• Tue. Sep 22nd, 2020



Mangrove loss has fallen dramatically, but the forests are still in danger

Lately, however, mangrove researchers have been delivering some rare good news: According to satellite data, mangrove loss has fallen dramatically in the past two decades, with human-caused mangrove destruction declining even faster than loss from natural causes.

“We’re definitely headed in the right direction,” says Liza Goldberg, a researcher at the University of Maryland and NASA, who led a recent study of satellite data.

But that doesn’t mean mangroves are out of hot water — figuratively or literally. Both Goldberg’s study and another paper using different methods found that sea-level rise and erosion could soon reverse the positive trend and, in the long term, possibly wipe mangroves off the map altogether.

If you were tasked with choosing just one ecosystem to safeguard the future of humans and the planet, mangroves would be a strong contender. These mucky, tangled coastal forests, although perhaps uninviting to humans, absorb and blunt storm surges, nurture a stunning array of life — from young fish to crabs to shorebirds — and store vast amounts of carbon in their complex root systems.

Acre per acre, mangroves “are the world champions of carbon sequestration,” says Neil Saintilan, an ecologist at Macquarie University in Sydney.

But figuring out how, where and why mangrove forests are disappearing has been hampered by a lack of good global-scale data.

Goldberg began working on the problem with mentors at NASA’s Goddard Space Flight Center in Greenbelt, Md., in 2016 — despite being 14 at the time, which was two years younger than the official minimum age for NASA’s internship program. She soon took the lead on a project to map, at the global scale, what was causing mangroves to disappear. She wrote computer algorithms to analyze 16 years’ worth of data from Landsat, the United States’ satellite program that aims to photograph every spot on Earth every eight days, generating images with pixels roughly the size of a baseball diamond.

To crunch the mass of data, Goldberg enlisted Google’s Earth Engine, a free cloud-based service frequently used by scientists for computationally demanding analyses.

Goldberg’s analysis was published in July in the journal Global Change Biology, making it an exceptionally rare scientific paper with a high school student as a lead author. (Goldberg graduated in the spring from Atholton High School in Columbia, Md., and is attending Stanford University.)

She and her colleagues found that mangrove loss rates had plummeted 73 percent globally since 2000. In some places, such as Senegal, virtually no loss had occurred. Elsewhere, such as Southeast Asia, loss rates were worryingly high, but still down from the peak of two decades ago.

In addition to their paper, the team has released an interactive global map allowing users to zoom in on causes of mangrove loss anywhere on Earth.

Others had mapped mangroves, says Dan Friess, an ecologist at the National University of Singapore, but no one had quantified globally why mangroves were disappearing.

“This paper has put numbers to what we previously had as a gut feeling,” says Friess.

With two other recent studies confirming a dramatically lower mangrove loss rate since 2000, Friess sees cause for cautious optimism.

After the catastrophic losses of mangroves in the ’80s and ’90s, conservationists swung into action, helping farmers raise shrimp while conserving mangroves. In many places, particularly in Southeast Asia, farmers often cut down mangroves to create offshore shrimp ponds.

“That’s something we should celebrate,” Friess says. “It’s a conservation success story, which we don’t often hear.”

Mangroves’ long-term prospects may be less rosy.

A few years ago, Saintilan became curious about how well mangroves can cope with sea-level rise. Recent studies, including ones Saintilan led in eastern Australia, had found that at today’s rates of sea-level rise, mangrove roots can capture sediment and build new roots fast enough to keep up. That constant soil and root building is what allows mangroves to vacuum huge volumes of carbon out of the air and store it underground.

But what if oceans were to rise faster than anyone alive has seen, as climate scientists believe could happen as early as 2050?

For that, Saintilan’s team turned to studies of coastal sediments laid down at the end of the last Ice Age, when ice sheets melted rapidly and raised sea levels more than 100 meters. Because such sediments are protected from oxygen, they can contain nearly-intact mangrove remains thousands of years old. But during periods when sea level rise topped around 7 millimeters per year, mangroves disappeared from every coastline on Earth. That result, Saintilan and authors wrote in the journal Science in June, seems to represent a hard limit on how much sea rise mangroves can stand.

Even before that limit is reached, increasingly powerful storms, wave erosion and drought may become mangroves’ biggest problems. In their study, Goldberg and her colleagues found that mangrove death because of such “natural” causes fell more slowly than directly from human-driven losses, and in some places was starting to tick upward.

Worse, as sea-level rise and extreme weather powered by global warming increasingly batter coastlines, mangroves will need to retreat inland. But in many places, they will hit hard barriers such as roads and buildings, possibly setting in motion a whole new cascade of destruction. Mitigating this “coastal squeeze” is Goldberg’s top recommendation for protecting mangroves.

The shift to an era dominated by nonhuman threats rings true for Vo Quoc Tuan, a researcher at Can Tho University in Vietnam who studies mangroves in the Mekong Delta, a vast, low-lying region vulnerable to storms and flooding. An organic shrimp farming initiative and a mangrove protection law have cut human-caused destruction in Vietnam, but waves and storms are eating away at the delta’s coastal forests, exacerbated by dams that cut off sediment flows from the Mekong River. “Erosion now is the biggest problem for mangrove losses,” he says.

To reverse the trend, researchers and conservationists are installing walls of bamboo poles off the coast. Over time, the poles trap sediment, building new mud flats where mangrove seedlings can be planted. If everything goes right, the mangroves quickly take off, building new forests.

Tuan also believes the government should reward farmers who keep mangroves intact. A similar “payment for ecosystem services” scheme has allowed Costa Rica to double its forest cover, turning the country from a poster child for deforestation into a global conservation success story.

While national policies may be crucial in the short term, mangroves’ fate will ultimately depend on whether the global community tackles climate change, Saintilan says.

“The conditions we have at the moment, 3.5 millimeters of sea level rise per year, is mangrove heaven,” he says. But unless carbon dioxide emissions are stabilized at the levels specified in the Paris agreement, “it’s only a matter of time before mangroves go into catastrophic retreat.”

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