The relative impact of the CO2 concentration on climate over the last 200 kyr is investigated here using the Louvain-la-Neuve 2-dimension Northern Hemisphere (LLN 2-D NH) climate model forced by different constant insolations and the variable CO2 concentration reconstructed from the Vostok ice-core record. Under such conditions the 100-kyr cycle in the Northern Hemisphere ice volume cannot be reproduced. Either a warm orbit prevents the ice sheets from forming or a present-day and colder orbits do not allow them to melt after being formed. The global hemispheric temperature, however, continues to respond directly to the atmospheric CO2-concentration changes. ¿ 2000 American Geophysical Union