Long-term variability in the global carbon cycle inferred from a high-precision CO2 and δ13C ice-core record

Abstract

The new high precision Law Dome ice core record of CO2 and δ13CO2 is used with a 1-D global carbon cycle model to investigate natural variability in the carbon cycle and the anthropogenic CO2 perturbation, focusing on variations on time-scales of centuries. A major feature of the ice core record is the decrease in CO2, and increase in δ13C, through the “Little Ice Age” period (roughly 1550–1800). We show that this observed decrease in CO2 is consistent with the effect of decreased temperature on either terrestrial or oceanic exchange, however the increase in δ13C favors a terrestrial response to cooling. We perform single deconvolution model calculations which generally give good agreement with observed variations in CO2, δ13C and Δ14C data for different reservoirs and due to both natural and anthropogenic causes. The fit to prebomb Δ14C is improved by using an ice core 10Be record to represent the natural production of 14C due to cosmic rays, however, the uncertainties in interpreting the 10Be are as yet too large to use prebomb Δ14C to better constrain the model parameters.