The ocean is acidifying rapidly and at an unprecedented rate in Earth’s history

Modelled global sea-surface pH from 1870 to 2100. The blue line reflects estimated pH change resulting from very low CO₂ emissions to the atmosphere (IPCC Representative Concentration Pathway, RCP* 2.6). The red line reflects pH from high CO₂ emissions (the current emissions trajectory, RCP* 8.5). Credit: Adapted from Bopp et al., 2013 (reference 9 in Ocean Acidification Summary for Policy Makers).

The chemistry of ocean acidification is well understood, and scientists have well-constrained models that can predict changes in the surface ocean chemistry as CO₂ increases in the atmosphere. When CO₂ gas dissolves in seawater, carbonic acid is formed, changing the chemical composition of the ocean: ocean acidification.

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, 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

Ocean acidification is caused by CO₂ emissions from
human activity to the atmosphere that end up in the
ocean [VERY HIGH CONFIDENCE]
The ocean currently absorbs approximately a quarter of the CO₂ added to the
atmosphere from human activities each year¹⁸, greatly reducing the impact of
this greenhouse gas on climate.

Anthropogenic ocean acidification is currently in
progress and is measurable [VERY HIGH CONFIDENCE]
Anthropogenic CO₂ emissions are causing chemical changes in the ocean that
are observable now and are highly predictable at a global scale into the future.
The acidity of surface ocean waters has increased about 26% since the
beginning of the industrial revolution¹. With increasing dissolved CO₂,
calcifying organisms will find it more difficult to build their shells.

The ocean is acidifying more rapidly than it has in
millions of years [HIGH CONFIDENCE]
Today’s human-induced acidification is a unique event in the geological
history of our planet due to its rapid rate of change.
An analysis of ocean acidification over the last 300 million years highlights
the unprecedented rate of change of the current acidification³¹. The most
comparable event 55 million years ago was linked to mass extinctions of
calcareous deep-sea organisms and significant changes to the surface ocean
ecosystem³¹. At that time, though the rate of change of ocean pH was rapid, it
may have been 10 times slower than current change³².

The legacy of historical fossil fuel emissions on ocean
acidification will be felt for centuries [VERY HIGH CONFIDENCE]
The increase in atmospheric CO₂ is occurring too quickly to be stabilised by
natural feedbacks such as the dissolution of deep-sea carbonates, which acts
on time-scales of thousands of years, or the weathering of terrestrial carbonate
and silicate rocks, which operates on time-scales of tens to hundreds of
thousands of years.
Global-scale projections of the changing chemistry of seawater can be
made with high accuracy from scenarios of atmospheric CO₂ levels. Even if
anthropogenic CO₂ emissions stopped today, the ocean’s pH would not recover
to its preindustrial level for centuries³³.

Reducing CO₂ emissions will slow the progress of
ocean acidification [VERY HIGH CONFIDENCE]
The concentration of atmospheric CO₂ is approximately 395 parts per million
(ppm; global average, as of 2013), which is more than 40% higher than the
preindustrial level of 280 ppm. Half of this increase has occurred in the last 33
years³⁴. If CO₂ emissions are reduced, less CO₂ will enter the ocean, limiting
the extent of ocean acidification impacts³³.
Reducing CO₂ emissions is possible with existing or developing technology.
Currently, there are agreements to stabilise CO₂ emissions to limit the global
mean temperature increase to 2°C above preindustrial levels. These levels may
still jeopardise the stability of some marine ecosystems. Current emissions are
tracking a much higher global temperature increase (see page on Future CO₂ emissions).

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