Low atmospheric carbon dioxide (CO 2 ) concentration 1 during the Little Ice Age has been used to derive the global carbon cycle sensitivity to temperature 2 . Recent evidence 3 confirms earlier indications 4 that the low CO 2 was caused by increased terrestrial carbon storage. It remains unknown whether the terrestrial biosphere responded to temperature variations, or there was vegetation re-growth on abandoned farmland 5 . Here we present a global numerical simulation of atmospheric carbonyl sulfide concentrations in the pre-industrial period. Carbonyl sulfide concentration is linked to changes in gross primary production 6 and shows a positive anomaly 7 during the Little Ice Age. We show that a decrease in gross primary production and a larger decrease in ecosystem respiration is the most likely explanation for the decrease in atmospheric CO 2 and increase in atmospheric carbonyl sulfide concentrations. Therefore, temperature change, not vegetation re-growth, was the main cause of the increased terrestrial carbon storage. We address the inconsistency between ice-core CO 2 records from different sites 8 measuring CO 2 and δ 13 CO 2 in ice from Dronning Maud Land (Antarctica). Our interpretation allows us to derive the temperature sensitivity of pre-industrial CO 2 fluxes for the terrestrial biosphere ( γ L = −10 to −90 Pg C K −1 ), implying a positive climate feedback and providing a benchmark to reduce model uncertainties 9 .