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.