evidence: Over a decade of CO2 emissions since 2005 show that the Representative Concentrations Pathway (RCP2.6) scenario is unlikely to adequately represent the future atmospheric CO2 level. Consequently, here we focus discussion instead on the RCP4.5 and RCP8.5 scenarios, as these are now the plausible low-end and high-end concentration pathways. Under the RCP 8.5 "business-as-usual" scenario, surface ocean pHT changes at about –0.02 units decade−1 (~6% [H+] decade−1) at the beginning of this century, close to what was observed by Lauvset et al.27, and gradually accelerates to an average of about −0.04 units decade−1 (~10% [H+] decade−1) towards the end of the century (Fig. 5). From 2000 to 2100, the globally and annually-averaged surface ocean pHT decreases by an average of about ~0.33 ± 0.04 (spatial variability) units (Figs. 5a, 6, and Supplementary Fig. S5), consistent with research based on ensembles of Earth System Models20,21. This is equivalent to an average hydrogen ion increase of ~114% (Figs. 5b, 7, and Supplementary Fig. S6), and slightly greater than the entire modern surface ocean pH range (Fig. 1a). The globally and annually-averaged Revelle Factor increases by 3.7 ± 0.9 (~34%) under the RCP8.5 scenario over the same period (Figs.