What’s the cost of an average shrimp-and-steak dinner? If it comes from the loss of mangrove forests to aquaculture and agriculture, it’s 1,795 pounds of carbon dioxide emissions. That’s about the same amount of greenhouse gases produced by driving a fuel-efficient car from Los Angeles to New York City.
Clearcutting of tropical mangrove forests to create shrimp ponds and cattle pastures contributes significantly to greenhouse gases and global warming, according to findings recently reported in the journal Frontiers in Ecology and the Environment.
“About 1,603 pounds of carbon dioxide are released for every pound of shrimp and 1,440 pounds of carbon dioxide for each pound of beef” from mangrove forest conversion to ponds and fields, says J. Boone Kauffman, an ecologist at Oregon State University who led the project.
New measurement: The land-use carbon footprint
The numbers were obtained with a new measurement called the land-use carbon footprint. It records the amount of carbon stored in an intact mangrove forest, the greenhouse gases released from conversion of that forest to aquaculture or agriculture, and the quantity of the shrimp or beef produced over the life of the land’s use.
“What we found was astounding,” Kauffman says. “When you convert mangrove forests to shrimp ponds or cattle pastures, a remarkable amount of carbon is being emitted into the atmosphere. And the food productivity of these sites is not very high.”
How best to convey the information to a wide audience? “We scaled the atmospheric carbon emissions from mangrove deforestation to the level of an individual consumer,” Kauffman says.
The research took place in 30 mangrove forests and 21 adjacent shrimp ponds or cattle pastures. The sites were in Costa Rica, the Dominican Republic, Honduras, Indonesia and Mexico. Shrimp ponds were sampled in all countries except Mexico, where the predominant use was conversion to cattle pastures.
“We determined that mangrove conversion results in greenhouse gas emissions ranging between 1,067 and 3,003 megagrams of carbon dioxide equivalent per hectare,” says Kauffman.
The increase in greenhouse gases exceeded the scientists’ estimates. Mangroves represent less than one percent of the world’s tropical forests, but their degradation accounts for as much as 12 percent of the greenhouse gas emissions that come from tropical deforestation.
Inside a mangrove forest
Enter a mangrove forest. In this dark water world, trees with twisted limbs live double lives—one foot on land, the other in the sea.
Some 80 species of mangroves, also called mangals, thrive in salty coastal habitats in the tropics and subtropics. All take root in waterlogged soils where slow-moving currents allow sediment to accumulate. Red, black and white mangrove trees, along with buttonwoods, may grow along the same shoreline. Where these species are found together, each stakes out a spot.
Red mangroves are closest to the sea’s edge; their prop roots extend into the water from branches above. The roots capture sediment, stabilizing the shore. Farther inland are black mangroves with oxygen-supplying pneumatophores pointing upward from the soil. White mangroves, with no special root adaptations, are found in interior mangrove forests, followed by buttonwoods in upland transition zones.
These forests-of-the-tide collectively cover a worldwide area of about 20,535 square miles in 118 nations—about 0.6 percent of all tropical forests. And that number is dropping. Rates of mangrove deforestation over the past three decades have been dramatic, says Kauffman. “Mangroves are disappearing at the rate of about one percent per year.”
In places such as Southeast Asia, mangrove conversion to shrimp ponds is the largest cause of these intertidal forests’ decline.
Mangroves: Top ecosystem services providers
Mangroves provide ecosystem services worth up to $57,000 USD per hectare per year, and collectively sustain more than 100 million people, according to the United Nations Environment Programme report The Importance of Mangroves: A Call to Action. Deforestation of the world’s mangroves results in annual economic damages of up to $42 billion USD, the document states.
Mangroves’ most important ecosystem service, scientists have found, may be mitigating climate change by removing greenhouse gases from the atmosphere. Like other plants, mangroves capture carbon dioxide and store it in their leaves, roots, and trunks (biomass) and in the soil below.
But unlike most other forests, mangroves don’t have a maximum carbon dioxide storage capacity. They continuously amass carbon in soil, where it can remain for millennia. “Mangroves are extremely productive ecosystems that can increase their biomass relatively quickly,” says Kauffman, “trapping more carbon than other forest types.” The upper several feet of mangrove soils are primarily anaerobic—missing the organisms that decompose organic material and release carbon into the environment, so the carbon stays put.
When mangrove forests are converted for agriculture or aquaculture, the majority of the carbon they’ve stored is released into the atmosphere, joining other sources of greenhouse gases. Clearing even small tracts of mangroves generates high volumes of carbon dioxide. “These forests have been absorbing carbon for the last 4,000 or 5,000 years, but now through deforestation have become significant sources of greenhouse gas emissions,” Kauffman says.
Which countries’ mangroves store the most “blue carbon”–carbon captured by the world’s coastal and ocean ecosystems? Mangroves in Sumatra, Borneo, and New Guinea, and along the coasts of Colombia and northern Ecuador, scientists have discovered. The findings were published in the journal Conservation Letters. The results can help guide decisions about priority areas for mangrove conservation and rehabilitation.
How much is a mangrove forest worth?
An important question, says Kauffman, is whether the value of the shrimp or beef produced from a former mangrove forest exceeds the value of the ecosystem services lost as a result of mangrove conversion. Those services include maintaining biodiversity, fisheries production, protection against storms and erosion, and carbon storage.
“Addressing this trade-off is the responsibility of governments and is the personal choice of the consumer, who should have access to information on the true costs and impacts of food production,” the researchers write in Frontiers in Ecology and the Environment. “A better understanding of land-use carbon footprints would provide context to make informed decisions about how our everyday lives affect land use and climate change.”
And whether that surf-and-turf dinner is worth the price—in mangrove currency.