A new study might make you think twice about ordering that shrimp cocktail.

You’re having dinner with your date. You both order the ‘surf and turf’ special: a shrimp appetizer and a steak. You might not know it, but the carbon footprint of your meal is mind-boggling massive.

If the beef and seafood came from the tropics, where mangroves once grew, the greenhouse gas emissions produced by the two dinners alone would be roughly equivalent to driving from Los Angeles to New York City and back – a massive 1632 kilograms of carbon dioxide.

Or, to put it another way, those greenhouse gas emissions would weigh about as much as the car you drove to the restaurant.

To come up with these numbers, scientists – including some from the Center for International Forestry Research (CIFOR) – spent seven years working in muddy mangrove forests from Southeast Asia to Central America.

Across the tropics, mangrove forests are being cleared to make way for agriculture and aquaculture. Found on the frontier of land and sea, their seaward sides are converted to shrimp ponds, while their drier edges are claimed and drained to become rice fields or cattle pastures.

The scientists examined 55 sites where that conversion is happening, in Indonesia, Costa Rica, Honduras, Mexico and the Dominican Republic. It’s the first time that a carbon-footprint study has taken into account the greenhouse gas emissions that result from deforestation.

When the researchers made their final calculations, even they were surprised.

For every kilogram of beef produced on land that was converted from mangrove forest, 1440 kilograms of climate-altering greenhouse gases are pumped into the atmosphere. For shrimp (more widely known as ‘prawns’ in the U.K. and Australia), it’s even worse: 1603 kg of emissions per kilo of crustacean.

“We were astounded that the carbon footprints were as high as they were,” says lead author Boone Kauffman, a mangrove expert from Oregon State University.

OUT-SIZED EMISSIONS

So why the out-sized emissions?

Mangrove forests store a lot more carbon than terrestrial tropical forests, because they sequester a huge amount in the soil – in some cases up to 98 percent of the carbon stocks in a mangrove ecosystem can be underground.

When those forests are cut and drained, carbon isn’t just lost through the breakdown of leaves, twigs and branches. All that carbon in the soil is also released – and not just at the surface. The study found that deforestation could release carbon stored up to three meters below ground.

That’s why mangroves may account for as much as 12 percent of the total emissions for all tropical deforestation, Kauffman says, even though they only make up 0.6 percent of the land area occupied by tropical forests.

“You’re losing centuries of carbon sequestration in just a few years of land use,” says Kauffman.

That’s the other big problem with these conversions – shrimp ponds in particular have very short life spans. Disease, soil acidification, pollution, and market conditions tend to limit their use to just three to nine years (the scientists assumed a conservative nine years for the purposes of the study, meaning that the actual carbon footprint of some shrimp may be even higher).

Once the area is exhausted, the ponds are abandoned – and the farmers move on to next patch of mangroves. 

A SIMPLE QUESTION

CIFOR Principal Scientist Daniel Murdiyarso’s research in Indonesia has shown just how much carbon mangrove ecosystems can lock away.

“They store twice as much carbon per hectare compared with terrestrial forests – and in some cases five to six times as much,” he says.

Carbon stocks

Carbon storage in mangrove and peatland ecosystems: a preliminary account from plots in Indonesia

New research is showing that emissions can be reduced during mangrove conversion by limiting the exposure of excavated soil to the air, but finding ways to reduce rampant mangrove deforestation is even more important.

Murdiyarso helped to conceptualise the carbon footprint study with Kauffman.

They wanted to find a way to make the climate impact of mangrove deforestation more easily understood.

“When scientists talk about the role that deforestation plays in climate change, scientists tend to talk about the global picture – petagrams, gigatons, a billion metric tonnes of carbon – and the public can’t really grasp that,” Kauffman says.

“So instead of scaling up to the global, we decided we would try to scale it down to an individual dinner – to report the influences of deforestation at the personal scale.”

To make the calculations, the researchers compared the carbon stocks in shrimp ponds or cattle pastures with nearby patches of intact mangrove forest.

That was harder than it sounds – they had to clamber through aerial mangrove roots to measure trees, gather every stick of downed wood, and collect muddy soil samples to take back to the lab.

Video

Dirty Science Part 1: Excavating the truth about mangroves and carbon

“It brings the child out in you if you like being in the mud,” jokes Kauffman.

But that hard work had a very serious objective.

“We spent seven years on this project to make sure that we got it right,” Kauffman says.

“We are faced with such unprecedented environmental problems, particularly the threats of climate change and its possible environmental and social ramifications.”

“So it’s really important that we convey our science in a way in which the public can comprehend, so they can see how their daily activities affect climate change, and they can manage their lives accordingly.”

The result is a study that uses solid, real-world data from a broad range of sites across the tropics, with the aim of making people think about one simple question: Is a kilogram of shrimp worth 1600 kilos of greenhouse emissions?