finding 13.3 : key-finding-13-3

The world’s oceans are currently absorbing more than a quarter of the CO2 emitted to the atmosphere annually from human activities, making them more acidic (very high confidence), with potential detrimental impacts to marine ecosystems. In particular, higher-latitude systems typically have a lower buffering capacity against pH change, exhibiting seasonally corrosive conditions sooner than low-latitude systems. Acidification is regionally increasing along U.S. coastal systems as a result of upwelling (for example, in the Pacific Northwest) (high confidence), changes in freshwater inputs (for example, in the Gulf of Maine) (medium confidence), and nutrient input (for example, in agricultural watersheds and urbanized estuaries) (high confidence). The rate of acidification is unparalleled in at least the past 66 million years (medium confidence). Under the higher scenario (RCP8.5), the global average surface ocean acidity is projected to increase by 100% to 150% (high confidence).

This finding is from chapter 13 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.

Process for developing key messages: There is very high confidence in evidence that the oceans absorb about a quarter of the carbon dioxide emitted in the atmosphere and hence become more acidic. The magnitude of the ocean carbon sink is known at a high confidence level because it is estimated using a series of disparate data sources and analysis methods, while the magnitude of the interannual variability is based only on model studies. There is medium confidence that the current rate of climate acidification is unprecedented in the past 66 million years. There is also high confidence that oceanic pH will continue to decrease.

Description of evidence base: Evidence on the magnitude of the ocean sink is obtained from multiple biogeochemical and transport ocean models and two observation-based estimates from the 1990s for the uptake of the anthropogenic CO2. Estimates of the carbonate system (DIC and alkalinity) were based on multiple survey cruises in the global ocean in the 1990s (WOCE – now GO-SHIP, JGOFS). Coastal carbon and acidification surveys have been executed along the U.S. coastal large marine ecosystem since at least 2007, documenting significantly elevated pCO2 and low pH conditions relative to oceanic waters. The data are available from the National Centers for Environmental Information ( Other sources of biogeochemical bottle data can be found from HOT-DOGS ALOHA ( or CCHDO ( Rates of change associated with the Palaeocene-Eocene Thermal Maximum (PETM, 56 million years ago) were derived using stable carbon and oxygen isotope records preserved in the sedimentary record from the New Jersey shelf using time series analysis and carbon cycle–climate modelling. This evidence supports a carbon release during the onset of the PETM over no less than 4,000 years, yielding a maximum sustained carbon release rate of less than 1.1 GtC per year.a7eaa271-3c43-4c6f-add7-305f4e0ce52e The projected increase in global surface ocean acidity is based on evidence from ten of the latest generation earth system models which include six distinct biogeochemical models that were included in the latest IPCC AR5 2013.

New information and remaining uncertainties: In 2014 the ocean sink was 2.6 ± 0.5 GtC (9.5 GtCO2), equivalent to 26% of the total emissions attributed to fossil fuel use and land use changes.e10cd7fc-e821-474a-bbde-74426a52b1d0 Estimates of the PETM ocean acidification event evidenced in the geological record remains a matter of some debate within the community. Evidence for the 1.1 GtC per year cited by Zeebe et al.,a7eaa271-3c43-4c6f-add7-305f4e0ce52e could be biased as a result of brief pulses of carbon input above average rates of emissions were they to transpire over timescales ≲ 40 years.

This finding was derived from figure -.2: Confidence / Likelihood

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