The observed decrease in ocean pH resulting from increasing concentrations of CO₂ is another indicator of global change. As discussed in AR4, the ocean’s uptake of CO₂ is having a significant impact on the chemistry of sea water. The average pH of ocean surface waters has fallen by about 0.1 units, from about 8.2 to 8.1 (total scale) since 1765 (Section 3.8). Long time series from several ocean sites show ongoing declines in pH, consistent with results from repeated pH measurements on ship transects spanning much of the globe (Sections 3.8 and 6.4; Byrne et al., 2010;[1] Midorikawa et al., 2010).[2] Ocean time-series in the North Atlantic and North Pacific record a decrease in pH ranging between –0.0015 and –0.0024 per year (Section 3.8). Due to the increased storage of carbon by the ocean, ocean acidification will increase in the future (Chapter 6). In addition to other impacts of global climate change, ocean acidification poses potentially serious threats to the health of the world’s oceans ecosystems (see AR5 WGII assessment).


  1. Byrne, R., S. Mecking, R. Feely, and X. Liu, 2010: Direct observations of basin-wide acidification of the North Pacific Ocean. Geophys. Res. Lett., 37. CCSP, 2009: [ Best Practice Approaches for Characterizing, Communicating, and Incorporating Scientific Uncertainty in Climate Decision Making]. U.S. Climate Change Science Program, Washington, DC, USA, 96 pp.
  2. Midorikawa, T., et al., 2010: Decreasing pH trend estimated from 25-yr time series of carbonate parameters in the western North Pacific. Tellus B, 62, 649–659.
ES 1.1 1.2.1 1.2.2 1.2.3 1.3 1.3.1 1.3.2 1.3.3 1.3.4 1.4.1 1.4.2 1.4.3 1.4.4 1.5 1.5.1 1.5.2 1.6 Box 1 FAQ Refs