Robust constraints on past CO₂ climate forcing from the boron isotope proxy

Abstract

The atmospheric concentration of the greenhouse gas carbon dioxide, CO₂, is intimately coupled to the carbon chemistry of seawater, such that the radiative climate forcing from CO₂ can be changed by an array of physical, geochemical and biological ocean processes. For instance, biological carbon sequestration, seawater cooling and net CaCO3 dissolution are commonly invoked as the primary drivers of CO₂ 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 CO₂ radiative forcing (ΔF) on orbital timescales, as is evident from independent late Pleistocene reconstructions of pH and CO₂. In short, all processes relevant for CO₂ on orbital timescales, including temperature change, cause pH to change to bring about fractional CO₂ change so as to yield a linear relationship of ΔpH to CO₂ climate forcing. Further, we show that ΔpH and CO₂ climate forcing can be reconstructed using the boron isotope pH-proxy more accurately than absolute pH or CO₂, 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 CO₂ 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.

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Identifiers

ORCID for G.L. Foster: orcid.org/0000-0003-3688-9668

ORCID for T. Chalk: orcid.org/0000-0002-2880-3847

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