HR: 09:30h
AN: PP41B-07    [Abstracts]
TI: A Significant Thirteenth-century CO$_{2}$ Increase in Stomatal Frequency an ice core Records
AU: * van Hoof, T
EM: T.B.vanHoof@bio.uu.nl
AF: Palaeoecology,Laboratory of Palaeobotany and Palynology, Utrecht University, Budapestlaan 4, Utrecht, 3584CD Netherlands
AU: Kaspers, K
EM: K.A.Kaspers@phys.uu.nl
AF: Institute for Marine and Atmospheric Research Utrecht, Utrecht University, Princetonplein 5, utrecht, 3584CC Netherlands
AU: Wagner, F
EM: r.wagner@bio.uu.nl
AF: Palaeoecology,Laboratory of Palaeobotany and Palynology, Utrecht University, Budapestlaan 4, Utrecht, 3584CD Netherlands
AU: van de Wal, R
EM: r.s.w.vandewal@phys.uu.nl
AF: Institute for Marine and Atmospheric Research Utrecht, Utrecht University, Princetonplein 5, utrecht, 3584CC Netherlands
AU: Kuerschner, W
EM: w.m.kurschner@bio.uu.nl
AF: Palaeoecology,Laboratory of Palaeobotany and Palynology, Utrecht University, Budapestlaan 4, Utrecht, 3584CD Netherlands
AU: Visscher, H
EM: h.visscher@bio.uu.nl
AF: Palaeoecology,Laboratory of Palaeobotany and Palynology, Utrecht University, Budapestlaan 4, Utrecht, 3584CD Netherlands
AB: The climate of the first half of the last millennium is characterized by a transition from a relative warm period during medieval times (the Medieval Climatic Optimum) towards a more cooler period during the Little Ice Age (~AD 1300-1850). In several global and Northern-Hemispheric air-temperature reconstructions for the last millennium, this natural climate variability is represented by an air-temperature anomaly in the range of 0.2 to 1 $^{O}$C. In contrast to the strongly increased atmospheric CO$_{2}$ levels of the last century, ice-core CO$_{2}$ measurements constrain the pre-industrial CO$_{2}$ variability to a maximum of 12 ppmv, precluding a significant role for CO$_{2}$ as the primary forcing factor of air-temperature changes during the last millennium. As an alternative to ice-core measurements, atmospheric CO2 reconstructions are currently available for the last millennium from stomatal frequency analysis performed on fossil leaves.A period where both methods consistently provide evidence for natural CO$_{2}$ changes is the 13th century.The results of the two independent methods differ significantly in the amplitude of the estimated CO$_{2}$ changes (10 ppmv ice, versus 34 ppmv stomatal frequency). Here, we compare stomatal frequency and ice core results by using a firn-diffusion model in order to assess the potential influence of smoothing during enclosure on the temporal resolution as well as the CO$_{2}$ mixing ratios. The seemingly large discrepancies between the CO$_{2}$ levels estimated by the contrasting methods, diminish when effects of natural smoothing of the ice-core record is simulated for the raw data of the stomatal frequency record. Results indicate that the differences derived by the two methods may be less significant than previously thought. Climate model calculations show that the 34 ppmv shift as detected in the stomata record could generate Northern-Hemisphere and global air-temperature responses that stay well within the constrained range of reconstructed air-temperature variability of the last millennium. To assess the exact influence of a dynamic CO$_{2}$ regime on the air-temperature history of the last millennium, model studies simulating the air-temperature response to fluctuating CO$_{2}$ levels in interaction with the other forcing factors are needed. However, this study already indicates that CO2 should be reconsidered as a significant pre-industrial climate forcing factor during the last millennium.
DE: 4806 Carbon cycling
DE: 3309 Climatology (1620)
DE: 1610 Atmosphere (0315, 0325)
DE: 1620 Climate dynamics (3309)
DE: 0400 Biogeosciences
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting