Including GHG emissions from mangrove forests LULUC in LCA: a case study on shrimp farming in the Mekong Delta, Vietnam

Abstract

Purpose Mangrove forests have been recognized as important regulators of greenhouse gases (GHGs), yet the resulting land use and land-use change (LULUC) emissions have rarely been accounted for in life cycle assessment (LCA) studies. The present study therefore presents up-to-date estimates for GHG emissions from mangrove LULUC and applies them to a case study of shrimp farming in Vietnam. Methods To estimate the global warming impacts of mangrove LULUC, a combination of the International Panel for Climate Change (IPCC) guidelines, the Net Committed Emissions, and the Missed Potential Carbon Sink method were used. A literature review was then conducted to characterize the most critical parameters for calculating carbon losses, missed sequestration, methane fluxes, and dinitrogen monoxide emissions. Results and discussion Our estimated LUC emissions from mangrove deforestation resulted in 124 t CO 2 ha −1 year −1 , assuming IPCC's recommendations of 1 m of soil loss, and 96% carbon oxidation. In addition to this, 1.25 t of carbon would no longer be sequestered annually. Discounted over 20 years, this resulted in total LULUC emissions of 129 t CO 2 ha −1 year −1 (CV = 0.441, lognormal distribution (ln)). Shrimp farms in the Mekong Delta, however, can today operate for 50 years or more, but are 1.5 m deep (50% oxidation). In addition to this, Asian tiger shrimp farming in mixed mangrove concurrent farms (the only type of shrimp farm that resulted in mangrove deforestation since 2000 in our case study) resulted in 533 kg methane and 1.67 kg dinitrogen monoxide per hectare annually. Consequently, the LULUC GHG emissions resulted in 184 and 282 t CO 2 -eq t −1 live shrimp at farm gate, using mass and economic allocation, respectively. These GHG emissions are about an order of magnitude higher than from semi-intensive or intensive shrimp farming systems. Limitations in data quality and quantity also led us to quantify the uncertainties around our emission estimates, resulting in a CV of between 0.4 and 0.5. Conclusions Our results reinforce the urgency of conserving mangrove forests and the need to quantify uncertainties around LULUC emissions. It also questions mixed mangrove concurrent shrimp farming, where partial removal of mangrove forests is endorsed based upon the benefits of partial mangrove conservation and maintenance of certain ecosystem services. While we recognize that these activities limit the chances of complete removal, our estimates show that large GHG emissions from mangrove LULUC question the sustainability of this type of shrimp farming, especially since mixed mangrove farming only provide 5% of all farmed shrimp produced in Vietnam.

The devil is in the details – the carbon footprint of a shrimp

“…We share Kauffman et al's concern about mangrove forest loss, but we believe that their land-use carbon footprint for farmed shrimp has been overestimated. Two previous studies -conducted by various authors of this letter -found LULUC-associated GHG emissions from shrimp farming to be one to two orders of magnitude lower (Jonell and Henriksson 2015;Järviö et al 2017) than that of the Kauffman et al study.…”

“…Jonell and Henriksson (2015) estimated a carbon stock of 406 metric tons of carbon per hectare (t C ha −1 ) down to 1-m sediment depth based on a global estimate by Pendleton et al (2012), and assumed 63% of that carbon to be oxidized into CO 2 (with alternative values in the sensitivity analysis). Likewise, Järviö et al (2017) concluded a total carbon stock of 724 t C ha −1 down to 1.5 m depth based upon a review of geographically diverse sources from the literature, and assumed 55% of the belowground carbon to be oxidized (50% of sediments and 100% of roots). By way of comparison, Kauffman et al (2017) measured carbon contents in mangrove forests in Mexico, Central America, and Indonesia, and reported values between 269 and 1663 t C ha −1 down to 3-m depth.…”

“…Extensive mangrove-integrated shrimp farms in Ca Mau, Mekong Delta (investigated by Jonell andHenriksson 2015 andJärviö et al 2017) have been in operation since the early 1980s (Ha et al 2012). These systems produce only 250-300 kg shrimp ha −1 yr −1 (Phan et al 2011) The assumed lifetime of shrimp ponds is important because emissions will be annualized or amortized over this time period (IPCC 2006).…”

And details for land‐use carbon footprints arise from quantitative and replicated studies

“…This broad range in ecosystem carbon stocks underscores the importance of its measurement in order to reliably determine land-use carbon footprints. Apparently, Jonell and Henriksson (2015) and Järviö et al (2017) did not perform such a measurement. We report that the land-use carbon footprint among our study sites ranged from 959 to 2498 kg CO 2 e for every kilogram of shrimp produced (see Figure 4 in .…”

Indonesian aquaculture futures—identifying interventions for reducing environmental impacts

“…The unit process dataset for the LCA was supplemented with methane emissions from freshwater ponds, assuming 533 kg ha −1 yr −1 (coefficient of variation (CV)=0.4; lognormal distribution) (Astudillo et al 2015). Emissions resulting from land use and land-use change of mangroves were derived from Järviö et al (2018), assuming 129 tonnes CO 2 eq. ha −1 yr −1 (CV=0.441, lognormal distribution), but not included in the global warming estimates due to the difficulty of assigning land use and land-use change to specific species.…”