Research ArticleENVIRONMENTAL SCIENCES

Methane emissions partially offset “blue carbon” burial in mangroves

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Science Advances  13 Jun 2018:
Vol. 4, no. 6, eaao4985
DOI: 10.1126/sciadv.aao4985
  • Fig. 1 Mangrove NPP versus mangrove carbon loss.

    The mangrove NPP of 191.9 Tg C year−1 and tidal exports of POC (25.6 Tg C year−1), DOC (13.7 Tg C year−1), and DIC (78.6 Tg C year−1) are derived from Alongi and Mukhopadhyay (13) and scaled to the total global mangrove area estimate of Giri et al. (25). Global mangrove CO2 emissions of 34.1 ± 5.4 Tg C year−1 are from Rosentreter et al. (14). Global mangrove CH4 emissions are further presented as CO2 equivalent emissions using the GWP20 (Table 2). The values presented in brackets are the latitudinal scaled total CH4 emission and burial rates (Table 3). Photo used with permission from A. McMahon.

  • Fig. 2 CH4 concentrations over two tidal cycles.

    CH4 concentration (nM) at the mangrove sites in the BR, FR, and JR estuaries in the wet and dry season.

  • Fig. 3 CH4 concentrations versus salinity.

    CH4 concentration (nM) over the salinity range during the 24-hour time series in the wet and dry season in the BR, FR, and JR mangrove creeks. The salinity in the BR dry season is only available for the first 3 hours of the time series. Note the different scale of salinity in each system.

  • Fig. 4 Latitudinal average mangrove CH4 flux rates and carbon burial rates.

    (A) Combined average sediment-atmosphere and water-atmosphere CH4 flux rate (±SE) of the latitudinal regions 0° to 40° (in 5° steps), including this study and previously published studies. Note that in latitudinal regions where no water-atmosphere or sediment-atmosphere CH4 flux data were available, we used the next closest latitudinal region or the average of the next higher and lower latitudinal region (see Materials and Methods and table S4). The black line indicates the quadratic regression line (r2 = 0.49, n = 8, P < 0.5). (B) Average carbon burial rate (±SE) of the latitudinal regions 0° to 30° (in 5° steps) of previously published studies. The black line indicates the linear regression line (r2 = 0.69, n = 6, P < 0.05). The dashed lines indicate the 95% confidence interval.

  • Fig. 5 Map of CH4 emission studies and carbon burial studies in mangroves.

    Gray triangles are mangrove carbon burial study sites (table S5). Black triangles are mangrove water-atmosphere CH4 flux study sites (table S2). Open squares are mangrove sediment-atmosphere CH4 flux study sites (table S3). Mangrove forest distribution is in green. Map based on the global mangrove forest distribution of Giri et al. (25).

  • Table 1 Average (± confidence interval) water-atmosphere CH4 flux rates and gas transfer velocities using different empirical k models.

    na, not available.

    k modelVariables*BR wetBR dryFR wetFR dryJR wetJR dry
    flux
    (μmol
    m−2 day−1)
    k600
    (cm hour−1)
    flux
    (μmol
    m−2 day−1)
    k600
    (cm hour−1)
    flux
    (μmol
    m−2 day−1)
    k600
    (cm hour−1)
    flux
    (μmol
    m−2 day−1)
    k600
    (cm hour−1)
    flux
    (μmol
    m−2 day−1)
    k600
    (cm hour−1)
    flux
    (μmol
    m−2 day−1)
    k600
    (cm hour−1)
    R17 (27)v236.5 ±
    7.4
    5.7103.7 ±
    3.6
    7.31059.0 ±
    48.9
    15.095.3 ±
    6.4
    8.3nana484.4 ±
    9.4
    9.8
    R17 (27)v, u283.7 ±
    8.3
    6.8104.6 ±
    4.3
    6.91110.7 ±
    49.5
    16.499.2 ±
    6.8
    8.9nana460.4 ±
    9.6
    9.3
    R17 (27)v, u, h324.7 ±
    8.8
    7.9115.7 ±
    4.3
    7.7979.8 ±
    40.7
    15.594.8 ±
    6.3
    9.0nana535.3 ±
    8.7
    10.9
    Average
    flux (R17)
    301.0 ±
    8.7
    6.8108.0 ±
    4.0
    7.31049.8 ±
    46.3
    15.696.5 ±
    6.5
    8.7826.210.2493.3 ±
    9.1
    10.0
    H16 (26)v, u, h275.0 ±
    9.8
    6.569.1 ±
    3.6
    3.8601.4 ±
    28.3
    8.169.6 ±
    4.9
    6.0nana167.0 ±
    4.5
    3.4
    RC01 (54)u385.0 ±
    13.8
    9.293.5 ±
    4.9
    5.0474.7 ±
    17.1
    8.174.8 ±
    5.1
    7.1375.2 ±
    8.7
    4.7224.0 ±
    5.8
    4.6
    B04 (55)v, u, h623.2 ±
    18.0
    14.7187.4 ±
    7.8
    10.91542.7 ±
    75.7
    19.3191 ±
    13.2
    15.6nana494.7 ±
    10.1
    10.2

    *Current velocity (v) in cm s−1, water depth (h) in m, and wind speed (u) in m s−1.

    †Calculated based on the % difference of RC01 and average R17 in the JR dry season.

    • Table 2 Global CH4 emissions, carbon burial rates, and offsets in mangroves.

      Global emission and carbon burial rates are upscaled to the global mangrove forest area of 137,760 km2 (25). n refers to the number of study site locations (see tables S2, S3, and S5).

      CH4 flux rate
      (μmol
      m−2 day−1)
      Global CH4
      emission
      (Tg CH4
      year−1)
      Global CH4
      emission
      (Tg C year−1)
      CH4 (CO2 eq.)
      emission,
      GWP20
      (Tg C year−1)
      CH4 (CO2 eq.)
      emission,
      GWP100
      (Tg C year−1)
      Sequestration
      rate (g C m−2
      year−1)
      Global
      carbon burial
      (Tg C year−1)
      Offset
      (%)*
      Sensitivity
      analysis
      offset range
      (%)
      Water-air
      CH4 flux
      (n = 9)(n = 9)(n = 9)(n = 9)(n = 9)(n = 27)(n = 27)
      Min35.100.0280.0210.660.2656.607.80
      Max659.80.5310.39812.454.92651.089.68
      Average ± SE288.0 ± 73.20.232 ± 0.0590.174 ± 0.0445.43 ± 1.382.15 ± 0.55227.2 ± 30.6731.30 ± 4.2217.3(15.7–18.5)
      Sediment-air
      CH4 flux
      (n = 14)(n = 14)(n = 14)(n = 14)(n = 14)(n = 27)(n = 27)
      Min0000056.607.80
      Max2,127.21.711.2840.1415.87651.089.68
      Average ± SE391.2 ± 153.40.315 ± 0.1230.236 ± 0.0937.38 ± 2.902.92 ± 1.14227.2 ± 30.6731.30 ± 4.2223.6(22.0–25.8)
      Global total(n = 23)(n = 23)(n = 23)(n = 23)(n = 23)(n = 27)(n = 27)
      Average ±
      scaled error
      339.6 ± 106.20.273 ± 0.0530.205 ± 0.0406.41 ± 1.242.53 ± 0.49227.2 ± 30.6731.30 ± 4.2220.5(19.8–21.0)

      *Offset is calculated from carbon burial (Tg C year−1) and the CH4 flux using the GWP20 (Tg C year−1).

      †Carbon burial (Tg C year−1) and CH4 emissions (GWP20; Tg C year−1) are adjusted up and down by their errors.

      ‡Accounting for mangrove ecosystems being inundated 50% of the time (water-atmosphere flux) and exposed 50% of the time (sediment-atmosphere flux).

      • Table 3 Latitudinal CH4 emissions, carbon burial, and offsets in mangroves.

        Mangrove latitudinal areas are based on Giri et al. (25). Error (±) is scaled error (see text) unless marked with “a” (in these cases, the SE is used).

        LatitudeMangrove
        area (km2)
        Average
        CH4 flux
        rate
        (μmol m−2
        day−1)*
        Latitudinal
        CH4
        emission
        (Tg CH4
        year−1)
        Latitudinal
        CH4
        emission
        (Tg C
        year−1)
        Latitudinal
        CH4
        (CO2 eq.)
        emission,
        GWP20 (Tg
        C year−1)
        Latitudinal
        CH4
        (CO2 eq.)
        emission,
        GWP100
        (Tg C year−1)
        Average
        burial
        rate
        (g C m−2
        year−1)
        Latitudinal
        carbon
        burial
        (Tg C year−1)
        Offset
        (%)
        Sensitivity
        analysis
        offset
        range (%)
        0–5°44,00040.26
        (± 40.3)a
        0.010
        (± 0.005)
        0.008
        (± 0.004)
        0.24
        (± 0.12)
        0.10
        (± 0.05)
        396.6
        (± 108.5)a
        17.45
        (± 4.78)
        1.4(1.0–1.6)
        5–10°30,000143.8
        (± 78.3)a
        0.025
        (± 0.013)
        0.019
        (± 0.009)
        0.59
        (± 0.29)
        0.23
        (± 0.12)
        651.0
        (± 298.0)a
        19.53
        (± 8.94)
        3.0(2.8–3.1)
        10–15°23,500251.0
        (± 72.8)a
        0.034
        (± 0.006)
        0.026
        (± 0.004)
        0.81
        (± 0.14)
        0.32
        (± 0.05)
        286.8
        (± 63.3)a
        6.74
        (± 1.49)
        12.0(11.5–12.7)
        15–20°16,500265.1
        (± 95.5)a
        0.026
        (± 0.001)
        0.019
        (± 0.001)
        0.60
        (± 0.03)
        0.24
        (± 0.01)
        104.4
        (± 10.4)a
        1.72
        (± 0.17)
        34.8(33.4–36.5)
        20–25°18,800800.5
        (± 310.4)a
        0.088
        (± 0.032)
        0.066
        (± 0.024)
        2.06
        (± 0.75)
        0.81
        (± 0.30)
        166.6
        (± 50.7)a
        3.13
        (± 0.95)
        65.8(63.2–67.2)
        25–30°4,300264.1
        (± 50.3)a
        0.007
        (± 0.003)
        0.005
        (± 0.002)
        0.16
        (± 0.06)
        0.06
        (± 0.02)
        182.8
        (± 19.7)a
        0.79
        (± 0.08)
        19.8(13.5–24.8)
        30–35°310194.9
        (± 18.9)a
        0.0004
        (± 0.0001)
        0.0003
        (± 0.0001)
        0.008
        (± 0.003)
        0.003
        (± 0.001)
        182.8
        (± 19.7)a
        0.057
        (± 0.006)
        14.6(10.0–18.3)
        35–40°350125.7
        (± 88.1)a
        0.0003
        (± 0.0001)
        0.0002
        (± 0.0001)
        0.006
        (± 0.002)
        0.002
        (± 0.001)
        182.8
        (± 19.7)a
        0.064
        (± 0.007)
        9.4(6.4–11.8)
        TotalAverageTotalTotalTotalTotalAverageTotalAverageRange
        Latitudinal
        total
        137,760260.6
        (± 94.3)a
        0.191
        (± 0.057)
        0.143
        (± 0.043)
        4.47
        (± 1.34)
        1.77
        (± 0.53)
        269.2
        (± 73.7)a
        49.48
        (± 16.4)
        20.1(17.7–22.0)

        *Combined sediment-atmosphere and water-atmosphere flux accounting for mangrove ecosystems being inundated 50% of the time (water-atmosphere flux) and exposed 50% of the time (sediment-atmosphere flux; see table S4).

        †Offset is calculated from carbon burial (Tg C year−1) and the CH4 flux using the GWP20 (Tg C year−1).

        ‡Carbon burial rates (Tg C year−1) and CH4 emissions (GWP20, Tg C year−1) are adjusted up and down by their scaled errors.

        Supplementary Materials

        • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/6/eaao4985/DC1

          table S1. Wet and dry season physicochemical parameters and CH4 concentration in the three studied mangrove creeks.

          table S2. Mangrove water-atmosphere CH4 fluxes from this study and published studies.

          table S3. Published mangrove sediment-atmosphere CH4 fluxes.

          table S4. Combined mangrove sediment-atmosphere and water-atmosphere CH4 fluxes for each latitude region (5° steps).

          table S5. Published mangrove carbon burial rates.

          fig. S1. Average mangrove CH4 flux rates over latitudes (5° steps).

          References (5991)

        • Supplementary Materials

          This PDF file includes:

          • table S1. Wet and dry season physicochemical parameters and CH4 concentration in the three studied mangrove creeks.
          • table S2. Mangrove water-atmosphere CH4 fluxes from this study and published studies.
          • table S3. Published mangrove sediment-atmosphere CH4 fluxes.
          • table S4. Combined mangrove sediment-atmosphere and water-atmosphere CH4 fluxes for each latitude region (5° steps).
          • table S5. Published mangrove carbon burial rates.
          • fig. S1. Average mangrove CH4 flux rates over latitudes (5° steps).
          • References (59–91)

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