Iolanda reef in Ras Muhammad nature park (Sinai, Egypt) is home to reef fish (these may be Pseudanthias, and were photographed 7 February 2006 by Mikhail Rogov).

Societies depend on the ocean for various ecosystem services:

• – provisioning services, such as food;
• – regulating services, such as carbon absorption from the atmosphere;
• – cultural services, such as recreation; and
• – supporting services, such as nutrient cycling.

While much is known about the effects of ocean acidification on individual organisms, the potential responses of whole ecosystems are largely unknown. Thus, although deleterious consequences are expected for shellfish and warm water corals (high confidence) and fisheries (low confidence), it is difficult to quantify how the ecosystems and fisheries will change and how societies will adapt to and manage the changes.

Confidence levels

Confidence levels are expressed in this document with the qualifiers “low”,
“medium”, “high” and “very high”. These qualifiers synthesise the authors’
judgments about the validity of findings as determined through evaluation
of evidence and agreement. The analysis builds on statements of confidence
derived from peer-reviewed synthesis such as the European Project on
Ocean Acidification synthesis book²⁸ and the Intergovernmental Panel on
Climate Change (IPCC) Fifth Assessment Report. The most recent metaanalyses,
of 228 ocean acidification studies on marine organisms²⁹ and 167
studies on marine animals¹⁴, provided further evidence to aid the authors
in analysing and summarising the outcomes of the experimental evidence.
Increasing levels of evidence and degrees of agreement are correlated with
increasing confidence (see figure), as outlined in the IPCC’s guidance note on
the treatment of uncertainties³⁰ in the Fifth Assessment Report.

• VERY HIGH CONFIDENCE
• HIGH CONFIDENCE
• MEDIUM CONFIDENCE
• LOW CONFIDENCE

The capacity of the ocean to act as a carbon sink
decreases as it acidifies [VERY HIGH CONFIDENCE]
The ocean provides a vast sink for anthropogenic CO₂ emissions. Around
one quarter of annual CO₂ emissions from human activities currently end
up in the ocean¹⁸. This service cannot be relied on in the future. Atmospheric
CO₂ is rising faster than the ocean can respond. The capacity of the ocean to
absorb CO₂ decreases as ocean pH decreases; that is, the buffering capacity
of seawater decreases¹⁹. This reduced capacity is a concern for stabilising
CO₂ emissions and implies that larger emissions cuts will be needed to meet
targets to mitigate climate change.

Declines in shellfisheries will lead to economic
losses [MEDIUM CONFIDENCE], but the extent of the losses
is uncertain
By 2100, estimated global annual economic losses due to declines in mollusc
production from ocean acidification could be more than $130 billion (US dollars,
at 2010 price levels) for a business-as-usual CO₂ emissions trend, according
to one estimate²⁰ [LOW CONFIDENCE]. For the United States, a 13% reduction
in revenue is estimated by 2060 from declines in mollusc harvests due to
acidification²¹ [LOW CONFIDENCE]. Economically important shellfish species may
respond in different ways to ocean acidification (see table on p. 9 of the full report), but we do not
know enough to make quantitative economic predictions for all fisheries yet.
Molluscs appear to be one of the most sensitive groups of organisms studied
under ocean acidification regimes. Indeed, oyster larvae in hatcheries in the
northeast Pacific Ocean region are very sensitive to ocean acidification and are
already affected by low pH waters^22,23.

Negative socio-economic impacts of coral reef
degradation are expected [MEDIUM CONFIDENCE],
but the size of the costs is uncertain
Substantial economic losses are likely to occur due to the loss of tropical coral
reef extent from ocean acidification (by 2100, the scarcity of corals will push
the value of their losses over $1 trillion per year, in US dollars at 2010 price
levels, according to one estimate²⁴) [LOW CONFIDENCE]. A large proportion
of these losses will occur in vulnerable societies and small island states
that economically rely on coral reefs. Coral reef losses will negatively affect
tourism, food security, shoreline protection and biodiversity. But ocean
acidification is not the only stressor. Reefs are already under pressure from
warmer temperatures (which cause coral bleaching), habitat destruction,
overfishing, sedimentation and pollution.
Actions that slow the rate of ocean acidification will reduce impacts and
maximise the potential for coral reefs to recover, and even adapt, to other
stressors. Thus, additional human stressors, such as destructive fishing
practices, pollution and sedimentation, will not only have immediate
ecological effects, they will also reduce the potential of coral reefs to adapt to
warmer, more acidified conditions.
In addition to global climate policy, local reef management strategies
– implemented using tools such as Marine Protected Areas, fisheries
management, and marine spatial planning – also increase the potential of
coral reefs to cope with ocean acidification^4,25.

Impacts of ocean acidification on ecosystems may
affect top predators and fisheries [LOW CONFIDENCE]
It is uncertain how changes in phytoplankton and zooplankton abundance
and distribution will propagate through marine ecosystems to affect fish
and fisheries, on which many societies depend. Also, very little is known
about the direct effects of ocean acidification on fish that are the target of
commercial and subsistence fishing, which results in high uncertainties in
predicting changes in fisheries in the future. However, this area is key for
research, as fisheries support the livelihoods of about 540 million people, or
8% of the world’s population²⁶.

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