Abstract
Abstract. The number and quality of ocean pH measurements have increased substantially over the past few decades such that trends, variability, and spatial patterns of change are now being evaluated. However, comparing pH changes across domains with different initial pH values can be misleading because a pH change reflects a relative change in the hydrogen ion concentration ([H+], expressed in mol kg−1) rather than an absolute change in [H+]. We recommend that [H+] be used in addition to pH when describing such changes and provide three examples illustrating why.
Highlights
In 1909, Danish biochemist Søren Peter Lauritz Sørensen proposed using a logarithmic scale to display the wide range of natural hydrogen ion concentrations (CH, expressed in mol L−1) in a more compact numerical form (Sørensen, 1909).pH = log10 (1/CH) (1)The logarithmic scaling of hydrogen ion concentration derives from the Nernst equation, which relates the potential of an electrochemical cell to ion concentrations in solution, while the reciprocal form ensured predominantly positive values for pH in an aqueous solution
While we focus here on pH changes in the open ocean, pH changes occur in coastal waters where they tend to be larger (Carstensen and Duarte, 2019)
The community often refers to changes in pH along with changes in other CO2 system variables, such as pCO2, total dissolved inorganic carbon (DIC), and the saturation state of seawater with respect to aragonite
Summary
In 1909, Danish biochemist Søren Peter Lauritz Sørensen proposed using a logarithmic scale to display the wide range of natural hydrogen ion concentrations (CH, expressed in mol L−1) in a more compact numerical form (Sørensen, 1909). Once dissolved in the ocean, CO2 reacts with water to form a weak acid that loses a hydrogen ion, which is largely neutralized through reaction with a carbonate ion to form bicarbonate, causing the seawater [H+] to increase and the pH to decrease (Millero, 2007) This overall process is commonly referred to as ocean acidification (Caldeira and Wickett, 2003; Doney et al, 2009), and it may have far reaching effects on marine life (Boyd et al, 2016; Doney et al, 2014; Hofmann et al, 2010; Kleypas et al, 2006) and on the rates of a variety of carbon cycle feedback processes within the ocean (e.g., Archer et al, 1998; Boudreau et al, 2018; Passow and Carlson, 2012; Revelle and Suess, 1957). These examples include an evaluation of (1) modern sea surface trends, (2) the evolution of seasonal cycle amplitudes over the 21st century, and (3) changing interior ocean chemistry
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