Ocean acidification in the IPCC Special Report: Global Warming of 1.5°C

Ocean acidification is mentioned in the Intergovernmental Panel on Climate Change (IPCC) Special Report: Global Warming of 1.5°C, released today, 8 October 2018. An overview of ocean acidification is given in chapter 3 section 3.10 titled Ocean Chemistry.

Ocean acidification is also included in a global synthesis table summarising the assessments of global and regional climate changes and associated hazards in section 3.3.11.

Below is the Ocean Chemistry chapter section:

Ocean chemistry includes pH, salinity, oxygen, CO2, and a range of other ions and gases, which affected by precipitation, evaporation, storms, river run-off, coastal erosion, up-welling, ice formation, and the activities of organisms and ecosystems (Stocker et al., 2013). Ocean chemistry is also changing with global temperature, with impacts projected at 1.5°C and, more so, at 2°C (high agreement, medium evidence). Projected changes in the upper layers of the ocean include changes to pH, carbonate ion and oxygen content. Despite its many component processes, ocean chemistry has been relatively stable for long periods of time prior to the Industrial Period (Hönisch et al., 2012). Ocean chemistry is changing under the influence of human activities and rising greenhouse gases (virtually certain, Rhein et al., 2013; Stocker et al., 2013). About 30% of CO2 emitted by human activities, for example, has been absorbed by the ocean where it has combined with water to produce a dilute acid that dissociates and drives ocean acidification (Cao et al., 2007; Stocker et al., 2013). Ocean pH has decreased by 0.1 pH units since the Pre-Industrial Period, which is unprecedented in the last 65 Ma (high confidence, Ridgwell and Schmidt, 2010) or even 300 Ma of Earth history (medium confidence, Hönisch et al., 2012). Ocean acidification is most pronounced where temperatures are lowest (e.g. Polar regions) or where CO2rich water is brought to the ocean surface by upwelling (Feely et al., 2008). Acidification can also be influenced by effluents from natural or disturbed coastal land use (Salisbury et al., 2008), plankton blooms (Cai et al., 2011), and the atmospheric deposition of acidic materials (Omstedt et al., 2015). These sources may not be directly attributable to climate change, yet may amplify the impacts of ocean acidification (Bates and Peters, 2007; Duarte et al., 2013). Ocean acidification also influences the ionic composition of seawater by changing the organic and inorganic speciation of trace metals (e.g. 20-fold increases in free ion concentrations such as Al) which may have impacts although these are poorly understood (Stockdale et al., 2016).

Oxygen varies regionally and with depth, and is highest in Polar regions and lowest in the eastern basins of the Atlantic and Pacific Oceans, and the northern Indian Ocean (Doney et al., 2014; Karstensen et al., 2015; Schmidtko et al., 2017). Increasing surface water temperatures have reduced oxygen in the ocean by 2% since 1960 with other variables such as ocean acidification, sea level rise, precipitation, wind, and storm patterns playing roles (Schmidtko et al., 2017). Changes to ocean mixing and metabolic rates (due to increased temperature and supply of organic carbon to deep areas) has increased the frequency of ‘dead zones’, areas where oxygen levels no longer support oxygenic life (Diaz and Rosenberg, 2008). Drivers are complex and include both climate change and other factors (Altieri and Gedan, 2015) with increases in tropical as well as temperate regions (Altieri et al., 2017).

Ocean salinity is changing in directions that are consistent with surface temperatures and the global water cycle (i.e. evaporation and inundation). Some regions (e.g. northern oceans and Arctic regions) have decreased salinity (i.e. due to melting glaciers and ice sheets) while others are increasing in salinity due to higher sea surface temperatures and evaporation (AR5 WGII Ch30, Durack et al., 2012). These changes in salinity (density) are also potentially driving changes to large scale patterns of water movement (Section 3.3.8)

Hoegh-Guldberg O., Jacob D., Taylor M., Bindi M., Brown S., Camilloni I., Diedhiou A., Djalante R., Ebi K., Engelbrecht F., Guiot J., Hijioka Y., Mehrotra S., Payne A., Seneviratne S. I., Thomas A., Warren R. & Zhou G., 2018. Chapter 3: Impacts of 1.5ºC global warming on natural and human systems. In: Marengo J. A., Pereira J. & Sherstyukov B. (Eds.), IPCC Special Report: Global Warming of 1.5 °C. Report.

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