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Ocean Acidification

What is already happening?
  • Atmospheric CO2 exceeded 414 ppm in 2021 and has continued to increase by approximately 2.4 ppm per year over the last decade. The global ocean absorbs approximately a quarter of anthropogenic carbon dioxide (CO2) emissions annually. 
  • The North Atlantic Ocean contains more anthropogenic CO2 than any other ocean basin, and surface waters are experiencing an ongoing decline in pH (increasing acidity). In some locations, rates of acidification in bottom waters are occurring at a faster rate than surface waters. 
  • Some species are already showing effects from ocean acidification when exposed to short term fluctuations and could be used as indicator species for long-term impacts on marine ecosystems.  

High evidence, high consensus

The overall confidence is stated as high, but it should be noted this relates to the fundamental chemistry underlying air–sea CO2 exchange and the ocean pH decline from anthropogenic CO2 uptake, which are both well established. In open oceans, the change in chemistry is observable, having been investigated by a multitude of independent techniques (observations, modelling and reanalysis assessments) with concordant results (medium evidence, high agreement). There are still significant knowledge gaps for shelf sea carbonate chemistry, including many of the factors affecting local and short-term variability relevant to ecosystem services.

What could happen in the future?
  • Models project that the average continental shelf seawater pH will continue to decline to year 2050 at similar rates to present day, with rates then increasing in the second half of the century, depending on emissions scenario. 

  • The rate of pH decline in coastal areas is projected to be faster in some areas (e.g. Bristol Channel) than others (e.g. Celtic Seas). 

  • Under high-emission scenarios, it is projected that bottom waters on the north-west European shelf seas will become corrosive to more-soluble forms of calcium carbonate (aragonite). Episodic undersaturation events are projected to begin by 2030.  

  • By 2100, up to 90% of the north-west European shelf seas may experience undersaturation for at least one month of each year. 

  • High levels of nearshore variability in carbonate chemistry may mean that some coastal species have a higher adaptative capacity than others. However, all species will be at an increased risk from extreme exposure episodes. 


Medium (medium evidence, medium agreement)

As above, it is not easy to give a single confidence value to ‘what could happen in the future’ and there are additional inherent uncertainties relating to societal behaviour and emission pathways. There is very high confidence that global mean seawater pH and saturation states of carbonate minerals will continue to decrease in response to increasing atmospheric CO2. However, specific details of regionally resolved decadal trends and changes in interannual and seasonal variability are less certain, due to the complex interplay of multiple pressures. The high importance of relatively small-scale processes in near-coastal environments adds uncertainty to model results for these regions, although this is improving. These uncertainties, including which future scenario might prevail, and the lack of regional assessment under the lower RCP2.6 scenario, means the overall confidence level is at ‘medium’ for understanding of the future of ocean acidification and its impacts in UK Shelf seas.

Key Challenges and Emerging Issues

1.    A lack of data hinders our understanding of near-shore dynamics and trends as well as being able to relate physical and chemical changes to biological change. To aid this there is a need to:  

  • Increase near-shore and shallow coastal environment monitoring activities. 
  • Sustain long-term time–series observations of the marine carbonate system to ascertain the scale and magnitude of changes.
  • Increase coupling of physicochemical and biological monitoring to support management. 
  • Develop of accurate and stable autonomous observing technologies for pH and related variables, deploying in difficult-to-sample regions, and comparing with other data streams.  

2.    Conducting ocean acidification studies in relation to other stressors (e.g. temperature, oxygen, metals, etc.). This requires: 

  • Local ecosystem and societally relevant experimental work together with an improved ability to assess changes in multiple parameters in the field. 
  • Improved spatial and temporal model resolution, with better descriptions of biogeochemical processes, to capture small-scale controls on the marine carbonate system in complex systems. 

3.    Capacity building to make data available, and compatible, for large scale assessments of trends and impacts. This includes: 

  • Co-ordinating intercomparison and intra-calibration activities and development of quality control schemes to enhance monitoring across UK laboratories. 
  • Continued support for a centralised Data Hub, with shared standards for  vocabulary and metadata, enabling data harvesting and sharing between data archiving centres and data product generators (e.g. IOC, SOCAT, GLODAP).     

Image credit: Helen Findlay