Over the course of the Neogene, the Earth underwent profound climatic shifts from the sustained warmth of the middle Miocene to the development of Plio-Pleistocene glacial–interglacial cycles. Major perturbations in the global carbon cycle have occurred alongside these shifts, however the lack of long-term carbonate system reconstructions currently limits our understanding of the link between changes in CO 2 , carbon cycling, and climate over this time interval. Here we reconstruct continuous surface ocean pH, CO 2 , and surface ocean aragonite saturation state using boron isotopes from the planktonic foraminifer Trilobatus trilobus and we perform a sensitivity analysis of the key variables in our calculations (e.g. δ 11 B sw , [Ca] sw , CCD). We show that the choice of δ 11 B sw influences both seawater pH and CO 2 while [Ca] sw reconstructed dissolved inorganic carbon exerts a significant influence only on CO 2 . Over the last 22 Myr, the lowest pH levels occurred in the Middle Miocene Climate Optimum (MMCO; 17–14 Myr ago) reaching ∼ 7.6 ± 0.1 units in all our scenarios. The extended warmth of the MMCO corresponds to mean CO 2 and aragonite saturation state levels of 470–630 ppm and 2.7–3.5, respectively. Despite a general correspondence between our CO 2 record and climate, all CO 2 scenarios show a peak at ∼9 Ma not matched by corresponding changes in climate reconstructions. This may suggest decoupling (i.e. significant CO 2 change without a discernible climate response) for a limited interval in the Late Miocene (11.6–8.5 Ma), although further refinement of our understanding of the temporal evolution of the boron isotopic composition of seawater is necessary to fully evaluate the nature of the relationship between CO 2 and climate. Nonetheless, from our long-term view it is clear that low-latitude open ocean marine ecosystems are unlikely to have experienced sustained surface pH and saturation levels below 7.7 and 1.7, respectively, during the past 14 million years (66% CI).