Covariation of carbon dioxide and temperature from the Vostok ice core after deuterium-excess correction

Ice-core measurements of carbon dioxide(1,2) and the deuterium palaeothermo meter reveal significant covariation of temperature and atmospheric CO2 con centrations throughout the climate cycles of the past ice ages. This covari ation provides compelling evidence that CO2 is an important forcing factor for climate(3-5). But this interpretation is challenged by some substantial mismatches of the CO2 and deuterium records, especially during the onset o f the last glaciation, about 120 kyr ago. Here we incorporate measurements of deuterium excess from Vostok(6,7) in the temperature reconstruction and show that much of the mismatch is an artefact caused by variations of clima te in the water vapour source regions. Using a model that corrects for this effect, we derive a new estimate for the covariation of CO2 and temperatur e, of r(2) = 0.89 for the past 150 kyr and r(2) = 0.84 for the period 350-1 50 kyr ago. Given the complexity of the biogeochemical systems involved, th is close relationship strongly supports the importance of carbon dioxide as a forcing factor of climate. Our results also suggest that the mechanisms responsible for the drawdown of CO2 may be more responsive to temperature t han previously thought.