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Perspective

Global Significance of Mangrove Blue Carbon in Climate Change Mitigation

by
Daniel M. Alongi
Tropical Coastal & Mangrove Consultants, 52 Shearwater Drive, Pakenham, VIC 3810, Australia
Sci 2020, 2(3), 67; https://doi.org/10.3390/sci2030067
Submission received: 9 July 2020 / Accepted: 13 July 2020 / Published: 21 August 2020
(This article belongs to the Special Issue Climate Change Impacts on Mangrove Ecosystems)
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Abstract

Mangrove forests store and sequester large area-specific quantities of blue carbon (Corg). Except for tundra and peatlands, mangroves store more Corg per unit area than any other ecosystem. Mean mangrove Corg stock is 738.9 Mg Corg ha−1 and mean global stock is 6.17 Pg Corg, which equates to only 0.4–7% of terrestrial ecosystem Corg stocks but 17% of total tropical marine Corg stocks. Per unit area, mangroves sequester 179.6 g Corg m−2·a−1 and globally about 15 Tg Corg a−1. Mangroves sequester only 4% (range 1.3–8%) of Corg sequestered by terrestrial ecosystems, indicating that mangroves are a minor contributor to global C storage and sequestration. CO2 emissions from mangrove losses equate to 0.036 Pg CO2-equivalents a−1 based on rates of C sequestration but 0.088 Pg CO2-equivalents a−1 based on complete destruction for conversion to aquaculture and agriculture. Mangrove CO2 emissions account for only 0.2% of total global CO2 emissions but 18% of CO2 emissions from the tropical coastal ocean. Despite significant data limitations, the role of mangrove ecosystems in climate change mitigation is small at the global scale but more significant in the tropical coastal ocean and effective at the national and regional scale, especially in areas with high rates of deforestation and destruction.

1. Introduction

The concept of blue carbon was introduced in 2009 in an assessment report to a special collaboration of the United Nations Environmental Programme (UNEP), Food and Agriculture Organization of the United Nations (FAO) and the Intergovernmental Oceanographic Commission of the United Nations Educational, Scientific and Cultural Organization (IOC/UNESCO) [1] with the idea that the role of coastal ecosystems such as salt marshes, mangroves and seagrass meadows in absorbing carbon (C) to reduce emissions is of global significance and they should be protected and, if necessary, restored in order to maintain and expand their ability as critical C sinks. ‘Blue carbon’, defined as the coastal carbon sequestered and stored by ocean ecosystems [1], has been increasingly used as a concept to justify numerous studies describing C stocks and rates of C sequestration, especially in salt marsh, mangrove and seagrass ecosystems.
A detailed assessment was commissioned by the International Union for Conservation of Nature (IUCN) [2] to document the C management potential of salt marshes, mangrove forests, seagrass meadows, kelp forests and coral reefs. The report found that these coastal habitats are quantitatively and qualitatively important for numerous reasons, including a high potential for C management [2]. The report concluded that (1) sediments and soils in these ecosystems, while small in geographical extent, sequester proportionally more C than terrestrial ecosystems due to lower potential for emissions of greenhouse gases (CH4, CO2); (2) there is therefore a critical need for comprehensive C inventories from these habitats to properly assess their role in absorbing C emissions; (3) anthropogenic greenhouse gas emissions are being underestimated because such emissions from these coastal habitats are not being accounted for in national and international inventories, meaning their C savings from sequestration do not count towards meeting climate change commitments; and (4) these habitats continue to be destroyed and need to be protected and restored.
Subsequently published policy reports [3,4,5] indicated that when these habitats are converted their C is released back into the atmosphere, thus reversing the effect of fostering carbon sequestration in REDD+ (Reducing Emissions from Deforestation and Forest Degradation; + refers to conservation and sustainable management and enhancement of carbon stocks) and other rehabilitation projects. Policymakers need to understand that there are three components involved in C sequestration: (1) the annual sequestration rate, that is, the annual flux of organic carbon (Corg) transferred to anaerobic soils and sediments where it cannot undergo oxidation to CO2 and be released into the atmosphere; (2) the amount of C stored in above- and below-ground biomass; and (3) the total ecosystem C stock stored below-ground as a result of prior sequestration, that is, historical sequestration over a habitat’s lifetime.
Since the publication of these seminal publications, there has been an explosion of subsequent papers on blue carbon, with over 1000 papers published since 2009 [6]. This impressive growth reflects the need of NGOs and various agencies around the globe for more data, as well as a lot of enthusiasm for the idea that blue carbon storage and sequestration is of national and international significance in reducing carbon emissions.
Two publications have estimated that mangrove forests, especially if converted to aquaculture ponds, cattle pastures and infrastructure upon deforestation, would account for more than one half of the carbon lost (0.09–0.45 Pg CO2 a−1) [7] from coastal ecosystems to the atmosphere and account for at least as much buried C as salt marshes and seagrasses [8]. However, two more recent publications [6,9] have cast doubt on the global significance of mangroves as C sinks, while at least one other publication [10] concluded that mangrove C is nationally important to Indonesia, due in part to the nation’s large mangrove biomass and forest area.
This paper is an attempt to clarify the global and regional significance of mangrove forest C storage and sequestration in reducing and mitigating anthropogenic CO2 gas emissions. The most recent data will be used to better pinpoint the range of rates of C sequestration, C stocks and potential and actual losses from deforestation.

2. Carbon Stocks

Mangrove C stocks have been measured in 52 countries in Africa, Southeast Asia, South and East Asia, Central and North America, the Caribbean, South America, the Middle East, Australia, New Zealand and some Pacific Islands (Table 1). Total ecosystem Corg stocks average 738.9 ± 27.9 Mg Corg ha−1 (±1SE) with 224 measurements and a median value of 702.5 Mg Corg ha−1; above-ground biomass C (living and dead) averages 109.3 ± 5.0 Mg Corg ha−1 (±1SE) with 272 measurements, below-ground biomass C (live and dead roots) averages 80.9 ± 9.5 Mg Corg ha−1 (±1SE) with 76.5% of total C stocks vested in mangrove soils (mean = 565.4 ± 25.7 Mg Corg ha−1) to a depth of at least 1 m (Table 1). These values are considerably lower than the estimates of Alongi [11] and Kauffman et al. [12]. In most cases, minimum and maximum estimates varied by an order of magnitude. Above-ground and below-ground biomass C accounted for 14.8% and 8.7% of total ecosystem C stocks. There is considerable variability in these estimates, reflecting the wide range of ages and geomorphological types of forests, from young plantations to mature undisturbed forests. Also, it is highly likely that the soil C stocks are underestimated in most studies as other studies have measured considerable soil C stocks below 1 m depth (Supplementary Materials Table S1). Further, these data do not include possible inorganic C stocks, particularly in arid mangroves and those near coral reef and mixed terrigenous-carbonate environments [12].
Using the median of 702.5 Mg Corg ha−1 and the most recent estimate of global mangrove area of 83,495 km−2 [13], we derive a global C stock estimate for mangroves of 5.85 Pg C. This estimate is higher than the estimates of 5.0 Pg C by Jardine and Siilamäki [14] and 4.19 Pg C by Hamilton and Friess [15], lower than the estimates by Sanders et al. [16] of 11.2 Pg C and Alongi [6] but within the range (3.7–6.2 Pg C) estimated by Ouyang and Lee [17]. While some of these differences are due to the use of different ecosystem C stock estimates, the main difference is due to the large disparity in the use of estimates of global mangrove area. The higher estimates used the global area estimate of Giri et al. [18] of 137,760 km2 while the lower estimates used the global area estimate of 83,495 km of Hamilton and Casey [13]. The latter estimate is based on the newest and most accurate databases of the Global Forest Change database, the Terrestrial Ecosystems of the World database and the Mangrove Forests of the World database to extract mangrove forest cover at high spatial and temporal resolutions.
Regionally, total ecosystem C stocks are, on average, greatest on the Pacific Islands (mean = 987.4 Mg Corg ha−1) of Kosrae, Yap and Palau, followed by mangroves in Southeast Asia (mean = 860.9 Mg Corg ha−1), Central and North America and the Caribbean (mean = 777.7Mg Corg ha−1) and Africa (mean = 664.2 Mg Corg ha−1). Total ecosystem C stocks were considerably lower in Australia and New Zealand (mean = 563.4 Mg Corg ha−1), South America (mean = 424.0 Mg Corg ha−1), South and East Asia (mean = 395.5 Mg Corg ha−1) and the Middle East (mean = 248.4 Mg Corg ha−1). The size of mangrove C stocks is obviously related to climate, with higher estimates in forests of the humid tropics and lower estimates in the dry tropics and in subtropical and warm temperate regions. This interpretation is supported by the analysis of Sanders et al. [16] who found that 86% of observed variability in mangrove C stocks is associated with annual rainfall, which is the best predictor of mangrove ecosystem C stocks.
At the individual forest level, the smallest C stocks occur in small stands that occur in the arid tropics or are young plantation forests. As forests age, forest biomass and thus C stocks increase. A clear example is the mangrove forests of known age in French Guiana [19]. As the forests age, C stocks in above- and below-ground biomass, soil and the forest ecosystem increase with increasing age (Figure 1). Each of the four C stocks shows significant linear regression (r2 = 0.959, p < 0.001 for AGBCorg; r2 = 0.618, p = 0.039 for BGBCorg; r2 = 0.982, p < 0.001 for soil Corg; and r2 = 0.979, p < 0.001 for total ecosystem Corg). These data indicate that mangrove Corg levels increase in soils as roots grow and die and accumulate and the canopy continues to grow in size with increasing age, suggesting that mangrove C is best preserved if mature mangrove forests are conserved and left undisturbed. Plantation data from Vietnamese and Indonesian [20,21,22] mangroves similarly indicate increased C storage with increased stand age.

3. Carbon Sequestration Rates

Rates of carbon sequestration, derived from soil accretion rates, in mangroves average 179.6 g Corg m−2·a−1 and a median of 103 g Corg m−2·a−1, with rates varying widely from 1 to 1722.2 g Corg m−2·a−1 (Figure 2). Half of all observations were in the range of 1–100 g Corg m−2·a−1 (Figure 2). The mean value is greater than the estimates of Breithaupt et al. [23], McLeod et al. [24] and Alongi [11]. Assuming a global area of 83,495 km−2 [13] and multiplying by the median value, carbon sequestration in the world’s mangrove forests equates to 8.6 Tg Corg a−1. This value is lower than the 23–25 Tg Corg a−1 calculated by Twilley et al. [25], Jennerjahn and Ittekot [26] and Duarte et al. [27] and the recent estimate of 14.2 Tg Corg a−1 by Alongi [6]. The standard deviation is greater than the mean, reflecting the high level of variability in soil accretion rates and rates of carbon sequestration among mangroves of different ages, types and locations. There was no clear relationship with latitude as it is likely that these rates are a function of several interrelated factors such as forest age, tidal inundation frequency, tidal elevation, geomorphology, species composition, soil grain size, catchment and river input and extent of anthropogenic inputs; most of the highest rates were measured in mature forests in close proximity to river deltas and in forests in highly impacted catchments.

4. Carbon Losses

Blue carbon storage in mangroves may be underestimated by considering soil Corg pools only to a depth of 1m but may be offset by losses of CH4 and oxidation of ancient Corg stored in deep soils [49,50]. Some of the soil Corg is decomposed and returned to the atmosphere as CH4. As CH4 has a higher global warming potential than CO2, it can offset the CO2 removed via Corg burial. Rosentreter et al. [49] calculated that high CH4 emissions from mangroves can partially offset blue carbon burial rates on average by 20% using the 20-year global warming potential. Corg buried in mangrove deposits not only releases CH4 but also century-old sequestered carbon in the form of exported dissolved inorganic carbon (DIC). In a subtropical mangrove system, ∆14C was measured in the DIC exported from the pore water and soil ∆14C profiles. Porewater exchange released isotopically depleted, old DIC to adjacent creek waters [50]. The DIC came from an average depth of 40 cm, equivalent to about a century of soil accumulation. Thus, 100-yr old DIC is still susceptible to remineralization and tidal export via pore water exchange or submarine groundwater discharge.
The loss of mangroves, irrespective of cause, results in significant loss of Corg inventory, especially if the soil horizon is removed or disturbed. This removal can be converted to CO2-eq (equivalent) emissions back to the atmosphere. Immediate removal of biomass and soil of destroyed mangrove forests to convert the area to aquaculture ponds, cattle pastures and other land uses results in extremely high losses (Table 2), with CO2 eq emissions averaging 1802.2 Mg ha−1·a−1 and ranging from 407.9 to 2781.5 Mg ha−1·a−1 [51,52] as estimated in Brazil, Mexico, the Philippines, Honduras, Dominican Republic, Indonesia and Costa Rica. Most of these emissions come from loss of the soil pool to a depth of 1 m. If soils deeper than 1 m are dredged, the estimated CO2 eq will be greater.
Hurricanes and typhoons can destroy significant areas of mangroves, as estimated in the Philippines, Honduras, Vietnam and in Florida (Table 2). Averaging the remaining estimates (n = 20), we derive an average emission of 65.2 ± 10.6 Mg CO2 eq ha−1·a−1 (±1SE) with a median of 46 Mg CO2 eq ha−1·a−1 (Table 2). Assuming total deforestation of mangroves (biomass + soils to 1 m depth) and using the mean CO2 eq emission of 1802.2 Mg CO2 eq ha−1·a−1 and multiplying by an annual average deforestation rate of 0.16% [13,15] and a global mangrove area of 83,495 km−2 [13], we can estimate an annual loss of 24.08 Tg CO2 eq a−1 or 0.0024 Pg CO2eq a−1. This estimate is considerably less than those of Pendleton et al. [7] and Alongi [6] mostly due to lower recent estimates of annual deforestation and less global mangrove area. Mangrove losses are small on a global scale, equating to just 2.2% of CO2 losses due to losses (1.1 Gt C a−1) of the world’s tropical terrestrial forests [69] and offsetting just 1.8% of the carbon sink (1.32 Pg a−1) in the global ocean’s continental margins [70]. However, mangrove losses offset 148.6% of total CO2-air-sea exchange (−16.21 Tg C a−1) by the world’s tropical coastal zone [71].

5. Assessment of Global Significance

Are mangrove blue C stocks and C sequestration rates globally significant? The global mean C stock for mangroves is estimated to be 6.17 Pg Corg, which is the largest C stock of any ecosystem in the global tropical ocean, constituting ~17% of total tropical marine C stocks (Table 3). Although mean mangrove C stocks per unit area are the largest among the world’s ecosystems (except tundra and peatlands), global mangrove C stocks equate to only 1.6% (range: 0.4–7%) of individual terrestrial ecosystem global C stocks (Table 3). Regarding C sequestration among coastal environments, seagrass meadows sequester slightly more than twice (35.3 Tg Corg a−1) the amount of mangroves (15Tg Corg a−1). Mangroves sequester ~50% of tropical peatlands globally but only 4% compared to other terrestrial ecosystems (range: 1.3–8%). CO2 emissions due to deforestation and other destructive land use practices result in large returns of CO2 to the atmosphere, for a total of roughly 51 Pg CO2-eq a−1 (Table 3). While the same calculations for mangroves result in an estimate of 0.036 Pg CO2-eq a−1, in some regions mangrove biomass and soils are entirely removed (Section 4) resulting in mean C losses of 1802.2 Mg Corg·ha−1 a−1. Assuming that all mangroves are so destroyed at a rate of 0.16% per year, total CO2 emissions equate to 0.088 Pg CO2-eq a−1 rather than the lower estimate based solely on losses of global C sequestration (see footnote b in Table 3).
While there is no doubt that mangroves store and sequester large amounts of carbon relative to their small global area, a perusal of Table 3 indicates that they play only a minor global role in storing Corg and in mitigating CO2 emissions. However, mangrove CO2 emissions were significant throughout the tropical coastal ocean (TCO). Mangrove CO2 emissions account for roughly 0.2% of total global CO2 emissions, but account for about 18% of CO2 emissions from the tropical coastal ocean (seagrasses account for 29% and coral reefs 0.1% of TCO emissions; the remaining 52.9% is accounted for by nearshore coastal waters and subtidal benthos). It must be noted that these C stock and C rate estimates are crude and can only point to relative differences, as there are significant data limitations.
Climate change mitigation is likely to be more significant and effective at the national scale especially in countries losing mangroves rapidly, such as in Indonesia and Brazil [9,10]. Taillardat et al. [9] estimated national mangrove sequestration potential showing that they can contribute to mitigation of CO2 emissions if deforestation rates remain low. For example, mangroves in countries such as Nigeria, Colombia, Bangladesh, Ecuador, and Cuba accounted for >1% of national CO2 emissions. In countries with high deforestation rates such as Malaysia and Myanmar the carbon storage potential of remaining mangroves was less than the carbon emissions generated by deforestation of mangroves. In some countries mangrove mitigation potential is a significant percentage of national losses, such as Papua New Guinea (34.9%), Gabon (11.3%), Panama (8.3%), Mozambique (8.3%), and Cameroon (8.4%) underscoring the importance of mangrove mitigation at the national scale.

Supplementary Materials

The following is available online at https://www.mdpi.com/2413-4155/2/3/67/s1, Table S1: Estimates of organic carbon stocks (Mg Corg ha−1) in mangrove above-ground (AGBCorg ha−1) and below ground root biomass (BGBCorg ha−1) and soils (SCorg ha−1) to a depth of 1 m, except where noted.

Funding

This research received no external funding.

Conflicts of Interest

The author declares no conflict of interest.

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Figure 1. The relationship of mangrove above- (AGBCorg) and below-ground (BGBCorg) biomass C, soil Corg and total ecosystem Corg stocks in different aged forests in French Guiana [19].
Figure 1. The relationship of mangrove above- (AGBCorg) and below-ground (BGBCorg) biomass C, soil Corg and total ecosystem Corg stocks in different aged forests in French Guiana [19].
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Figure 2. Annual rates of carbon sequestration in mangrove forests globally. Refs. [6,11,19,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48].
Figure 2. Annual rates of carbon sequestration in mangrove forests globally. Refs. [6,11,19,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48].
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Table 1. Estimates of organic carbon stocks (Mg Corg ha−1) in mangrove above-ground (AGBCorg) and below-ground root biomass (BGBCorg) and soils (SCorg) to a depth of 1 m. SCorg stock estimates taken from cores < 1 m depth are not presented. Some SCorg stocks were taken from cores > 1 m depth (see Supplementary Table S1). ND = no data. Only references with sufficiently detailed methods and replication were used. References are provided in Supplementary Table S1.
Table 1. Estimates of organic carbon stocks (Mg Corg ha−1) in mangrove above-ground (AGBCorg) and below-ground root biomass (BGBCorg) and soils (SCorg) to a depth of 1 m. SCorg stock estimates taken from cores < 1 m depth are not presented. Some SCorg stocks were taken from cores > 1 m depth (see Supplementary Table S1). ND = no data. Only references with sufficiently detailed methods and replication were used. References are provided in Supplementary Table S1.
CountryAGBCorgBGBCorgSCorgTotal Corg Stock
Africa
Benin41.615.8NDND
Cameroon102.238.81961.12102.1
Congo537.715.1967.41520.2
Gabon130.0372.0504.3786.3
Ghana165.137.5310.9466.0
Guinea59.622.7NDND
Ivory Coast99.838.8NDND
Kenya101.168.8643.6806.7
Liberia50.0297.5342.0950.0
Nigeria69.226.3NDND
Madagascar70.635.8368.3457.3
Mozambique95.836.5216.3348.6
Senegal34.0401.0240.0675.0
Sierra Leone62.723.8NDND
South Africa6.7ND228.1234.8
Tanzania55.750.2293.4397.1
Togo42.916.3NDND
Southeast Asia
CambodiaNDNDND657.4
Indonesia142.0335.9420.1794.9
Malaysia119.75.9763.0894.4
Myanmar20.718.4167.0206.1
Philippines161.463.1450.2549.0
Singapore105.039.9307.3452.3
Thailand68.0108.7604.7754.1
Vietnam120.021.8768.0968.7
South and East Asia
Bangladesh81.442.3438.9565.6
China89.530.3380.1499.9
India88.033.681.3248.5
Japan57.927.0154.2239.1
Pakistan93.339.0NDND
Sri Lanka151.730.0362.1543.7
Central and North America and Caribbean
Belize42.4725.0333.4738.3
Costa Rica101.4484.0480.5845.0
Dominican Republic50.5112.3690.8853.5
Honduras85.5509794.01222.4
Mexico109.188.8643.1810.7
Panama33.0365.0531.0929.0
USA62.712.6201.4272.5
South America
Brazil87.933.8310.6432.3
Colombia84.2382.2159.0648.2
Ecuador100.7ND407.0507.7
French Guiana91.231.8149.2272.1
Guyana176.5NDNDND
Middle East
EgyptNDND389.4ND
Iran46.165.6227.3339.0
Saudi ArabiaNDND92.0ND
United Arab Emirates25.431.7123.2180.4
Australia and New Zealand
Australia84.8177.0726.6870.3
New Zealand17.021.473.5103.0
Pacific Islands
Hawaii179.378.3197.1464.0
Kosrae256.4237.9694.11188.0
Palau117.9100.0522.1739.9
Yap249.9201.6714.11165.7
Global Means
AGBCorgBGBCorgSCorgTotal Corg Stock
Mean109.380.9565.4738.9
±1SE5.09.525.727.9
n274176243224
Median94.134.1500.5702.5
Min1.90.337.046.3
Max537.7866.02102.72205.0
Table 2. Losses of blue carbon via CO2 eq (Mg ha−1·a−1) emissions from degraded mangroves worldwide. ND = no data. a = Mg CO2 eq ha−1 lost immediately upon conversion/hurricane disturbance; b = above-ground biomass C losses only.
Table 2. Losses of blue carbon via CO2 eq (Mg ha−1·a−1) emissions from degraded mangroves worldwide. ND = no data. a = Mg CO2 eq ha−1 lost immediately upon conversion/hurricane disturbance; b = above-ground biomass C losses only.
DisturbanceLocationMethod for Estimating CO2 EmissionYears Since DisturbanceCO2 eq EmissionReference
DeforestationBelizeFlux chambers1106[53]
2030
New Zealand0.1–821.4[54]
Cambodia10–1548[55]
Conversion to aquaculture and/or cattle pasturesIndonesia2516[56]
Indonesia2544
Dominican RepublicChange in SCorg2982[57]
NE BrazilChange in ecosystem Corg stock8–121392 a[51]
Mexico7–302610 a[52]
HondurasND1068.4 a
Costa RicaND1811.9 a
IndonesiaND2544.0 a
Dominican RepublicND2781.5 a
Thailand10179[58]
Mahakam delta, Borneo16120[59]
Tree mortalityKenyaChange in soil volume and gas flux225.3–35.6[60]
Hurricane/typhoon damageHondurasDifference in C inventory between disturbed and undisturbed mangroves218.7[61]
Global data3033.9
3027.2 [62]
3020.4
VietnamDifference in C inventory between disturbed and undisturbed mangroves14106.3[63]
SW FloridaLoss total ecosystem Corg1425.7–216.5 a[64]
Natural erosion, conversion to agricultureRufiji deltaC inventory and remote sensing16119.7[65]
Zambezi delta1698.9
Ganges delta1698.6
Mekong delta1688.4
Abandoned fishpondsPhilippines∆in C inventory abandoned and natural mangroves11–15407.9 a[66]
Various land use changesMexico∆in C inventory, loss of mangroves2014.8[67]
Sundarbans, India383.7 b[68]
Table 3. Estimated area-specific and global C stocks, C sequestration rates and CO2 emissions due to losses from mangrove forests, salt marshes, seagrass meadows, coral reefs, the tropical coastal ocean and terrestrial ecosystems.
Table 3. Estimated area-specific and global C stocks, C sequestration rates and CO2 emissions due to losses from mangrove forests, salt marshes, seagrass meadows, coral reefs, the tropical coastal ocean and terrestrial ecosystems.
EcosystemArea (106 ha)Mean C Stock
(Mg Corg ha−1)		Global Mean C Stock
(Pg Corg)		Mean C Sequestration
(g Corg m−2 a−1)		Global C Sequestration
(Tg Corg a−1)		Current Conversion Rate (% a−1)Carbon Emissions
(Pg CO2-eq a−1)
Mangrove8.34 [6]738.9 a6.17 a179.6 a14.980.16 [13,15]0.088 b (0.036)
Salt Marsh5.50 [72]317.2 [6]1.74212.0 [6]11.661.32 [73]0.084
Seagrass16.0 [74]163.3 [6]2.61220.7 [6]35.311.5 [7]0.144
Coral Reef52.7 [75]0.6 [76,77]0.035.69 [78]3.00.43 [79]0.0005
Tropical coastal ocean710.0 [71]50.7 [80]36.00.55 [71]3.90.93 c0.5
Tropical forest1760 [81]314.2 [81]553.062.51100.0 [82]0.53 [83]10.8
Temperate forest1040 [81]280.8 [81]292.128.9300.0 [83]0.70 [84]7.5
Boreal forest1370 [81]288.3 [85]395.018.0 [85]246.60.80 [84]11.6
Tropical grassland/savanna2250 [81]202.4 [86]455.414.0 [86]315.00.70 [86]11.7
Temperate grassland1250 [81]181.1 [86]226.416.8210.0 [86]0.55 [87]4.6
Desert and xericshrub land4550 [81]26.3 [88]119.79.5 [88]432.30.3 [88]1.3
Montane grasslands/forests519 [89]173.9 [90,91]90.3NDND0.49 [92,93,94,95,96,97,98]1.6
Mediterraneanforest322 [89]271.4 [99,100,101,102,103,104]87.465.8 [101,102,103]212.8NDND
Tundra835 [89]1779.6 [105,106,107,108,109,110,111]1486.063.2 [112,113,114,115,116]528.0NDND
Boreal peatlands361 [117]1182.8427.0 [117]53.1 [117]191.7ND0.26 [117]
Tropical peatlands58.7 [117]2030.7119.2 [117]54.2 [117]31.8ND1.48 [117]
a = from Table 1 and Table 2; b = estimated assuming total forest biomass and soil losses to a depth of 1 m (see Section 4). CO2 emissions based on global sequestration rate are in parentheses. c = weighted average of conversion rates for mangroves, seagrasses and coral reefs.

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Alongi, D. M. (2020). Global Significance of Mangrove Blue Carbon in Climate Change Mitigation. Sci, 2(3), 67. https://doi.org/10.3390/sci2030067

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