PREDICTING A CO2 RECORD FOR THE LAST MILLION YEARS

Atmospheric CO₂ played an important role in the climate of the past. Up to now only the Vostok ice core provides a direct record of CO₂ concentration over the last 400 kyr although the new Dome C EPICA ice core will extend this record. Meanwhile we are looking for methods that could provide a prediction of the CO2 changes over the last million. Some of them could also be used for future natural CO₂ scenarios. Several attempts were already made in the past. It includes statistical approaches based on the correlation between marine ice volume and Vostok CO₂ records. The statistical scenario from Li et al (1998) assumes that the correlation between Vostok CO₂ concentration (Jouzel et al., 1993) and SPECMAP (Imbrie et al., 1984) over the last 200 kyr can be extended over the last million year. The same procedure is applied here by using the 400 kyr Vostok record (Petit et al., 1999) and the marine record from Bassinot (1994). Multiple regressions using insolation sea level, marine d¹⁸0 and d¹³C records were used by Berger (1997). Another reconstruction for the last million years is presented here, using spectral analysis. The major periodical components of the Vostok CO₂ record are extracted using the Thomson multi-taper method. These terms are then used to reconstruct a signal for the last million years, which contains the low frequency part of the CO₂ record. Our reconstruction assumes, in a first attempt, the stationarity of the periods in the CO₂ record over time. However climatic reconstruction, in particular marine records, indicates a change in the major period of variation, from a dominating 40-kyr to a 100-kyr period, occurring around 700 kyr BP, or earlier. As CO₂ concentration is strongly influenced by climate, the reconstruction technique for the CO₂ time series must also take into account this transition between these two periods. Although empirical and descriptive, conceptual models can account, in some way, for this transition. They consider different states. Thresholds, depending on the continental ice volume, the insolation and the climate states, are crossed following a simple time evolution of the CO₂ concentration