[A-CG35] グローバル炭素循環の観測と解析

[ACG35-09] 太平洋のCO₂吸収トレンド

*石井 雅男¹、遠山 勝也¹、飯田 洋介²、北村 佳照¹、辻野 博之¹ (1.気象研究所、2.気象庁)

キーワード：太平洋、全球炭素循環、海洋炭素吸収

Ocean is thought to be absorbing approximately a quarter of CO₂ released by human industrial activities such as fossil fuel combustion and land-use change and is mitigating the growth of CO₂ concentration in the atmosphere. The rate of net CO₂ uptake by the ocean is supposed to increase with the growth of CO₂ concentration in the atmosphere while being perturbed by the variability in ocean climate, CO₂ storage in the ocean by itself due to the reduction of CO₂ buffering capacity of seawater and the change in the ventilation of the ocean interior, i.e., the transport of CO₂ from surface layer into the ocean interior and vice versa.
Here we present the trend of CO₂ uptake by the ocean focusing on the Pacific to the north of 45°S. We use the outputs of air-to-sea CO₂ flux from 8 pCO₂ observation-based data products and those from 14 global ocean biogeochemistry models (GOBMs) that have been shared among the participants of the Global Carbon Project's Regional Carbon Cycle Assessment and Processes Phase 2 (RECCAP2) Ocean. The outputs of GOBMs include those from the run with contemporary physical forcings and atmospheric CO₂ increase (run A) and that with contemporary physical forcings and preindustrial atmospheric CO₂ level (run D). Therefore “run A – run D” represents the increase of ocean CO₂ uptake due to CO₂ increase in the atmosphere.
The sea-to-air CO₂ flux in the Pacific is characterized by a strong CO₂ release to the atmosphere in the central and eastern tropics and strong CO₂ uptake in the western subtropical-to-sub(ant)arctic transition zones. Mean contemporary uptake of CO₂ by the Pacific for the period 1985-2018 was 0.39 ±0.15 PgC yr^-1 for data products and 0.45 ±0.16 PgC yr^-1 for GOBMs. The difference might be more or less ascribed to the natural air-to-sea CO₂ flux associated with land-to-ocean carbon transport being evaluated to be 0.14 PgC yr^-1 for the Pacific that has not been necessarily taken into account in GOBMs. However, 0.07 PgC yr^-1 smaller CO₂ release from GOBMs than data products in the tropics appears to be another major source of discrepancy between these products.
Ocean CO₂ uptake in the Pacific showed a trend of increase for 1985-2018 at rates of 0.08±0.06 PgC yr^-1 decade^-1 for data products and 0.07±0.02 PgC yr^-1 decade^-1 for GOBMs. The rates were faster for 2001-2018 at all zones from subarctic to southern subtropics and 0.17 ±0.04 PgC yr^-1 decade^-1 for both data products and GOBMs in the whole Pacific.
The anthropogenic CO₂ uptake as evaluated by “run A – run D” of GOBMs is everywhere positive and amounts to 0.70 ±0.09 PgC yr^-1 in the Pacific to the north of 45°S for the period 1985-2018. Namely, ocean CO₂ sink in the extra-tropics is strengthened and ocean CO₂ source in the tropics is reduced with the increase of atmospheric CO₂ concentration. The anthropogenic CO₂ sink is stronger in the flanks of tropics and in the subtropical-to-subarctic western North Pacific off of Japan. In the tropics, the combination of higher rate of delta-pCO₂ (= pCO₂^air – pCO₂^sea) increase and meridional gradient of gas exchange coefficient result in the stronger anthropogenic CO₂ sink in the flanks of the tropics. The slower pCO₂^sea increase than pCO₂^air is consistent with the trends of inorganic carbon increase determined by its measurements in the Equatorial Undercurrent over the past decades. On the other hand, a larger anthropogenic CO₂ sink in the subtropical-to-subarctic western North Pacific is attributable to the moderately higher rate of delta-pCO₂ increase due to the increase in the winter CO₂ uptake by the increasing amplitude of pCO₂ seasonal variability and a larger gas exchange coefficient in this zone.