Report on ocean acidification monitoring in the Western Indian Ocean

Carbon dioxide (CO2) emissions from human activities are largely absorbed by the ocean, accounting for about one-third of the total emissions over the past 200 years from the combustion of fossil fuels, the production of cement, and changes in land use (Sabine et al., 2004). The uptake of CO2 by the ocean benefits society by moderating the rate of climate change but also causes unprecedented changes to ocean chemistry, decreasing the pH of the water and leading to a suite of chemical changes collectively known as ocean acidification. Like climate change, ocean acidification is a growing global problem that will intensify with continued CO2 emissions and has the potential to change marine ecosystems and affect benefits to society.

The chemistry of the ocean is changing at an unprecedented rate and magnitude due to anthropogenic carbon dioxide emissions; the rate of change exceeds that which has occurred for at least the past hundreds of thousands of years. Unless anthropogenic CO2 emissions are substantially curbed, or atmospheric CO2 is controlled by some other means, the average pH of the ocean will continue to fall. Ocean acidification has demonstrated impacts on many marine organisms. While the ultimate consequences are still unknown, there is a risk of ecosystem change that may threaten coral reefs, fisheries, protected species, and other natural resources of value to society.

Since the start of the Industrial Revolution, the average pH of ocean surface waters has decreased by about 0.1 units, from about 8.2 to 8.1. Model predictions show an additional 0.2–0.3 drop in pH by the end of the century, even under optimistic scenarios. Perhaps more important is that the rate of this change exceeds any known change in ocean chemistry for at least 800,000 years. The major changes in ocean chemistry caused by increasing atmospheric CO2 are well understood and can be precisely calculated, despite some uncertainty resulting from biological feedback processes.

However, the direct biological effects of ocean acidification are less certain and will vary among organisms, with some coping well and others not at all. The long-term consequences of ocean acidification for marine biota are unknown, but changes in many ecosystems and the services they provide to society appear likely based on current understanding.

In response to these concerns, WIOMSA launched ocean acidification projects in six countries: Kenya, Mauritius, Mozambique, Seychelles, South Africa and Tanzania, with the support of the Swedish International Development Cooperation Agency and institutional partners in the WIO region. The research provides a baseline that will foster the development of an integrated science strategy for ocean acidification monitoring, research and impact assessment. It presents a review of the current state of knowledge on ocean acidification in the WIO region and identifies the gaps in information required to improve understanding and address the consequences of ocean acidification.

The report consists of seven chapters. Chapter 1 introduces the subject of ocean acidification and chapters 2 to 7 summarize the esults of ocean acidification monitoring in the six countries that participated in the four-year monitoring project. Lessons learned and recommendations are presented for each country.

WIOMSA, 2022. Report on ocean acidification monitoring in the Western Indian Ocean. WIOMSA. Report.

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