Mangroves, Global Warming and Rising Sea Levels

August 10, 2015 – Developers of new beach resorts in tropical locations are very familiar with mangroves and they hate them because they take up valuable beachfront real estate. But hating mangroves makes little sense in a world facing many environmental challenges because these trees perform an amazing series of functions.

There are 70 species of mangrove in the world today. They occupy coastal wetlands largely in tropical and subtropical environments (see map below). They serve as nurseries for shellfish, crustaceans, fish, birds and other wildlife. They represent a significant renewable wood source. They absorb storm surges and prevent coastal erosion. They are salt tolerant filters. Their biomass is greater below ground than above which makes them effective sequesters of carbon. And with all these good things they are increasingly endangered by us.


Mangrove world map

The largest concentration of mangroves is found in Indonesia. In that country’s latest study researchers calculated the amount of carbon stored by these forests to be 3.14 billion tons with each tree storing 3 to 5 times the amount of carbon of any other tree species. And yet Indonesia is seeing massive mangrove deforestation. With 25% of the world’s remaining mangrove forests estimated at 2.9 million hectares (over 4.94 million acres) Indonesia is losing 52,000 hectares (128,500 acres) every year largely to shrimp ponds. The loss of mangrove acreage is contributing 42% to Indonesia’s carbon emissions contribution annually. If the country could stop this deforestation it could immediately see carbon emissions decline from 10 to 35%.

What is happening in Indonesia has already occurred in South Florida where mangrove forests are under pressure from real estate developers seeking beachfront locations for high-rise condominiums. The Florida Department of Environmental Protection describes mangroves as significant contributors “to the overall health of the state’s southern coastal zone.”

For Florida’s commercial fisheries mangroves are essential to the health of snapper, tarpon, jack, sheepshead and other fish populations. In fact without mangroves Florida might not even be there. And yet in many areas along the state’s coast mangroves have been destroyed. In Tampa Bay the loss amounts to 44% of the total habitat. Near West Palm Beach the loss is as much as 87%.

And now climate change is threatening mangroves as weather patterns change and the atmosphere warms. While a warming climate might appear to be beneficial to the range of the mangrove spreading it closer to the poles, with atmospheric warming has come ocean warming, and the latter is contributing to sea level rise and more destructive storm surges. For isolated mangrove forests it is becoming more difficult to hold back the sea.

Add incidents like the Deepwater Horizon explosion and spill of 2010 have seen mangrove mutation rates skyrocket along the Louisiana and other Gulf Coast state wetlands. In addition Deepwater Horizon has contributed to die offs and the weakening of mangrove forests further exacerbated by polluted runoff from coastal factories, cities and farms chemically impairing the trees. The consequence for much of the Gulf Coast – negatively impacted freshwater aquifers extending several kilometers inland.

Other impacts from mangrove duress in the Gulf, Florida and other coastal areas include:

  • changes to tidal flow.
  • changes to the water table.
  • greater coastal water siltation and turbidity.
  • increased coastal erosion.
  • alteration of natural food chains.

The carbon sequestering property of mangroves is being further compromised as more forests get cleared. In leaving fewer mangroves the trees get larger increasing above-ground biomass. This makes them less efficient for capturing carbon because smaller more dense forests produce much larger sub-surface systems with better sequestering capability. In addition the larger trees means less of a living canopy and less biodiversity below the forest floor in associated waterways. The loss of biodiversity means less filter feeders to clean pollutants. It means the decline in water and soil quality and fewer juvenile fish which rely on the mangrove root systems for shelter from predators.

But there is a change in attitude beginning to make its presence felt in the world of mangroves. For example:

  • In the Philippines an effort is underway to restore mangrove forests. Jurgenne Primavera, a researcher in that country has been campaigning for years and showing coastal communities how to exploit mangrove forests rather than destroy them. In Thailand researchers are studying how entire mangrove ecosystems function and recording the dynamics of these environments. What they are finding is that young and more extensive mangrove forests are better at carbon sequestration than older exploited coastal forests. This is encouraging replanting on a significant scale. The scientists cite the reason, the volume of biomass both in wood and sediments which trap the carbon effectively.
  • Sri Lanka is the first nation to pass legislation protecting all of its mangrove forests. This happened in May 2015 and the government is implementing replanting projects to increase its mangroves. In the past century Sri Lanka saw 90% destruction of its mangrove population. And when in 2004 the Indian Ocean tsunami struck villages with intact mangroves were far less damaged than those without. The comparison is rather stark. A village with dense mangroves lost two of its citizens. A comparable one without witnessed the death of almost 6,000.

So what is the future for mangroves? With the rise in human population greatest in the tropical and semitropical world, and with most of humanity living at or near coastlines, mangroves are highly endangered. Aquaculture is expected to significantly increase to meet food demand. And where aquaculture sets up shop mangroves vanish.

As for climate change and its byproduct, rising sea levels, scientists are still studying potential impacts. Will forests move further inland as wave erosion and storm surges deplete coastal areas? In one 1997 study a rise of 16 centimeters (approximately 6 inches) in sea level initially caused seedling growth spurts but slowed once the trees reached the sapling stage after 2.5 years. That study didn’t look at erosion impacts from storm surges and coastal flooding.

I leave you with these three final notes and hope you have a renewed appreciation for the unique value that mangroves bring to our planet:

  • one hectare (just under 2.5 acres) of mangrove forest yields 0.45 tons of marine fish caught per year.
  • a mangrove stand averaging 30 trees per 0.01 hectares (0.025 acres) can reduce the destructive force of a tsunami or storm surge by 90%.
  • and the living biomass of a mangrove forest ranges between 100 to 400 tons per hectare which rivals the sequestration capability of the Amazon tropical rainforests.

We need robust mangrove forests to grow and prosper in this 21st century, an important piece in our chess set of options to combat climate change.


Len Rosen lives in Toronto, Ontario, Canada. He is a researcher and writer who has a fascination with science and technology. He is married with a daughter who works in radio, and a miniature red poodle who is his daily companion on walks of discovery. More...