Oceans acidify at unprecedented rate

Shellfish are particularly sensitive to acidification. Photo: Flickr.com/Marine Stewardship Council/CC BY-NC-ND

The rate of ocean acidification is the highest in 300 million years. The predicted effects on food webs and biodiversity will impact aquaculture and societies around the world.

Atmospheric carbon dioxide (CO₂) levels are rising as a result of human activities, such as fossil fuel burning, and are increasing the acidity of seawater. This process is known as ocean acidification. Historically, the ocean has absorbed approximately 30 per cent of all CO₂ released into the atmosphere by humans since the start of the industrial revolution, resulting in a 26 per cent increase in the acidity of the ocean.

The current rate of acidification is over ten times faster than at any time in the last 55 million years. The projected increase in ocean acidity by 2100 compared with preindustrial levels if high CO2 emissions continue is about 170 per cent. This was concluded in a statement by 540 experts from 37 countries during a symposium on the “Ocean in a High-CO₂ World” in Monterey, California in 2012. The scientists also stated the following selected conclusions from 15 years of research on ocean acidification problems:

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

Reinhold Pape

This article is based on the following text: IGBP report, Ocean Acidification Summary for Policymakers 2013,