Avoiding mangrove destruction by avoiding carbon dioxide emissions.

T ens of millions of tons of CO2 emissions from mangrove deforestation may be avoided each year at an average cost of less than $10 per ton CO2, conclude Siikamäki et al. in their study (1) in PNAS. Avoiding CO2 emission from mangroves would have the beneficial side effect of protecting biodiversity. With careful selection of areas to be protected, increased protection of biodiversity could be obtained at relatively little additional cost. However, a major challenge to mangrove protection is the availability of good governance. It is one thing to pay for mangrove protection and another thing entirely for those mangroves to be protected in practice. Mangrove forests are found in tropical and subtropical coastal areas throughout the world. Approximately 30% to 50% of mangrove forests have already been lost globally, primarily as a result of coastal development and overharvesting (2). The study of Siikamäki et al. (1) is directly relevant to those interested in developing practical approaches to protect mangroves. However, it raises important issues about the value of CO2 emissions avoidance, environmental protection, and good governance. Siikamäki et al. (1) find that mangroves are among the most carbon-rich of tropical forest, storing on average ∼50 kg C·m, mostly in soils. Mangroves typically accumulate carbon at a rate of approximately 0.1 kg·m·y (3). Because they cover less than 0.03% of Earth’s surface, despite large fluxes and amounts stored on a perunit-area basis, they play a relatively small role in the global carbon cycle, taking up approximately 0.06 gigatons (Gt) CO2 annually from the atmosphere. Total carbon storage in mangroves globally could be as much as 20 Gt of C (2), or the equivalent of approximately 2 y of total global anthropogenic carbon emissions (4). Siikamäki et al. (1) estimate that mangrove deforestation generates approximately 0.12 Gt of CO2 emissions annually, approximately 0.3% of total anthropogenic CO2 emissions. They estimate that the measures they propose could reduce these emissions by approximately 25%—on the order of 35 megatons CO2 y −1 —approximately 1/1,000 of total anthropogenic CO2 emissions. [In their analysis, Siikamäki et al. (1) assume that the area in which mangroves are protected can be increased annually by ∼0.7% total mangrove area. This may represent an achievable goal, but there is no fundamental reason why more mangrove area should not be protected sooner.] Because the estimated costs per ton CO2 emissions avoided (Fig. 1) are typically lower than prices of carbon credits traded on in the European Union Emissions Trading System, mangrove protection could be a costeffective way of avoiding at least some carbon emissions. Thus, policies aimed primarily at solving the carbon-climate problem may be key to protecting mangroves, even if mangrove protection by itself is a minor contributor to solving the carbon-climate problem. Siikamäki et al. (1) propose that the amount of carbon credits should be equal to how much CO2 emissions from deforestation would have been generated, on average, per square kilometer of mangrove forest in that country over that 25-y period. They assume that, to protect the land for 25 y, funds must be provided to attain the land and protect it from deforestation for that period. [At a 3% discount rate and constant real costs of protection, the cost of permanent protection is approximately twice that of 25-y protection, but we do not have institutions that can assure permanent protection of mangroves.] The cost of emissions avoidance is then calculated as the amount of CO2 that would have been emitted from a typical piece of mangrove forest in that country over that 25-y period divided by the cost of land plus the cost of 25 y of land protection. Siikamäki et al.’s well-executed study (1) raises a range of important issues, some of which can be illustrated with a simple example. Let’s say that I pay some money to purchase and protect a 1-km patch of mangrove forest for 25 y. How much carbon credit should I get for avoided CO2 emissions? Does it matter if my patch stored more carbon or less carbon than anybody else’s? Does it matter if all the CO2 is released right away, or if the CO2 is released from the soils over a period of many decades? Accounting systems must be usable in practice; thus, the best accounting scheme might not involve the most scientifically accurate answer. Let us assume that some accounting scheme could be agreed upon that would use a reasonable approach to attribute some 0 5 10 15 Carbon price ($ / t CO2) 0 2 6 8 10