Mangrove copse on Yellow Water, Kakadu National Park, Australia.
Photo by Rod Long
With sea levels beginning to submerge coastal communities and crops around the globe, drastic action is needed to protect coastlines from brine, storm surge, and inundation. Enter: the lowly mangrove.
Uniquely adapted to low-oxygen, high-saline environments, mangroves have historically populated exclusively equatorial and sub-tropical shorelines. From tropical South America, Africa, coastal India, the Pacific isles, Asia proper, to the U.S. Gulf coast, mangroves have played a critical role in ecosystem health.
They vacuum the salts from local freshwater sources, foster an incredible diversity of living organisms, keep coastal erosion at bay and – underappreciated until recently – capture and store about four times the CO2 of their glycophytic (salt-averse) cousins in the arboreal forests. Under climate change as we know it, all those qualities are important. But their most critical task as the century progresses will likely be protecting communities from the swelling, surging, brine-encrusted seas.
Partial image of the Sundarbans mangrove forests of India and Bangladesh.
Photo by SPOT satellite. Source: Wikipedia
But wait: the vast majority of mangroves cluster between 25˚ N and 25˚ S latitude, though some venture into subtropical zones such as Florida (where Valutus founder Daniel Aronson canoed among them) and the northern coasts of Australia. But could mangroves help elsewhere? Even the hardiest of the species cannot stand protracted cold and are limited to areas where frost, if not unknown, is both rare and brief. Could their range be expanded to help communities north and south of their usual habitat?
The answer, tentatively, is yes, and here’s a case where the solution is partially supported by the problem: mangroves can’t handle much freezing and, accommodatingly, the planet is warming. As it does, the more robust mangrove varieties are creeping up once-temperate coasts where they can infiltrate, and in some cases supplant, more traditional wetlands.
Florida mangroves as seen from a canoe. Photo by Daniel Aronson
But the balance is delicate: just as a little warming may help northerly mangrove migration, excessive warming is potentially destructive to mangrove habitat. “Emergent communities such as mangroves and salt marshes,” notes the IPCC, in its AR6 Special Report SR1.5˚C, “[are] most susceptible to sea level variability and temperature extremes,” whether hot or cold. Of the five potential carbon emissions / warming scenarios the report outlines for the future, even at the very low end (SSP1-1.9) some mangroves would suffer from excess heat and higher sea levels than even mangroves can handle. In the worst-case scenarios, with warming of +2˚C or +3˚C to the catastrophic RCP8.5 (+8.5˚C/+15˚F), though northern coastal mangroves might thrive, many current mangrove forests would be inundated or overwhelmed by heat.
Mangroves on Whiskey Island, Louisiana. Photo by William SooHoo
Source: US Geological Survey
Meanwhile, the migration process is actually underway. In Louisiana, where previously only a few stunted and lonely mangroves had taken root, a team from the U.S. Geological Survey (USGS) found that, as temperatures rose between 2000 and 2009, the number of mangrove stands in their study area saw a five-fold increase.
Florida, too, has seen mangroves sneaking north. Currently they range the eastern shores as far north as the Canaveral barrier seashore. But what if they could be coaxed up as high as Jacksonville, to protect another 120 miles of coast, or 150 miles up the Gulf coast from their current terminus at Clearwater to Cedar Key? That one tract is almost 400 miles (644 km) of coast that would have at least some protections from higher seas.
Mangroves in the Florida Everglades. Photo by Fabian FF16 / Pixabay
But could they go even higher up the southeast coast of the United States? A recent climate report from the North Carolina Institute for Climate Studies (NCICS), indicates that, after rising a measly 0.55˚C over the past 120 years, local temperatures are expected to increase dramatically over the next 8 decades.
Such heat will cause severe problems in the area. Without protection, over the next 80 years “increased flooding, due largely to sea level rise, will disrupt coastal and low-lying communities. By the end of the century, these areas will experience high tide flooding nearly every day and a substantial increase in the chance of flooding from coastal storms… heavy rains from hurricanes and other weather systems will become more frequent and more intense.” But such heat might also help mangroves come to the rescue. Right?
Frozen Mangrove roots in February 2018. Photo by Rolf Dietrich Brecher.
Source: Wikimedia Commons
Possibly. Even a few hours at freezing can kill even the hardier species. The key is not higher high temperatures but higher low temperatures: fewer and shorter frosts. In one Florida survey, “the expansion of mangroves did not match patterns of increased average temperatures. Instead, the largest increases in mangrove area occurred in regions that experienced fewer extremely cold nights (when the temperature dipped below 25 degrees F [-3.9˚C].” So, it is possible that warmer overall temperatures will allow mangroves to creep up the coast.
Take Elizabeth City, at the northeastern tip of South Carolina. It currently averages January lows of 0.55˚C (33˚F) – just above freezing – and highs around 10.5˚C (51˚F). The lows aren’t good enough to support mangroves and save the coasts. But the rise in average low temps need not be dramatic. Indeed, as the researchers explained, “these temperature changes were small, just one fewer freeze event per year on average, but they appear to have helped cause these large increases in mangrove area.”
Say temperatures were to rise as expected for the Elizabeth City area? Using the IPCC’s ‘low-emissions scenario’ for the coming decades, Elizabeth City’s winter lows would now be about 4.15˚C (39.5˚F) – well above freezing. For the high-emissions forecast – which is certainly looking likely at the moment – winter lows would top 18.75˚C (65.8˚F), well into the hardy mangrove comfort zone. Suddenly, those bulwarks against coastal erosion, storm surge, higher daily tides, crop salinization, and atmospheric carbon, could potentially weather North Carolina’s winters.
Elizabeth City, South Carolina. Source: Wikipedia
Frost, ice, and snow may wipe them out, true… though on the Gulf side, the last frost in coastal Louisiana occurred in 1989. Though the same cannot be said yet for Georgia and the Carolinas, it may be so in time. We’ll have to wait to see exactly how far north mangroves will take hold over the next 80 years or so.
The importance of maintaining carbon-sequestering, salt-tolerant, coast-protective mangroves is obvious. But regardless of mangrove migration, it’s important to know that extant mangrove forests have already been under stress: logging, clear cutting for crops, aquaculture, and more are hurting them even in their traditional ranges. Supporting and repopulating razed mangrove forests will be a critical part of coastal defenses throughout this century and beyond.
Many projects to rebuild native mangrove forest are currently in the works. In the Indian-Bangladeshi Sundarbans –“a network of muddy islands and waterways that extends roughly 3,860 square miles (10,000 square km), two times the size of the state of Delaware,” making it “the “largest contiguous mangrove forest in the world” – plantings are underway. Kenya is hosting a project wherein the growth of carbon-sequestering mangroves pays the community to plant – rather than harvest – the trees, garnering income through carbon credits. Papua, New Guinea is attempting to save communities vulnerable to encroaching seas with mangrove plantings as well, and even the U.S. state of Florida – where the critically vulnerable manatee population eats, lives, and finds protection in the area’s shrinking mangrove wetlands – has natural restoration and replanting efforts underway.
There are other issues and concerns, like the potential long-term effects of mangroves displacing more traditional temperate wetland ecosystems, such as salt marshes. Much is still to be learned, but this, at least, is clear: given the folly of mankind, having warmed the globe so quickly humanity can at least develop a symbiotic pact with mangroves wherein we help them thrive and they protect us from the very furies we have unleashed.