CO 2 radiative forcing during the Holocene Thermal Maximum revealed by stomatal frequency of Iberian oak leaves

Abstract

Here we analyse radiocarbon-dated Quercus leaf assemblages from northern Spain to obtain past atmospheric CO 2 mixing ratios for the time period 9000–1100 cal BP by means of stomatal frequency analysis. Normalized, stomata based CO 2 records show fluctuations of 20 ppmv during the Holocene that parallel Northern Hemisphere 5 palaeotemperature reconstructions. The calculated radiative forcing of CO 2 indicates a CO 2 contribution of +0.1 • C to the Holocene Thermal Maximum from 7 to 5 kyr BP, and −0.05 • C to the Neoglacial cooling around 4 kyr BP. Derived northern hemispheric air-temperature anomalies forced by atmospheric CO 2 variation suggest an active role of this trace gas as an amplifier of initial orbital forcing of Holocene climate.