Dining out? You may want to think twice before ordering the “surf and turf special.”
The carbon footprint of a steak and shrimp dinner — were it to come from shrimp farms and pasture formerly occupied by mangroves — is the same as driving a small car across the continental United States, according to a new study.
The research, published in the journal Frontiers in Ecology and the Environment, quantifies the full impacts of mangrove deforestation and puts it in terms of the consumer end product.
Mangrove forests have major implications for global climate. On average, they can store even more carbon than a similarly sized area of rainforest. When those systems are degraded or destroyed for other uses, that carbon is slowly released back into the atmosphere. What was not fully understood — until this study — was the full carbon footprint of food production in mangrove areas; other studies did not include the full effects of deforestation into their analyses.
“What we know about carbon footprints for many types of food products [grown in former mangrove areas] has not taken into account the losses of carbon that occur when you cut down the mangroves,” said Boone Kauffman, a scientist with Oregon State University and the study’s lead author. These new numbers, he said, “show the dramatic size of the carbon footprint coming of unsustainably farmed shrimp or beef cattle that comes from mangroves when we include the carbon losses from deforestation.”
Behind the numbers
The researchers measured carbon stocks in 30 undisturbed mangrove forests and 21 adjacent shrimp ponds or cattle pastures in five tropical countries — enabling comparisons between the carbon in the soils of healthy mangrove forests and in cleared forest areas nearby. What the researchers found shocked them, Kauffman said.
“We just had no idea that there would be such a dramatic loss of carbon when the mangrove ecosystems were deforested,” he said. “We had no idea that the greenhouse gas emissions from that land-cover change would be so high.”
Describing the emissions in terms of the food items derived from those areas makes the findings startlingly clear.
Consider two people who go to a restaurant and order the “surf and turf” special of 100 grams (4 ounces) of shrimp and a 450-gram (16-ounce) steak. If the shrimp and beef were grown in an area of converted mangrove forest, the carbon footprint of the two meals combined would be the same as burning 695 liters (182 gallons) of gasoline.
As the study’s authors write: “Driving a moderately fuel-efficient automobile across the U.S. (from Los Angeles to New York) and back would have a lower carbon footprint than that of these two meals.”
Science to policy
The new paper fills a major policy gap, according to one expert.
“The scientific community had for a decade been measuring carbon in these beautiful, pristine mangrove ecosystems,” said Emily Pidgeon, an expert on coastal carbon at Conservation International (CI) and who was not involved in the study. “But policymakers were not interested in pristine ecosystems — what they were worried about was what happens when those ecosystems are converted or degraded, and scientists didn’t have the data to speak to those concerns,” she said.
Those concerns have taken center stage in global climate policy: Under the Paris Agreement, countries are required to commit to actions to prevent climate change. The reforestation of mangrove forests and other so-called “blue carbon” ecosystems offer countries a “two-for-one solution,” both mitigating climate change (by absorbing carbon from the atmosphere) and adapting to it (by providing coastal buffers against rising seas).
The rise of coastal shrimp farming, especially in Southeast Asia, in particular has devastated mangroves globally; one-fifth of the world’s mangroves have been lost just since 1980, as global shrimp production has surged, from 1 million metric tons per year in the mid-1990s to nearly 5 million tons, according to the UN Food and Agriculture Organization.
“This study is critical for starting a conversation about the real impact of some of the ways we’re losing these critical ecosystems,” Pidgeon said. “For countries to account for this properly in their national greenhouse gas accounting, you need those numbers. They have huge policy significance.”
Kauffman and Pidgeon both say that attention has never been greater on the role of mangroves and blue carbon — but there’s still a long way to go to stop their destruction.
“Almost every country I’ve worked in with mangroves is still losing them at an alarming rate,” Kauffman said. “But I think there is growing awareness of the value of blue carbon and its role in mitigating climate change.”
The impetus for the research, he said, is thanks to the hard work of scientists and organizations like CI.
“This paper would not have come about if not for CI,” Kauffman said. “They played a very integral role in the conceptualization of this study.”
Bruno Vander Velde is Conservation International’s editorial director.