Research ArticleATMOSPHERIC SCIENCE

Shipborne eddy covariance observations of methane fluxes constrain Arctic sea emissions

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Science Advances  29 Jan 2020:
Vol. 6, no. 5, eaay7934
DOI: 10.1126/sciadv.aay7934
  • Fig. 1 SWERUS-C3 cruise in the Arctic Ocean during July to August 2014.

    The dotted purple lines indicate the approximate extent of shelf seas after (7). Dotted magenta lines divide shelf seas. Dotted yellow lines indicate the approximate location of the top of the continental slope in Laptev and East Siberian seas. (A) Atmospheric CH4 concentrations (ppm) during SWERUS-C3 (until DoY 240). This figure includes data from figure 1a from (4), extended with additional measurements after DoY 222 in the Chukchi and East Siberian seas. (B) Sea ice coverage (%) at Oden’s position during SWERUS-C3 from AMSR2 satellite retrievals.

  • Fig. 2 Measured sea-air EC CH4 flux (mg m−2 day−1) during SWERUS-C3 until DoY 240 with insets for selected seep regions.

    Red open stars on the main map show the approximate location of each seep area. The spatial extent of each inset map is shown above and to the right of each inset. Note that the color scale varies between the main plot and insets.

  • Table 1 EC average sea-air CH4 fluxes by region (in ng m−2 s−1 and mg m−2 day−1).

    Regions marked with * are the same regions as used in table 2 of (4) for calculated sea-air CH4 fluxes. Note that many of the regions are overlapping. “Seeps only” is defined as stations/areas with EC CH4 measurements >6 mg m−2 day−1 (see Table 2 for locations).

    Spatially normalized fluxes
    Average
    (ng m−2 s−1)
    Average
    (mg m−2 day−1)
    Maximum
    (mg m−2 day−1)
    No. of EC measurement
    locations
    Arctic Ocean*1.390.126.3349
    Shelf breaks and upper
    continental slope*
    1.010.096.4241
    Shelf seas (Laptev + East
    Siberian + Chukchi seas)
    17.31.506181851
    Laptev Sea (all)*53.14.58170265
    Laptev Sea, seeps only36321.417032
    East Siberian Sea (all)20.21.74618834
    East Siberian Sea, ice-covered
    /melt regions
    (<DoY 222.3)*
    23.52.02618695
    East Siberian Sea, ice-covered
    /melt regions
    24.12.09618677
    East Siberian Sea, ice-free3.170.278.6157
    East Siberian Sea, seeps only100463.361817
    Chukchi Sea, ice-free2.510.228.4429
    Chukchi Sea, ice-covered/melt
    regions
    0.390.038.3326
    Chukchi Sea (all)1.620.148.4755
  • Table 2 Locations and peak and average CH4 EC sea-air CH4 fluxes from Laptev Sea and East Siberian Sea seep areas sampled during SWERUS-C3.

    2σ error is ±2 mg m−2 day−1 for peak (single measurement) flux observations. Areal extent of each seep area is determined by the distance from peak flux of first measurement <6 mg m−2 day−1 CH4, a very conservative assumption that seep area extends this distance from peak. EC measurement locations refer to discrete measurement locations in the spatially normalized dataset, not the total number of EC measurements. All data are after filtering. LS1, LS2, ESS1, ESS2, and ESS7 are displayed in Fig. 2.

    Peak
    latitude (°N)
    Peak longitude (°E)Peak CH4 flux
    (mg m−2 day−1)
    Average CH4 flux
    (mg m−2 day−1)
    Estimated areal extent
    of fluxes >6 mg m−2
    day−1 (km2)
    EC measurement
    locations within
    enhanced area
    76.7742 (LS1)125.833117036.02.025
    76.8884 (LS2)127.776233.114.84.47
    74.9571 (ESS1)161.08396181429.44
    74.9913 (ESS2)161.142311429.618.87
    74.9099 (ESS3)160.424654.243.46.32
    74.4200 (ESS4)166.930311.811.80.51
    74.4376 (ESS5)167.343917.617.65.41
    73.8477 (ESS6)170.383017.217.23.91
    74.1982 (ESS7)171.39232212213.01
    Area weighted fluxes
      Laptev Sea seep areas21.46.432
      East Siberian Sea seep areas63.347.317
  • Table 3 ESAS CH4 sea-air flux results and comparisons with earlier studies.

    Annual EC fluxes assume that 100% of CH4 trapped in or under ice for part of the year eventually reaches the atmosphere. The whole-ESAS CH4 emission estimate from EC fluxes is lower than estimates given in three earlier measurement-based studies (2, 4, 24), shown at the bottom of this table. Part of this difference is due to the inclusion of the Chukchi Sea in the present study, which accounts for 29.4% of the ESAS area and had markedly lower sea-air fluxes than the Laptev or East Siberian seas, the two seas that the earlier three studies based their results solely on. Inclusion of low flux observations from the Chukchi Sea in the earlier measurement-based studies could have reduced pan-ESAS areal flux estimates in all of them. The flux values in (24) were reported in Tg- C- CH4 year1 and have been converted to Tg CH4 year1 here in Table 3. Extrapolating the whole-ESAS fluxes from only our observed Laptev Sea and East Siberian Sea regional fluxes, as in previous studies (2, 4, 24), we obtain the slightly higher annual CH4 flux estimate of 2.43 mg m−2 day−1 or 2.07 Tg year1. This is still the lowest whole-ESAS value yet reported based on in situ measurements but is closest to the recent bulk flux measurement from the same cruise (Table 3) and is within the range of the only inverse modeling study for the ESAS region (3). The highest value, 4.65 Tg year−1, is obtained by combining the data of (2, 24) for depths <35 m with (4) for deeper waters and accounting for bubble losses using the model of (9). Estimating small fluxes possibly missed by the EC system due to the EC noise levels, we add 1.53 Tg year1 to our total (see text for explanation). This is our best estimate for pan-ESAS CH4 fluxes based on SWERUS-C3 data alone and is close to the 2.9 Tg year−1 of (4).

    Shelf Sea*Sea area (×103 km2)Calculated
    bulk flux
    (mg m−2 day−1)
    (EC) CH4 flux
    (mg m−2 day−1)
    Annual (EC)
    CH4 flux
    (Tg year−1)
    Laptev4983.904.580.83
    East Siberian9873.7 (13.8)1.740.62
    Chukchi620No data0.140.03
    ESAS21051.941.49
    ESAS (extrapolation without
    Chukchi data as in previous
    studies)
    14853.8 (12.3)2.432.07
    ESAS [EC + estimated flux in
    EC noise (present study)]
    21053.02
    ESAS [0- to 35-m depth range
    based on (24) and (2) +
    depth-based bubble
    dissolution model; >35-m
    depths based on (4)].
    21054.65
    Previous studies (method used)
    Areal fluxAnnual flux
    ESAS (3) (inverse model)0–5.90–4.5
    ESAS (2) (ebullition only; bubble counting with sonar)22.19§
    ESAS (24) (various measurements, primarily surface water concentrations)13.910.6
    ESAS (4) (surface water and atmospheric concentrations; Laptev and East Siberian seas only)3.82.9

    *Sea areas defined as in (7).

    †Calculated bulk fluxes are from data presented in (4).

    ‡Calculated bulk flux values in parentheses include ice-covered areas, where calculated fluxes are hypothetical, and may represent temporary ice-out fluxes.

    §Total ESAS annual CH4 flux including diffusive emissions was reported as 17 Tg year−1 in (2).

    Supplementary Materials

    • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/5/eaay7934/DC1

      Fig. S1. Histogram of EC CH4 sea-air 20-min flux measurements away from gas seeps.

      Fig. S2. HYSPLIT back trajectory for Chukchi Sea enhanced CH4 (21 August 2014).

      Fig. S3. HYSPLIT back trajectory for Chukchi Sea enhanced CH4 (27 August 2014).

    • Supplementary Materials

      This PDF file includes:

      • Fig. S1. Histogram of EC CH4 sea-air 20-min flux measurements away from gas seeps.
      • Fig. S2. HYSPLIT back trajectory for Chukchi Sea enhanced CH4 (21 August 2014).
      • Fig. S3. HYSPLIT back trajectory for Chukchi Sea enhanced CH4 (27 August 2014).

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