The pH of the oceans is changing. From the cover of the Ocean Acidification Summary for Policy Makers, 2013.

Ocean acidification causes ecosystems and marine biodiversity to change. It has the potential to affect food security and it limits the capacity of the ocean to absorb CO₂ from human emissions. The economic impact of ocean acidification could be substantial. Reducing CO₂ emissions is the only way to minimise long-term, large-scale risks.

Summary of outcomes from the latest Ocean Acidification Summary for Policymakers

During the last 20 years, it has been established that the pH of the world’s oceans is decreasing as a result of anthropogenic CO₂ emissions to the atmosphere. The Third Symposium on the Ocean in a High-CO₂ World built on this knowledge.

•   The ocean continues to acidify at an unprecedented rate in Earth’s history. Latest research indicates the rate of change may be faster than at any time in the last 300 million years.
• As ocean acidity increases, its capacity to absorb CO₂ from the atmosphere decreases. This decreases the ocean’s role in moderating climate change.

• Species-specific impacts of ocean acidification have been seen in laboratory and field studies on organisms from the poles to the tropics. Many organisms show adverse effects, such as reduced ability to form and maintain shells and skeletons, as well as reduced survival, growth, abundance and larval development. Conversely, evidence indicates that some organisms tolerate ocean acidification and that others, such as some seagrasses, may even thrive.
• Within decades, large parts of the polar oceans will become corrosive to the unprotected shells of calcareous marine organisms.
• Changes in carbonate chemistry of the tropical ocean may hamper or prevent coral reef growth within decades.
• The far-reaching effects of ocean acidification are predicted to impact food webs, biodiversity, aquaculture and hence societies.
• Species differ in their potential to adapt to new environments. Ocean chemistry may be changing too rapidly for many species or populations to adapt through evolution.
• Multiple stressors – ocean acidification, warming, decreases in oceanic oxygen concentrations (deoxygenation), increasing UV-B irradiance due to stratospheric ozone depletion, overfishing, pollution and eutrophication – and their interactions are creating significant challenges for ocean ecosystems.
•   We do not fully understand the biogeochemical feedbacks to the climate system that may arise from ocean acidification.
•   Predicting how whole ecosystems will change in response to rising CO₂ levels remains challenging. While we know enough to expect changes in marine ecosystems and biodiversity within our lifetimes, we are unable to make reliable, quantitative predictions of socio-economic impacts.
•   People who rely on the ocean’s ecosystem services are especially vulnerable and may need to adapt or cope with ocean acidification impacts within decades. Shellfish fisheries and aquaculture in some areas may be able to cope by adjusting their management practices to avoid ocean acidification impacts. Tropical coral reef loss will affect tourism, food security and shoreline protection for many of the world’s poorest people.

[This text is from the Ocean Acidification Summary for Policy Makers, 2013, and is available online as a PDF with full references.]