Ocean Acidification

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Ocean acidification
What is already happening?
  • The global ocean absorbs approximately a quarter of anthropogenic carbon dioxide (CO2) emissions annually; atmospheric CO2 now exceeds 400 ppm and has continued to increase by approximately 2.3 ppm per year over the last decade. 
  • The North Atlantic contains more anthropogenic CO2 than any other ocean basin, and ocean surface measurements between 1995 and 2013 reveal a pH decline (increasing acidity) of 0.0013 units per year there. 
CONFIDENCE LEVEL
HIGH

High evidence, high agreement

The fundamental chemistry is very well established, and seasonal cycles and spatial patterns well studied. However, there are still significant knowledge gaps for shelf sea carbonate chemistry, including many of the factors affecting local and short-term variability.

What could happen in the future?
  • High-emission scenario models project that the average continental shelf pH could drop by up to 0.366 by 2100. Spatial variability in the rate of pH decline is projected with coastal areas declining faster. 
     
  • Under high-emission scenarios, it is projected that bottom waters will become corrosive to more-soluble forms of calcium carbonate (aragonite). Episodic undersaturation events are projected to begin by 2030. 
     
  • By 2100, up to 20% of the North-west European shelf seas may experience undersaturation for at least one month of each year.
     
CONFIDENCE LEVEL
MEDIUM

Medium evidence, medium agreement

There is very high confidence that global mean seawater pH and saturation states of carbonate minerals will decrease in response to increasing atmospheric CO2. However, specific details of regional decadal trends and changes in interannual and seasonal variability are much less certain.

Key Challenges and Emerging Issues
  • Sustaining time–series observations of the marine carbonate system at key point sites and transects and improving high resolution monitoring of the near-coastal marine environment. 
     
  • Developing accurate and stable autonomous observing technologies for pH and related variables, deploying them in difficult-to-sample regions, linking and analysing their measurements effectively with other data streams. 
     
  • Improving the spatial and temporal resolution of models, along with their descriptions of biogeochemical processes, to capture the relatively small-scale controls on the marine carbonate system in complex coastal and shelf sea environments.