定期貨物船を利用した太平洋温室効果ガスモニタリング

本事業の目的と概要

温室効果ガス濃度の大気観測は世界の100点以上の観測サイトで実施されていますが、多くの観測サイトが陸上に偏在するため、観測空白域となる海洋上での大気観測が求められてきました。また、海洋は大きな二酸化炭素（CO₂）の吸収源としての役割を果たしていますが、時間的にも空間的にも大きく変動するCO₂吸収量を精緻に評価するには海洋表層水のCO₂濃度（分圧）の直接観測が必要となるため、その意味でも船舶は重要な“動く観測サイト”といえます。本モニタリング事業では広範囲な海域を定常的に高頻度で往来する民間貨物船舶に“動く観測サイト”としての機能を担っていただき、1992年から日本－オセアニア航路でボトルサンプリングによる大気観測を開始しました。1995年からは日本－北米航路で洋上大気と海洋表層水の温室効果ガス連続観測を開始しています。2001年からは自動車の輸出を手掛けるトヨフジ海運（株）が本事業に参画して日本－北米航路での観測を引き継ぎ、さらに2005年からの日本－オセアニア航路（大気・海洋観測）、2007年からの日本－東南アジア航路（大気観測のみ）での観測展開に繋がり現在まで継続しています。これまでの大気中温室効果ガス濃度や海洋表層水中CO₂分圧の観測により、温室効果ガスの時空間分布を明らかにするだけでなく、温室効果ガス吸収・排出量の評価に大きく貢献しました。

現在運用中の船舶による2020年の観測航路を下図に示します。日本－北米航路と日本－オセアニア航路の各船舶は概ね年間8往復、日本－東南アジア航路の船舶は概ね年間12往復の航海を行なっており、観測域の季節変化や年々変化の把握に貢献しています。

観測方法

各船舶には大気と海洋の観測用に観測スペースを設けて、さまざまな測定機器を設置しています（写真）。

洋上大気の観測では、洋上大気中CO₂測定システムとともに、自動大気採取システムを搭載して洋上大気を採取しており、CO₂濃度やCO₂同位体の他に、メタンや一酸化二窒素などの温室効果ガス濃度の測定を行っており、近年ではメタン濃度の連続観測も実施しています。さらに、大気中の酸素窒素比やオゾン濃度、一酸化炭素濃度などの測定も本プログラムの中で同時に進められています。

海洋観測では、船底から引き込んだ海水を気液平衡器に導入して生成される平衡空気のCO₂濃度を大気中CO₂濃度と同様の方法で測定しています（詳しくは地球環境研究センターニュース「長期観測を支える主人公—測器と観測法の紹介—」3 海洋に溶ける温室効果気体の挙動を探る：海洋CO₂濃度測定システム https://www.cger.nies.go.jp/cgernews/201210/263006.html をご覧ください）。同時に海水温や塩分を高精度で連続測定し、1日3回海水試料をサンプリングして栄養塩等の測定を行なっています。

写真　船舶（TRANS FUTURE 5）に設置した大気観測室と海洋観測室の様子

各航路における観測の概要については下記のリンクからご覧ください。

代表的な論文成果

Ono H., Toyama K., Enyo K., Iida Y., Sasano D., Nakaoka S., Ishii M. (2023) Meridional Variability in Multi-Decadal Trends of Dissolved Inorganic Carbon in Surface Seawater of the Western North Pacific Along the 165°E Line. Journal of Geophysical Research: Oceans, 128, e2022JC018842
Friedlingstein P., O'Sullivan M., Jones W.M., Andrew M.R., Gregor L., Hauck J., Le Quere C., Luijkx I.T., Olsen A., Peters G.P., Peters W., Pongratz J., Schwingshackl C., Sitch S., Canadell J.G., Ciais P., Jackson B., Alin S.R., Alkama R., Arneth A., Arora V.K., Bates N.R., Becker M., Bellouin N., Bittig H.C., Bopp L., Chevallier F., Chini P., Cronin M., Evans W., Falk S., Feely R.A., Gasser T., Gehlen M., Gkritzalis T., Gloege L., Grassi G., Gruber N., Gurses O., Harris I., Hefner M., Houghton A., Hurtt G.C., Iida Y., Ilyina T., Jain A.K., Jersild A., Kadono K., Kato E., Kennedy D., Goldewijk K.K., Knauer J., Korsbakken J.I., Landschutzer P., Lefevre N., Lindsay K., Liu J., Liu Z., Marland G., Mayot N., McGrath M.J., Metzl N., Monacci N., Munro D.R., Nakaoka S., Niwa Y., O'Brien K., Ono T., Palmer P.I., Pan N., Pierrot D., Pocock K., Poulter B., Resplandy L., Robertson E., Rodenbeck C., Rodriguez C., Rosan T.M., Schwinger J., Seferian R., Shutler J.D., Skjelvan I., Steinhoff T., Sun Q., Sutton A.J., Sweeney C., Takao S., Tanhua T., Tans P.P., Tian X., Tian H., Tilbrook B., Tsujino H., Tubiello F., van der Werf G.R., Walker A.P., Wanninkhof R., Whitehead C., Wranne A., Wright R., Yuan W., Yue C., Yue X.Y., Zaehle S., Zeng J., Zheng B. (2022) Global Carbon Budget 2022. Earth System Science Data, 14 (11), 4811-4900
Niwa Y., Sawa Y., Nara H., Machida T., Matsueda H., Umezawa T., Ito A., Nakaoka S., Tanimoto H., Tohjima Y. (2021) Estimation of fire-induced carbon emissions from Equatorial Asia in 2015 using in situ aircraft and ship observations. Atmospheric Chemistry and Physics, 21, 9455-9473
Mueller A., Tanimoto H., Sugita T., Machida T., Nakaoka S., Patra P.K., Laughner J., Crisp D. (2021) New approach to evaluate satellite-derived XCO₂ over oceans by integrating ship and aircraft observations. Atmospheric Chemistry and Physics, 21, 8255-8271
Tokoro T., Nakaoka S., Takao S., Nojiri Y., Kuwae T., Kubo A., Endo T. (2021) Contribution of Biological Effects to Carbonate-System Variations and the Air -Water CO₂ Flux in Urbanized Bays in Japan. Journal of Geophysical Research-Oceans, 126 (6)
Hoshina Y., Tohjima Y., Katsumata K., Machida T., Nakaoka S. (2018) In situ observation of atmospheric oxygen and carbon dioxide in the North Pacific using a cargo ship. Atmospheric Chemistry and Physics, 18, 9283-9295
Quere C.L., Andrew R.M., Canadell J.G., Sitch S., Korsbakken J.I., Peters G.P., Manning A.C., Boden T.A., Tans P.P., Houghton R.A., Keeling R.F., Alin S., Andrews O.D., Barbero L., Bopp L., Chevallier F., Chini L.P., Ciais P., Currie K., Delire C., Doney S.C., Friedlingstein P., Gkritzalis T., Harris I., Hauck J., Haverd V., Hoppema M., Goldewijk K.K., Jain A.K., Kato E., Kortzinger A., Landschutzer P., Lefevre N.L., Lenton A., Lienert S., Lombardozzi D.L., Melton J.R., Metzl N., Millero F., Monteiro P.M.S., Munro D.R., Nabel J.E.M.S., Nakaoka S., Brien K.O., Olsen A., Omar A.M., Ono T., Pierrot D., Poulter B., Rodenbeck C., Salisbury J., Schuster U., Schwinger (2016) Global Carbon Budget 2016. Earth System Science Data, 8, 1-45
Dorothee C. E. Bakker, Benjamin Pfeil, Camilla S. Landa, Nicolas Metzl, Kevin M. O’Brien, Are Olsen, Karl Smith, Cathy Cosca, Harasawa S., Stephen D. Jones, Nakaoka S., Nojiri Y., Ute Schuster, Tobias Steinhoff, Colm Sweeney, Taro Takahashi, Bronte Tilbrook, Wada C., et al. (2016) A multi-decade record of high-quality fCO₂ data in version 3 of the Surface Ocean CO₂ Atlas (SOCAT). Earth System Science Data, 8, 383-413
Yasunaka S., Ono T., Nojiri Y., Whitney F.A., Wada C., Murata A., Nakaoka S., Hosoda S. (2016) Long-term variability of surface nutrient concentrations in the North Pacific. Geophysical Research Letters, 43, 3389-3397
Rodenbeck C., Bakker D. C. E., Gruber N., Iida Y., Jacobson A. R., Jones S., Landschutzer P., Metzl N., Nakaoka S., Olsen A., Park G.H. , Peylin P., Rodgers K.B., Sasse T.P., Schuster U., Shutler J.D., Valsala V., Wanninkhof R., Zeng J. (2015) Data-based estimates of the ocean carbon sink variability – first results of the Surface Ocean pCO₂ Mapping intercomparison (SOCOM). Biogeosciences, 12, 7251-7278
Zeng J., Nojiri Y., Nakaoka S., Nakajima H., Shirai T. (2015) Surface ocean CO₂ in 1990-2011 modelled using a feed-forward neural network. Geoscience Data Journal, 2, 47-51.
Nara H., Tanimoto H., Tohjima Y., Mukai H., Nojiri Y., and Machida T. (2014) Emissions of methane from offshore oil and gas platforms in Southeast Asia. Sci. Rep., 4(1), 6503, 10.1038/srep06503.
Yasunaka S., Nojiri Y., Nakaoka S., Ono T., Whitney A. F., Telszewski M. (2014) Mapping of sea surface nutrients in the North Pacific: Basin-wide distribution and seasonal to interannual variability. Journal of Geophysical Research: Oceans, 119, 7756-7771
Zeng J., Nojiri Y., Landschützer P., Telszewski M., Nakaoka S. (2014) A Global Surface Ocean fCO₂ Climatology Based on a Feed-Forward Neural Network. J. Atmos. Ocean. Tech., 31 (8), 1838-1849
Yasunaka S., Nojiri Y., Nakaoka S., Ono T., Mukai H., Usui N. (2014) North Pacific dissolved inorganic carbon variations related to the Pacific Decadal Oscillation. Geophys. Res. Lett., 41, 1005-1011
Nakaoka S., Telszewski M, Nojiri Y., Yasunaka S., Miyazaki C., Mukai H., Usui N. (2013) Estimating temporal and spatial variation of ocean surface pCO₂ in the North Pacific using a self-organizing map neural network technique. Biogeosciences, 10, 6093-6106
Yasunaka S., Nojiri Y., Nakaoka S., Ono T., Mukai H., Usui N. (2013) Monthly maps of Sea Surface Dissolved Inorganic Carbon in the North Pacific: Basin-wide Distribution and Seasonal Variation. J. Geophys. Res., 118 (8), 3737-3977
Yokouchi Y., Nojiri Y., Toom-Sauntry D., Fraser P., Inuzuka Y., Tanimoto H., Nara H., Murakami R., and Mukai H. (2012) Long-term variation of atmospheric methyl iodide and its link to global environmental change, Geophys. Res. Lett., 39, L23805, 10.1029/2012GL053695.
T. Aramaki, Y. Nojiri, H. Mukai, S. Kushibashi, M. Uchida, and Y. Shibata Y. (2010) Preliminary results of radiocarbon monitoring in the surface waters of the North Pacific, Nucl. Instrum. Methods Phys. Res. B, 268, 1222-1225, 10.1016/j.nimb.2009.10.138.
T. Takahashi, S. C. Sutherland, R. Wanninkhof, C. Sweeney, R. A. Feely, D. W. Chipman, B. Hales, G. Friederich, F. Chaves, C. Sabine, A. Watson, D. C. E. Bakker, U. Schuster, N. Metzl, H. Yoshikawa-Inoue, M. Ishii, T. Midorikawa, Y. Nojiri, A. Kortzinger, T. Steinhoff, M. Hoppema, J. Olafsson, T. S. Arnarson, B. Tilbrook, T. Johannessen, A. Olsen, R. Bellerby, C. S. Wong, B. Delille, N. R. Bates, and H. J. W. de Baar. (2009) Climatologocal mean and decadal change in surface ocean pCO₂ and net sea-air CO₂ flux over the global oceans, Deep-Sea Res. II, 56, 554-577, 10.1016/j.dsr2.2008.12.009.
Y. Yokouchi, K. Osada, M. Wada, F. Hasebe, M. Agama, R. Murakami, H. Mukai, Y. Nojiri, Y. Inuzuka, D. Toom-Sauntry, and P. Fraser. (2008) Global distribution and seasonal concentration change of methyl iodide in the atmosphere, J. Geophys. Res. Atmospheres, 113, D18, D18311, 10.1029/2008JD009861.
C. S. Wong, D. A. Timothy, C. S. Law, Y. Nojiri, L. X. S.-K. E. Wong, and J. S. Page. (2006) Carbon distribution and fluxes during the SERIES iron fertilization experiment with special reference to the fugacity of carbon dioxide (fCO₂), Deep-Sea Research II, 53, 2053–2074, 10.1016/j.dsr2.2006.05.036.
A. Fransson, M. Chierici, and Y. Nojiri. (2006) Increased net CO₂ outgassing in the upwelling region of the southern Bering Sea in aperiod of variable marine climate between 1995 and 2001, J. Geophys. Res. Oceans, 111, C8, C08008, 10.1029/2004JC002759.
V. V. S. S. Sarma, T. Saino, K. Sasaoka, Y. Nojiri, T. Ono, M. Ishii, H. Y. Inoue, and K. Matsumoto. (2006) Basin-scale pCO₂ distribution using satellite sea surface temperature, Chl a, and climatological salinity in the North Pacific in spring and summer. Global Biogeochem. Cycles, 20, GB3005, 10.1029/2005GB002594.
M. Chierici, A. Fransson, and Y. Nojiri. (2006) Biogeochemical processes as drivers of surface fCO₂ in contrasting provinces in the subarctic North Pacific Ocean, Global Biogeochem. Cycles, 20, GB1009, 10.1029/2004GB002356.
Y. Yokouchi, F. Hasebe, M. Fujiwara, H. Takashima, M. Shiotani, N. Nishi, Y. Kanaya, S. Hashimoto, P. Fraser, D. Toom-Sauntry, H. Mukai, and Y. Nojiri. (2005) Correlations and emission ratios among bromoform, dibromochloromethane, and dibromomethane in the atmosphere, J. Geophys. Res. Atmospheres, 110, D23, D23309, 10.1029/2005JD006303.
Y. Tohjima, H. Mukai, T. Machida, Y. Nojiri, and M. Gloor. (2005) First measurements of the latitudinal atmospheric O2 and CO₂ distributions across the western Pacific, Geophys. Res. Lett., 32, L17805, 10.1029/2005GL023311.
K. Watanabe, Y. Nojiri, and S. Kariya. (2005) Measurements of ozone concentrations on a commercial vessel in the marine boundary layer over the northern North Pacific Ocean, J. Geophys. Res. Atmospheres, 110, D11, D11310, 10.1029/2004JD005514.
H. Kitagawa, H. Mukai H, Y. Nojiri, Y. Shibata, T. Kobayashi T and T. Nojiri. (2004) Seasonal and secular variations of atmospheric CO₂-C-14 over the western Pacific since 1994, Radiocarbon, 46, 901-910.
C. S. Wong, N. A. D. Waser, Y. Nojiri, W. K. Johnson, F. A. Whitney, J. S. Page and J. Zeng. (2002) Seasonal cycles of nutrients and dissolved inorganic carbon at high and mid latitudes in the North Pacific Ocean: determination of new production and nutrient uptake ratios, J. Oceanogr., 58, 227-243.
T. Takahashi, S. C. Sutherland, C. Sweeney, A. Poisson, N. Metzl, B. Tilbrook, N. Bates, R. Wanninkhof, R. A. Feely, C. Sabine, J. Olafsson, and Y. Nojiri. (2002) Global sea-air CO₂ flux based on climatological surface ocean pCO₂, and seasonal biological and temperature effects, Deep-Sea Res. II, 49, 1601-1622.
J. Zeng, Y. Nojiri, P. P. Murphy, C. S. Wong, and Y. Fujinuma. (2002) A comparison of ΔpCO₂ distributions in the northern North Pacific using results from a commercial vessel in 1995-1999, Deep Sea Res. II, 5303-5315.
C. S. Wong, N. A. D. Waser, Y. Nojiri, F. A. Whitney, J. S. Page, and J. Zeng. (2002) Seasonal cycles of nutrients and dissolved inorganic carbon at high and mid latitudes in the North Pacific Ocean during the Skaugran cruises: determination of new production and nutrient uptake ratios, Deep Sea Res. II, 49, 5317-5338.
M. Mochida, Y. Kitamori, K. Kawamura, Y. Nojiri, and Suzuki K. (2002) Fatty acids in the marine atmosphere: Factors governing their concentrations and evaluation of organic films on sea-salt particles, J. Geophys. Res. Atmospheres, 107, D17, D4325, 10.1029/2001JD001278.
Y. Obayashi, E. Tanoue, K. Suzuki, N. Handa, Y. Nojiri and C. S. Wong. (2001) Spatial and temporal variabilities of phytoplankton community structure in the northern North Pacific as determined by phytoplankton pigments, Deep-Sea Res. I, 48, 439-469.
P. P. Murphy, Y. Nojiri, D. E. Harrison and N. K. Larkin. (2001) Scales of spatial variability for surface ocean pCO₂ in the Gulf of Alaska and Bering Sea: toward a sampling strategy, Geophys. Res. Lett., 28, 1047-1050.
P. P. Murphy, Y. Nojiri, Y. Fujinuma, C. S. Wong, J. Zeng, T. Kimoto and H. Kimoto. (2001) Measurements of surface seawater fCO₂ from volunteer commercial ships: Techniques and experiences from Skaugran, J. Atmos. Ocean. Technol., 18, 1719-1734.
Y. Yokouchi, Y. Nojiri, L. A. Barrie, D. Toom-Sauntry and Y. Fujinuma. (2001) Atmospheric methyl iodide: High correlation with surface seawater temperature and its implications on the sea-to-air flux, J. Geophys. Res., 106, 12661-12668.
Y.Yokouchi, Y.Nojiri, L.A.Barrie, D.Toom-Sauntry, T.Machida, Y.Inuzuka, H.Akimoto, H.-J.Li, Y.Fujinuma and S.Aoki. (2000) A strong source of methyl chloride to the atmosphere from tropical coastal land, Nature, 403, 295-298.
Y.Yokouchi, T.Machida, L.A.Barrie, D.Toom-Sauntry, Y.Nojiri, Y.Fujinuma, Y.Inuzuka, H.-J.Li, H.Akimoto and S.Aoki. (2000) Latitudinal distribution of atmospheric methyl bromide measurements and modeling, Geophys. Res. Lett., 27, 697-700.
M.Uematsu, K.Kinoshita, and Y.Nojiri. (2000) Scavenging of insoluble particles from the marine atmosphere over the sub-arctic North Pacific, J. Atmos. Chem., 35, 151-164.

本モニタリング事業への協力機関

National Oceanic and Atmospheric Administration (NOAA, U.S.A.)-Pacific Marine Environmental Laboratory (NOAA/PMEL) and Atlantic Oceanographic and Meteorological Laboratory (NOAA/AOML)
Commonwealth Scientific and Industrial Research Organization (Australia)
Institute of Ocean Sciences (Canada)
National Institute of Water and Atmospheric Research (New Zealand)

終わりに

本モニタリング事業は、船主会社、運航会社、協力船舶の船長はじめ乗員の皆様、港湾代理会社などの多くの方々の協力により実現できるものであり、関係諸氏に感謝の意を表します。