Climate sensitivity to the carbon cycle modulated by past and future changes in ocean chemistry

The carbon cycle has a central role in climate change. For example, during glacial-interglacial cycles, atmospheric carbon dioxide has altered radiative forcing and amplified temperature changes. However, it is unclear how sensitive the climate system has been to changes in carbon cycling in previous geological periods, or how this sensitivity may evolve in the future, following massive anthropogenic emissions. Here we develop an analytical relationship that links the variation of radiative forcing from changes in carbon dioxide concentrations with changes in air-sea carbon cycling on a millennial timescale. We find that this relationship is affected by the ocean storage of carbon and its chemical partitioning in sea water. Our analysis reveals that the radiative forcing of climate is more sensitive to carbon perturbations now than it has been over much of the preceding 400 million years. This high sensitivity is likely to persist into the future as the oceans become more acidic and the bulk of the fossil-fuels inventory is tansferred to the ocean and atmosphere.