Reconciling ice core CO 2 and land-use change following New World-Old World contact.

Ice core records of carbon dioxide (CO 2 ) throughout the last 2000 years provide context for the unprecedented anthropogenic rise in atmospheric CO 2 and insights into global carbon cycle dynamics. Yet the atmospheric history of CO 2 remains uncertain in some time intervals. Here we present measurements of CO 2 and methane (CH 4 ) in the Skytrain ice core from 1450 to 1700 CE. Results suggest a sudden decrease in CO 2 around 1610 CE in one widely used record may be an artefact of a small number of anomalously low values. Our analysis supports a more gradual decrease in CO 2 of 0.5 ppm per decade from 1516 to 1670 CE, with an inferred land carbon sink of 2.6 PgC per decade. This corroborates modelled scenarios of large-scale reorganisation of land use in the Americas following New World-Old World contact, whereas a rapid decrease in CO 2 at 1610 CE is incompatible with even the most extreme land-use change scenarios.