Reassessment of Greenhouse Gas and Temperature Covariation From Vostok Ice Core Data

A major and widely recognized failure of the Milankovitch theory is its inability to predict the 100 kyr beat of the ice ages, and the synchroneity of major climate changes in the northern and southern hemispheres. Feedbacks between climate and biogeochemical cycling, and corresponding changes in atmospheric chemistry, have had a significant role in these aspects of ice-age climate cycling. This was first demonstrated by Barnola and colleagues using gas and water-isotope data from the Vostok ice core. We have re-evaluated the relationship between temperature and greenhouse gas forcings as revealed by Vostok ice, by using a temperature reconstruction model that incorporates deuterium excess to correct for artefactual source region climate effects. The covariance of carbon dioxide and temperature is significantly stronger in our new reconstructions than in the original analyses. Covariance of carbon dioxide and temperature is estimated to have been 89% over the last 150 kyr, and 84% over the past 350 kyr. Furthermore, much of the obliquity-period variations in temperature originally inferred from the Vostok ice core are shown to be artefacts of changes in atmospheric transport characteristics associated with changes in the meridional insolation gradient.