Avoiding mangrove destruction by avoiding carbon dioxide emissions

Tens 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⁻², mostly in soils. Mangroves typically accumulate carbon at a rate of approximately 0.1 kg⋅m⁻²⋅y⁻¹ (3). Because they cover less than 0.03% of Earth’s surface, despite large fluxes and amounts stored on a per-unit-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 cost-effective 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.

Fig. 1.

Amount of CO₂ emission avoidance available from mangrove protection in megatons (Mt) of CO₂ per dollar change in carbon price as a function of cost for different regions and for the world as a whole plotted from the data of Siikamäki et al. (1). Darker areas represent CO₂ emissions avoidance available in the 50% of the countries in the world with the best governance, and lighter areas represent the remainder of the world. There is substantial CO₂-emissions avoidance available for less than $10 per ton of CO₂, but most of it is located in countries with poor governance (5).

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-km² 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 amount of emissions to the deforestation of my patch of mangrove.

Let us posit that I contract to protect my 1-km² patch of mangrove for 25 y. The danger, of course, is that somebody comes in and cuts down the forest anyway. We need good governance to assure that contracts will be adhered to. Unfortunately, most mangroves of the world are in countries that rank in the bottom half of the World Bank index on governance effectiveness (5). Siikamäki et al. (1) find that approximately 80% of the global “emissions avoidance potential” from mangrove protection is located in the 50% of countries that rank lowest on the World Bank governance index (Fig. 2).

Fig. 2.

Amount of CO₂ emission avoidance available each year from mangrove protection as a function of total annual expenditure, plotted from data in the article by Siikamäki et al. (1). The different lines represent emission avoidance available in the top 50%, 90%, and 100% of the countries in the world ranked by best governance. Average cost of emissions avoidance is <$10 per ton of CO₂ (tCO₂) in all cases, but only ∼20% of the avoidable emissions is in the top 50% of countries ranked by quality of governance (5).

Siikamäki et al. (1) estimate that, on average, approximately 16% of mangrove would be expected to be lost in a 25-y period. Under their proposed accounting system, for 25-y protection of my 1-km² patch of mangrove, I would get credit for avoiding 16% of the emissions that would occur were my patch of mangrove cleared. However, is there any assurance or even expectation that, by protecting 1 km² of mangrove for 25 y, mangrove loss in that country will be diminished? Might not the deforestation just be displaced to other unprotected patches of land without any net reduction in rates of mangrove loss? It seems we need policies to assure that displacement does not occur (6) and that there is a real net reduction in mangrove loss.

Most economists would likely say that the most economically efficient way to reduce CO₂ emissions from mangrove deforestation would be to tax CO₂ emissions resulting from mangrove loss. The results of Siikamäki et al. (1) indicate that a $10-per-ton CO₂ tax on CO₂ emissions from mangrove loss would—if it could be implemented—be both a cost-effective CO₂ emission reduction strategy and an effective biodiversity protection strategy. (One could also imagine crediting owners of mangrove forests for the service provided by CO₂ uptake from the atmosphere.) Such a tax would, in a very simple way, address a wide range of issues, including within-country displacement, and, if universally applied, such a tax would also address displacement across international borders. Why the preference for crediting avoidance of CO₂ emissions rather than taxing production of CO₂ emissions? The answer to this question appears to be political. Carrots are more popular than sticks, and thus may be easier to implement. Nevertheless, sticks are sometimes more effective at producing desired results.

Siikamäki et al. (1) suggest that we should get full credit for all of the tons of CO₂ that would have been emitted on average from my 1-km² patch during its 25 y of protection. However, let us fast forward 25 y into the future when my patch of mangrove is once again vulnerable to deforestation. Without further protection, we might assume that, on average, ∼0.7% of my 1-km² patch will be lost annually to deforestation. My 25 y of protection has not avoided the risk of deforestation but has delayed this risk by 25 y. Therefore, it could be argued that I should be credited only for the value of a delay in emission, not the value of an emission avoided (7). For a constant value of carbon emissions avoided and a 3% discount rate, a 25-y delay is worth approximately half what permanent avoidance would be worth. However, economic theory says that if there is a fixed amount of carbon dioxide emissions allowable for the future, the price of emission should increase at the discount rate (a phenomenon known as “hoteling rent”). In this case, the net present value of a carbon emission is equal regardless of when it occurs, and there is no value to delay in emissions. Determining whether a policy avoids or merely delays emission is complex and shows up in other domains, such as in the discussion of the use of coal vs. natural gas a primary fuel (8).

The conclusions of Siikamäki et al. (1) are fundamentally correct and are not challenged by the issues raised here. Protection of mangroves presents a win-win situation. Protection of mangroves provides a cost-effective means of avoiding CO₂ emissions and, importantly, helps to maintain biodiversity. However, we need good policies and good institutions that can provide confidence that contracts can be enforced, displacement can be prevented, and some semblance of permanence can be a realistic expectation.

It is shameful that we do not simply find the resources to protect and sustainably manage all mangrove ecosystems—and consider avoided carbon emissions to be an ancillary cobenefit.