planning.
- Mangrove loss has major human and economic costs, jeopardizing flood protection worth $65 billion annually and threatening 775 million people dependent on coastal ecosystems.
- Urgent, dynamic conservation and emissions cuts are essential; restoring degraded areas, enabling inland migration, and reducing emissions could significantly reduce risk and buy adaptation time.
More than half of all mangrove areas worldwide may face “severe and widespread risk” from climate change-intensified tropical cyclones and rising sea levels by 2100, according to newly published research.
The study uses a first-of-its-kind, open-source risk index tool to predict risk for mangroves, according to warming estimates for three different scenarios of greenhouse gas emissions (intermediate, high, and very high) projected by the Intergovernmental Panel on Climate Change (IPCC) in 2021.
“Almost all regions will see a non-negligible increase in risk,” study lead researcher Sarah Hülsen, from ETH Zürich’s Institute for Environmental Decisions, told Mongabay.
But Southeast Asia is likely to be worst affected, with more tropical cyclones and higher wind speeds than at present, according to the study.
“Even at the lowest emission pathway [scenario] … Central America, Southeast Africa, East Asia, and Oceania, will become newly affected by very high intensity tropical cyclone winds,” the study says.
The tool predicts that the eastern Philippines may experience 254% more tropical cyclones than the historical maximum, regardless of the emissions scenario. This means 50- year storms could now occur up to four times per half-century, potentially becoming once-a-dozen-year storms.
Far-reaching impacts
Mangrove loss to superstorms isn’t just an ecological concern — it has major economic and human implications. The estimated annual worth of flood protection from mangroves globally is more than $65 billion, while more than 775 million people are thought to be “highly dependent” on coastal ecosystems, the study says.
Mangrove ecosystem services, or benefits to humans, include protecting coastlines from erosion and flooding, increasing biodiversity, storing carbon, and supporting fisheries.
“The mangrove ecosystems providing the highest levels of benefits to people … are also at the highest levels of risk,” Hülsen said.
The study’s estimates hint at large potential socioeconomic impacts: Up to 98% of mangroves protecting people in Southeast Asia, and 97% protecting assets, will be at high to severe risk under the high-emissions scenario, according to the index. The outlook is not much better in Central America, with 75% of asset-protecting mangroves in the same category.
To effectively ameliorate these risks to valuable mangrove ecosystems, Hülsen said “conservation planners need information on the type, magnitude and location of changes to expect.” Only then can they put effective dynamic conservation practices into place, she said.
“Risk indices such as ours make complex climate projections and natural hazard data more easily accessible,” she added.
Decisive action to cut greenhouse gas emissions should take priority, however, Hülsen noted. While all the scenarios show increasing risk, the intermediate-emissions pathway saw only 1% of mangroves in the highest risk category, dubbed “severe.” That jumps to 20% of all mangroves for the very-high-emissions pathway.
How is this risk index different and why does that matter?
Ecosystem risk indices usually address changes in a single stress factor. But the risk index created by Hülsen and her team is the first global, location-specific projection for mangrove ecosystems that combines two stressors: Climate change-driven tropical cyclones and sea level changes.
“Combining multiple model outputs on a global scale to extract usable information is generally a very complex and difficult task that I think is well implemented here,” Temilola Fatoyinbo, a research scientist with NASA’s Biospheric Sciences Lab, specializing in studying forest and coastal environments with remote sensing, told Mongabay. Fatoyinbo, who wasn’t involved in the recent research, called it “an important step” to improve policy change, conservation and restoration planning.
The risk framework can also be extended and adapted, according to the study. This could include other climate-driven mangrove ecosystem hazards, like marine heat waves and drought. It could also be applied to other sets of ecosystems and their stressors.
How does it work?
Hülsen and her colleagues created their tool, funded by ETH Zürich (the Swiss Federal Institute of Technology), using emissions scenarios developed by the IPCC in 2021. These predict likely socioeconomic outcomes for humanity, depending on fossil fuel emissions policy choices.
They took three scenarios from the IPCC: the intermediate or “middle-of-the-road” future where business as usual continues; a high-emissions future where nationalism resurges, resulting in a more fragmented world; and very-high-emissions, free-for-all future with unconstrained energy use and economic output. Known as Shared Socioeconomic Pathways, or SSPs, these scenarios help project how much Earth is likely to warm by 2100.
The researchers combined the warming projections with existing data on how temperature increase is likely to affect sea level rise and tropical cyclone frequency and intensity across the world. Then they used remote-sensing data from mangrove monitoring platform Global Mangrove Watch to identify mangrove areas likely to face significant challenges.
Finally, they cross-referenced these findings with mangrove ecosystem services ranked according to benefits provided, established in research published in 2023. This allowed the index to incorporate the value of mangroves into the risk equation.
However, planners should be careful when using global analysis like this to evaluate project feasibility, according to Fatoyinbo. “When working on the ground, the actual local site conditions and how they respond to storms and sea level rise can vary greatly, depending on smaller-scale conditions and locations” she said.
Her own research found that mangrove resilience was very different across parts of Florida hit by Hurricane Irma in 2017 in the United States. Local topography and storm surge factors had a “much greater effect,” she said, than wind speed and tree height on how well mangrove forests survived and recovered.
Compounded effects and adaptability
“The beauty of ecosystems lies in their adaptive capacity, which allows them to deal with and adjust to disturbances,” Hülsen said. “But this capacity is not infinite, and the higher the degree of warming and rate of changes, the less likely it becomes for the systems to be able to adapt.”
Mangroves grow in tidal zones and are used to having their roots submerged. They can therefore adapt to small increases in sea level — they just grow upward from the roots and, as they do so, trap more sediment beneath them, preventing drowning. But they can only keep up with small increases.
Once relative sea level rise exceeds around 7 millimeters (0.28 inches) per year, they die off, according to research on current mangrove ecosystems as well as evidence from around 10,000 years ago.
Mangroves typically grow in the tropics and subtropics, so they’ve evolved resilience to tropical cyclones, absorbing strong winds and buffering coastlines from storm surges. Severe storms are likely to cause structural damage, but they don’t happen very often, and mangrove ecosystems have generally had time to recover in the interim.
The risk estimates of the study indicate that tropical cyclones are likely to occur so often that mangrove ecosystems won’t have long enough to bounce back. At the very least, this will result in “considerable structural changes,” according to the study. Restoration efforts may support recovery, but it takes an estimated 55 years for restored forests to catch up with their natural counterparts, the study says.
Adding in sea level rise compounds challenges for mangroves. With tropical cyclones causing structural damage, potentially limiting mangroves’ upward growth, this is likely to affect their ability to adjust vertically to sea level change, increasing the likelihood of die-offs.
What should be done?
While sea level rise may outpace mangroves’ ability to adjust vertically, Hülsen said that if space is available, they could expand inland across existing and newly established floodplains.
Such a strategy is being piloted by Building With Nature, an international, sustainable coastal development program, in Java, Indonesia. The program is trying, among other things, to expand mangroves inland in an effort to keep their benefits despite rising sea levels.
Java experiences around 6 mm (0.24 in) average annual sea level rise, almost twice the global average. Excessive groundwater extraction is also causing the land to sink by a staggering 120 mm (4.7 in) per year. Villages and parts of cities have been lost, mangroves drowned, and aquaculture and farming areas flooded.
For mangroves, the sea rising or the ground sinking have much the same effect — they’re in deeper water. So, Java is both a cautionary tale and an opportunity to develop solutions ahead of time for the rest of the planet.
Where coastal and estuarine mangrove areas had previously been converted to shrimp farms, the project aims to reverse them, moving backward inland and “offering farmers sustainable shrimp farming options behind the mangroves,” Susanna Tol, a communications and advocacy officer at Wetlands International, a Building with Nature partner, told Mongabay.
It’s unclear how successful these pilots will be. The project previously constructed sediment capture structures along 20 kilometers (12 miles) of coastline, in an effort to facilitate natural mangrove regrowth. While this can work in less extreme circumstances, “the structures are not able to keep up with the sinking of land in the long term,” Tol said.
Hülsen also advocates for restoring cleared and degraded areas, as well as fighting pollution to give existing mangroves the highest resilience. High-risk areas should be monitored, she said, so that struggling areas can be spotted fast and actively restored.
“These are not all-or-nothing issues,” Hülsen said. The more we protect mangroves, the more time we buy for coastal populations and ecosystems to adapt, she said. But the more we reduce emissions, the less they will need to adapt.
“We are in this for the long run, so any fraction of warming we can avoid matters, and so does every piece of nature we can save,” Hülsen said.
Banner image: A snapper hunting silver fish in a mangrove patch. Image by Lorenzo Mittiga / Ocean Image Bank / Mangrove Photography Awards.
Citations:
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Lagomasino, D., Fatoyinbo, T., Castañeda-Moya, E., Cook, B. D., Montesano, P. M., Neigh, C. S. R., … Morton, D. C. (2021). Storm surge and ponding explain mangrove dieback in southwest Florida following Hurricane Irma. Nature Communications, 12(1). doi:10.1038/s41467-021-24253-y
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Saintilan, N., Khan, N. S., Ashe, E., Kelleway, J. J., Rogers, K., Woodroffe, C. D., & Horton, B. P. (2020). Thresholds of mangrove survival under rapid sea level rise. Science, 368(6495), 1118-1121. doi:10.1126/science.aba2656
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