Where humans have the money, we sometimes build storm defenses like seawalls to protect our coastal cities. But coastal development can often destroy natural defenses like coastal marshes or mangrove swamps. These ecosystems dampen waves and reduce storm surge flooding, and mangroves can even reduce wind speeds.

The protections provided by coastal ecosystem services are typically estimated by carefully looking at a single area or event. A new study led by East Carolina University’s Jacob Hochard took another tack, comparing the economic impacts of tropical cyclones around the world with satellite data.

Into the swamps

Mangrove swamps are ecosystems that develop thanks to a number of species of salt-tolerant trees, collectively termed mangroves, that flourish in the tropics. Their dense root systems trap sediments brought in on the tides and provide habitat for a variety of species. They also act to slow down storm surges and storms, providing a degree of resiliency for the ecosystem and any human infrastructure nearby.

To get a global perspective on the role of mangroves, the researchers quantified two things: mangrove extent along coastal communities and economic activity. A global, high-resolution mangrove map allowed the researchers to calculate a standardized swamp width by dividing mangrove area by the community’s coastline length. For example, the average extent in their analysis of 23 countries' sea coasts was 6.3 meters of mangrove per meter of coastline.

Measuring economic activity requires a little more ingenuity, as you can’t just look at reported statistics for many of these countries. Fortunately, the intensity of nighttime lighting in any location is easily determined from space and has been shown to be a reliable indicator of economic activity. Basically, if an area is doing well economically, there will be more lights on at night there.

To find out how well mangrove swamps protect nearby communities, the researchers measured the change in lighting for the years after a tropical cyclone made landfall. They compared communities with the average mangrove extent and above-average communities at the 68th percentile (one standard deviation above the mean). Instead of about 6 meters of mangrove buffer, these communities had about 25 meters of mangrove.

Protection

After a storm, the average-mangrove areas suffered a 6% to 8% drop in their economic growth rate and took at least six years to level back out. This impact isn’t seen in the first year after the storm—aid and recovery efforts seem to mask the change. That loss is equivalent to about six months of economic activity.

For the above-average mangrove areas, however, the economic growth rate only went down about 3% to 6%, equivalent to a loss of roughly four months of economic activity. So a modest increase in mangrove extent has a pretty significant benefit for these vulnerable economies. As the researchers put it, this also indicates that “mangrove restoration efforts for protective benefits may be more cost effective, and mangrove deforestation more damaging, than previously thought.”

From the 1970s to 1990s, about 35% of the world’s mangroves were lost due to urbanization, aquaculture, timber, and reductions of the freshwater flows that keep these swamps alive. The rate of loss has slowed down greatly since then, but it hasn’t fallen to zero.

The researchers made sure that they weren’t mistaking causation for mere correlation—like the economic recovery after a storm involving more clearing of mangroves if the economic damage was greater. But an interesting possibility they couldn’t evaluate is that the presence of more extensive mangroves might make recovery seem like a safer investment. It could be that given the choice between a community that seems unprotected from future storms and one that has a good mangrove buffer, more post-disaster money might flow into the latter—a case of “perception is reality” on top of the physical reality of storm waves coming through the trees.

Either way, the tangible benefits for humans provided by these mangroves—often called “ecosystem services”—are huge.

PNAS, 2019. DOI: 10.1073/pnas.1820067116 (About DOIs).