Global trends of ocean CO 2 sink and ocean acidification: an observation-based reconstruction of surface ocean inorganic
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ORIGINAL ARTICLE
Global trends of ocean CO2 sink and ocean acidification: an observation‑based reconstruction of surface ocean inorganic carbon variables Yosuke Iida1,2 · Yusuke Takatani3 · Atsushi Kojima1 · Masao Ishii1,2 Received: 26 December 2019 / Revised: 8 October 2020 / Accepted: 12 October 2020 © The Oceanographic Society of Japan and Springer Nature Singapore Pte Ltd. 2020
Abstract Ocean acidification is likely to impact marine ecosystems and human societies adversely and is a carbon cycle issue of great concern. Projecting the degree of ocean acidification and the carbon-climate feedback will require understanding the current status, variability, and trends of ocean inorganic carbon system variables and the ocean carbon sink. With this goal in mind, we reconstructed total alkalinity (TA), dissolved inorganic carbon (DIC), C O2 partial pressure (pCO2sea), sea–air CO2 flux, pH, and aragonite saturation state (Ωarg) for the global ocean based on measurements of pCO2sea and TA. We used a multiple linear regression approach to derive relationships to explain TA and DIC and obtained monthly 1° × 1° gridded values of TA and DIC for the period 1993–2018. These data were converted to pCO2sea, pH, and Ωarg, and monthly sea-air CO2 fluxes were obtained in combination with atmospheric CO2. Mean annual sea–air C O2 flux and its rate of change were estimated to be − 2.0 ± 0.5 PgC year−1 and − 0.3 (PgC year−1) decade−1, respectively. Our analysis revealed that oceanic CO2 uptake decreased during the 1990s and has been increasing since 2000. Our estimate of the globally averaged rate of pH change, − 0.0181 ± 0.0001 decade−1, was consistent with that expected from the trend of atmospheric C O2 growth. However, rates of decline of pH were relatively slow in the Southern Ocean (− 0.0165 ± 0.0001·decade−1) and in the western equatorial Pacific (− 0.0148 ± 0.0002·decade−1). Our estimate of the globally averaged rate of pH change can be used to verify Indicator 14.3.1 of Sustainable Development Goals. Keywords Ocean acidification · Ocean carbon sink · SOCAT · SOCOM · SDGs
1 Introduction The ocean has stored approximately 30% of the total C O2 released by human industrial activities during the industrial era (Khatiwala et al. 2013) and has therefore played a significant role in slowing the growth of C O2 concentrations in the atmosphere and mitigating global warming. However, this uptake of C O2 has endangered the health of the * Yosuke Iida iida‑[email protected] 1
Global Environment and Marine Department, Japan Meteorological Agency, 1‑3‑4 Otemachi Chiyoda‑ku, Tokyo 100‑8122, Japan
2
Climate and Geochemistry Research Department, Meteorological Research Institute, 1‑1 Nagamine, Tsukuba, Ibaraki 305‑0052, Japan
3
Forecast Department, Japan Meteorological Agency, 1‑3‑4 Otemachi Chiyoda‑ku, Tokyo 100‑8122, Japan
ocean through what is referred to as ocean acidification. The absorption of anthropogenic CO2 into seawater increases its acidity (reduces its pH), reduces the saturation lev
