Coupling between atmospheric CO2 and temperature during the onset of the Little Ice Age

Abstract

Present day global warming is primarily caused by the greenhouse effect of the increased CO2 emissions since the onset of the industrial revolution. A coupling between temperature and the greenhouse gas CO2 has also been observed in several ice-core records on a glacial-interglacial timescale as well as on a millennial timescale during the glacials. In marked contrast, no significant ice-derived CO2 fluctuations occur on centennial time scales contemporaneously with well-documented cooling events such as the Younger Dryas, Preboreal Oscillation, and the 8.2 kyr BP event. Intriguingly however, the Little Ice Age cooling event seems to be recorded in several Antarctic ice cores of the last millennium, showing changes in CO2 from 5-12 ppmv As CO2 fluctuations of these magnitudes only generate a minor temperature response the role of these small CO2 perturbations in climate forcing of the last millennium is considered to be non-significant.

An alternative methodology to assess palaeo-atmospherical CO2 concentrations, is based on the inverse relationship between the number of leaf stomata and the atmospheric CO2 concentration. Compared to the ice cores this stomatal frequency based CO2 proxy in general observes a much more dynamic CO2 regime throughout the Holocene and therefore implies a much larger role for CO2 in Holocene climate forcing. In order to corroborate the concept of coupling between atmospheric CO2 and temperature during the onset of the Little Ice Age, a palaeo-atmospheric CO2 reconstruction for the first half of the past millennium was developed by studying a high resolution stomatal frequency record from fossil Quercus robur (oak) leaves from the Netherlands.

The results of this study indicate that during the thirteenth century AD a 35 ppmv shift in atmospheric CO2 did occur. More evidence of this CO2 perturbation have previously been observed in one other stomatal frequency based record and two Antarctic ice core records. By applying a firn diffusion model on the in this study presented stomatal frequency based CO2 reconstruction, the lower amplitude of the recorded shift in the ice core D47 could be fully explained by smoothing of the ice core record due to the diffusion processes that take place in the firn layer of the ice. Radiative forcing calculations of the stomatal frequency based CO2 record suggest a more prominent role for CO2 in climate forcing during the onset of the Little Ice Age.