UC Santa Cruz

Abstract The atmospheric concentration of the greenhouse gas carbon dioxide, CO 2 , is intimately coupled to the carbon chemistry of seawater, such that the radiative climate forcing from CO 2 can be changed by an array of physical, geochemical, and biological ocean processes. For instance, biological carbon sequestration, seawater cooling, and net CaCO 3 dissolution are commonly invoked as the primary drivers of CO 2 change that amplify the orbitally paced ice age cycles of the late Pleistocene. Based on first‐principle arguments with regard to ocean chemistry, we demonstrate that seawater pH change (ΔpH) is the dominant control that effectively sets CO 2 radiative forcing (Δ F ) on orbital timescales, as is evident from independent late Pleistocene reconstructions of pH and CO 2 . In short, all processes relevant for CO 2 on orbital timescales, including temperature change, cause pH to change to bring about fractional CO 2 change so as to yield a linear relationship of ΔpH to CO 2 climate forcing. Further, we show that ΔpH and CO 2 climate forcing can be reconstructed using the boron isotope pH proxy more accurately than absolute pH or CO 2 , even if seawater boron isotope composition is poorly constrained and without information on a second carbonate system parameter. Thus, our formalism relaxes otherwise necessary assumptions to allow the accurate determination of orbital timescale CO 2 radiative forcing from boron isotope pH reconstructions alone, thereby eliminating a major limitation of current methods to estimate our planet's climate sensitivity from the geologic record. Key Points Radiative forcing by CO 2 linearly related to pH change, second carbonate system parameter is not required Using the boron isotope proxy, uncertainty of seawater boron isotopic composition has weaker effect on pH change than absolute pH Short time slices of high‐resolution boron isotope data better suited to reconstruct climate forcing than long‐term, low‐resolution records