FY 2025

CO2 uptake in the Pacific from 1985 to 2018: a comparative assessment of observation- and model-based estimates

Ishii, M., B.R. Carter, K. Toyama, K.B. Rodgers, R.A. Feely, T.-T.-T. Chau, F. Chevallier, F. Desmet, L. Gregor, Y. Iida, Y. Kitamura, J.D. Müller, and H. Tsujino

Global Biogeochem. Cycles, 39(5), e2024GB008355, doi: 10.1029/2024GB008355,

As a contribution to the second REgional Carbon Cycle Assessment and Processes effort, we compare net and anthropogenic sea-air CO₂ fluxes, CO₂ accumulation rates in the ocean interior and their trends in the Pacific Ocean by analyzing results from state-of-the-art observation-based estimates and global ocean biogeochemistry models (GOBMs) over the period 1985–2018. The ensemble-mean net CO₂ fluxes integrated over the Pacific (44°S–62°N) are −0.41 ± 0.12 PgC yr⁻¹ from pCO₂ products and −0.51 ± 0.16 PgC yr⁻¹ from GOBMs. The anthropogenic CO₂ flux from GOBMs (−0.71 ± 0.10 PgC yr⁻¹) is 1.4 times as large as the net CO₂ flux, with particularly large anthropogenic uptake in the equatorial region (−0.34 ± 0.03 PgC yr⁻¹) significantly offsetting the large natural CO₂ outgassing there (+0.72 ± 0.06 PgC yr⁻¹). The basin-wide net CO₂ uptake has increased at similar mean rates of −0.09 ± 0.06 and −0.08 ± 0.02 PgC yr⁻¹ decade⁻¹ in pCO₂ products and GOBMs, respectively, comparable with the increase in anthropogenic CO₂ uptake of −0.10 ± 0.01 PgC yr⁻¹ decade⁻¹ in GOBMs. However, a notable mismatch in the trend of the net CO₂ flux change that exists between pCO₂ products (+0.00 ± 0.02 PgC yr⁻¹ decade⁻¹) and GOBMs (−0.04 ± 0.01 PgC yr⁻¹ decade⁻¹) in the equatorial region is yet to be resolved. The rate of anthropogenic CO₂ accumulation from GOBMs is +0.76 ± 0.17 PgC yr⁻¹. This is nearly balanced with the anthropogenic CO₂ flux and is also encompassed by the previous observation-based estimates.

Plain Language Summary. A better understanding of carbon dioxide (CO₂) uptake is critical for understanding the role of the ocean in modulating the CO₂ rise in the atmosphere. In this study, we assess the uptake of CO₂ by the Pacific Ocean, how this uptake changes with the rise of atmospheric CO₂ and climate change, and how much CO₂ is accumulating in its interior, focusing on the period 1985–2018. For this purpose, we analyze the output of numerical models that simulate the ocean carbon sink, and compare them to the estimates derived from ocean CO₂ observations. The Pacific Ocean's uptake of excess CO₂ resulting directly from human CO₂ emissions is quantified to be approximately 0.7 peta-grams of carbon (PgC) per year. This CO₂ uptake increased over time on average by 0.1 PgC per year per decade. CO₂ accumulation rates within the interior of the Pacific are shown to be nearly balanced with the cumulative surface uptake rates. However, regional discrepancies in these quantities among model simulations and observation-based products are also found, in particular in the rate of change in the CO₂ uptake in the equatorial region and the rate of CO₂ accumulation in the interior of the South Pacific.