Reconciling ice core CO2 and land-use change following New World-Old World contact

Abstract

Ice core records of carbon dioxide (CO2) throughout the last 2000 years provide context for the unprecedented anthropogenic rise in atmospheric CO2 and insights into global carbon cycle dynamics on centennial and multidecadal timescales. Yet the atmospheric history of CO2 remains uncertain in some time intervals. A particular source of debate is the exact timing and magnitude of the decrease in atmospheric CO2 after 1550 CE. Here we present new ice core measurements of CO2 and methane (CH4) in the Skytrain Ice Rise ice core from 1450 to 1700 CE. The measurements, alongside analysis of the effects of gas record smoothing, suggest that a sudden decrease in ice core CO2 around 1610 CE in one widely used record is most likely an artefact of a small number of anomalously low values. Instead, our analysis suggests a more gradual decrease in CO2 of 0.5 ppm per decade between 1516 and 1670 CE, with an inferred land carbon sink of 2.7 PgC per decade. Furthermore, a rapid decrease in CO2 at 1610 CE is incompatible with even the most extreme modelled scenarios for land-use change, whereas our data support scenarios of large-scale reorganization of land use in the Americas following New World-Old World contact.