1. Department of Geography, and Department of Earth and Planetary Science, 507 McCone Hall, University of California, Berkeley, California 94720-4740, USA
2. Laboratoire des Sciences du Climat et de l'Environnement, UMR CEA-CNRS 1572, CEA Saclay, Batiment 709, Orme des Merisiers, 91191 Gif sur Yvette, France

Correspondence to: Kurt M. Cuffey¹ Correspondence and requests for materials should be addressed to K.M.C. (e-mail: Email: kcuffey@socrates.berkeley.edu).

Abstract

Ice-core measurements of carbon dioxide^{1, 2} and the deuterium palaeothermometer reveal significant covariation of temperature and atmospheric CO₂ concentrations throughout the climate cycles of the past ice ages. This covariation provides compelling evidence that CO₂ is an important forcing factor for climate^{3, 4, 5}. But this interpretation is challenged by some substantial mismatches of the CO₂ and deuterium records, especially during the onset of 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 climate 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 temperature, of r² = 0.89 for the past 150 kyr and r² = 0.84 for the period 350–150 kyr ago. Given the complexity of the biogeochemical systems involved, this 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 than previously thought.