Mangroves Won’t Survive Sea-Level Rise If Emissions Aren’t Cut

Mangrove swamps are coastal wetlands. This swamp is in the Florida Everglades. (Photo via Wikipedia Commons)

(CN) — Mangrove trees are some of the most valuable ecosystems on the planet, but they won’t survive past 2050 without significant reductions in greenhouse gas emissions, according to a study in the journal Science

The small trees that grow in warm coastal waters protect shorelines from storms while providing habitat for fish. They also store large amounts of carbon, making them some of the best natural carbon scrubbers. 

Using sediment data from the last 10,000 years, a scientific team led by Macquarie University in Australia found that mangroves face extinction if sea levels rise as expected in the coming decades.

“Under high-emissions scenarios, rates of sea-level rise on many tropical coastlines will exceed 7 millimeters per year, the rate at which we concluded there’s a 3.5 percent probability mangroves can sustain growth,” said co-author Erica Ashe, a post-doctoral scientist in the Department of Earth and Planetary Sciences in the School of Arts and Sciences at Rutgers University-New Brunswick. “The loss of these mangrove ecosystems could result in increased carbon dioxide in the atmosphere and fewer vital buffers against storm surges in the long run.”

When sea level rise rates exceeded 0.24 inches per year, scientists found mangroves struggled to keep pace. Mangroves can survive when sea-level rise is less than about 0.2 inches per year — the level projected under low-emissions scenarios.

Neil Saintilan, professor of environmental science of Macquarie University, led a team of scientists who spent two years studying organic sediments related to mangrove trees. 

“Mangroves are world champions of carbon sequestration,” Saintilan said during a phone interview from his home in Sydney.

In the past, mangroves have had a significant impact on carbon dioxide, Saintilan noted. Once mangroves grew enormous, there was a measurable decline in CO2 in the atmosphere as they absorbed carbon, contributing to lower greenhouse gas levels.

Soils in which mangroves thrive are saturated, so they are saline — which eliminates most bacteria — and low in oxygen, Saintilan explained.  And unlike freshwater wetlands, mangroves emit low levels of methane, leading scientists to consider mangroves “blue level” ecosystems — essential areas that provide coastal protection and nursery grounds for fish while sequestering and storing carbon from the atmosphere and oceans.

Saintilan said his study is the first of its kind to assess the global mangrove response to rising sea levels, utilizing two teams of scientists and 78 observation points across five continents to better understand mangroves.

The study found a strong relationship between mangrove survival and a rise in sea level.

“The extensive development of mangrove environments under (relative sea level rise rates) has exerted an influence on global carbon cycles over geologic scale times,” Saintilan and his colleagues wrote in the study.

Over the last century, sea levels have risen .06 to .07 inches per year. In the past couple of decades, that rise has reached 0.13 inches per year, leading to an average of about 0.11 millimeters per year. The rate of sea level rise has doubled under global warming, Saintilan noted. At its present rate, sea levels could rise by up to 0.28 inches per year by 2100.

Ultimately, Saintilan said his study adds to the weight of evidence that lower emission practices are better for the planet.

There are about 80 species of mangrove trees, which occupy 3% of the global forest canopy but are responsible for 19% of emissions due to deforestation. In fact, mangrove effectiveness to protect local environments and sequester carbon rises with sea level — up to a point, Saintilan explained.

“Mangroves are highly opportunistic species,” he stated. “Their seeds float so they can get on a roll with sea level rise.”

But rising sea levels are not the only obstacle mangroves encounter in the modern world. Currently they face what Saintilan called a “coastal squeeze,” which includes development — housing and fences and seawalls — that impede their ability to expand across coastal lowlands.

Starting in the 1990s, mangroves were heavily cleared for palm oil plantations and aquaculture, which included the establishment of shrimp farms that only last a few years.

In recent years there has been a focus on revegetating cleared mangrove areas, driven by private investment. But those efforts have not always been successful. 

Saintilan highlighted the positive outcome of the study, noting that people should not hesitate to support mangrove mediation because mangroves are “fairly resilient.”

“If we focus on mangrove restoration and low carbon economies, the future of mangroves is quite good.”

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