Indian Ocean Acidification and Its Driving Mechanisms over the Last Four Decades (1980–2019)

The oceans play a significant role in regulating the amount of carbon dioxide (CO₂) in the atmosphere. The increasing oceanic uptake of CO₂ counterbalances the increase in atmospheric CO₂. This uptake considerably impacts marine biogeochemistry, leading to pH and alkalinity imbalances in the water column, commonly referred to as ocean acidification. In an acidic ocean, excess CO₂ reacts with seawater to form carbonic acid, which is highly unstable and undergoes further reduction by releasing hydrogen ions (H⁺) and acidifying the seawater (reduces the pH). Several studies have projected a decline of upper ocean pH by 0.3-0.4 by the end of the 21st century, which has the potential to reduce oceanic biological production considerably. There is a critical need to understand the present status of Indian Ocean acidification and identify its key drivers. However, the number of spatially and temporally varying available observations to examine the present state of Indian Ocean acidification is limited. The numerical ocean models have a unique ability to integrate our empirical and theoretical understanding of the marine environment. Therefore, a comprehensive assessment of the present state of Indian Ocean acidification and its driving mechanisms has been carried out using a regional high-resolution model simulated outputs from 1980-2019. The major findings of this study are (i) the Indian Ocean has been acidifying at an average rate of 0.015 dec^-1 from 1980-2019, (ii) in the recent decade (2010-2019), the rate of surface acidification has been accelerated throughout the Indian Ocean compared to the previous decades, (iii) the increasing anthropogenic CO₂ uptake by the ocean primarily drives an increasing Indian Ocean acidification trend and (iv) the climatic events such as El Ninõ and positive Indian Ocean Dipole (IOD) lead to an enhancement of the Indian Ocean acidification. In summary, this research work consolidates the current level of understanding about the Indian Ocean acidification based on the available field observations, reconstructed data sets, and model simulations.

Reference: Chakraborty, K., Joshi, A.P., Ghoshal, P.K., et al. (2024). Indian Ocean acidification and its driving mechanisms over the last four decades (1980–2019). Global Biogeochem. Cycles, 38, e2024GB008139. https://doi.org/10.1029/2024GB008139