[A-CG35] グローバル炭素循環の観測と解析

[ACG35-P01] Global carbon budgets for a recent two decadal period based on atmospheric O₂ and CO₂ observations in the western Pacific region

*遠嶋 康徳¹、笹川 基樹¹、中岡 慎一郎¹、寺尾 有希夫¹、坪井 一寛²、向井 人史¹、町田 敏暢¹ (1.国立環境研究所、2.気象研究所)

キーワード：全球炭素収支、大気中酸素、大気中二酸化炭素

We present global ocean and land carbon sequestration rates for a recent two decadal period based on measurements of the atmospheric O₂/N₂ ratio and CO₂ mole fractions of air samples collected in the western Pacific region. The air samples have been collected into glass flasks at Hateruma Island (HAT; lat. 24.1°N, long. 123.8°E) since July 1997, at Cape Ochi-ishi (COI; lat. 43.2°N, long. 145.5°E) since December 1999, at Minamitorishima (MNM; lat. 24.3°N, long. 154.0°E) since September 2011, and onboard cargo ships on regular roundtrip cruises to North America and to New Zealand/Australia since December 2001. Combining the O₂/N₂ ratio and CO₂ mole fraction, we computed a value of atmospheric potential oxygen (APO=O₂+β×CO₂) for each sample, where β represents the −O₂/CO₂ exchange ratios for the land biosphere. The partitioning of the global ocean and land carbon sinks were evaluated by solving simultaneous budget equations of the APO and the atmospheric CO₂. In accordance with the previous study (Tohjima et al., 2019), we used the global CO₂ data derived from the NOAA/ESRL observation network and net ocean O₂ outgassing flux estimated from the change in the ocean heat content (0-2000m). Note that we corrected O₂/N₂ data of this study for the difference in the span sensitivity from the gravimetric scale (Aoki et al., 2021) and used a value of 1.05 for β.
The resulting average global ocean and land carbon sequestration rates for the 22-year period from 2000 to 2021 were 2.9 ± 0.7 PgC yr⁻¹ and 1.4 ± 1.0 PgC yr⁻¹, respectively. To examine the temporal trend in the sequestration rates, we computed those for an interval of 5-year period, which effectively reduced apparent variations in the sequestration rates derived probably from the imbalances between seasonal ocean O₂ outgassing and ingassing fluxes. The consecutive 5-year ocean sequestration rates showed an increasing trend at a rate of 0.08 ± 0.01 PgC yr⁻² during 2001-2019. In contrast, the relatively large land sequestration rate of 1.9 ± 0.3 PgC yr⁻¹ during 2004-2013 decreased to that of 1.3 ± 0.3 PgC yr⁻¹ during 2014-2019.