Résumé
Ice-core measurements of carbon dioxide (super 1,2) and the deuterium palaeothermometer reveal signigicant covariation of temperature and atmospheric CO (sub 2) concentrations throughout the climate cycles of the past ice ages. This covariation provides compelling evidence that CO (sub 2) is an inportant forcing factor for climate (super 3-5) . But this interpretation is challenged by some substantial mismatches of the CO (sub 2) and deuterium records. especially during the onset of the last glaciation, about 120 Kyr ago. Here we incorporate measurements of deuterium excess from Vostok (super 6,7) in the temperature reconstruction and show that much of the mismatch is an artefact caused by variations of climate in the water vapour source regions. Using a model that corrects for this effect, we derive a new estimate for the covariation of CO (sub 2) and temperature, of r (super 2) = 0.89 for the past 150 Kyr and r (super 2) = 0.84 for the period 350-150 Kyr ago. Given the complexity of the biogeochemical systems involved, this close relationship strongly supports the impportance of carbon dioxide as a forcing factor of climate. Our results also suggest that the mechanisms responsible for the drawdown of CO (sub 2) may be more responsive to temperature than previosly thought.