Transient eddy activity has been diagnosed for the winter season of two ten-year equilibrium GCM integrations carried out at the UK Meteorological Office, one with present-day levels of carbon dioxide concentration (control) and one with doubled carbon dioxide (2CO2). On synoptic timescales, indicators of storm-track activity such as eddy kinetic energy are shifted northwards and intensified downstream in the 2CO2 experiment relative to the control. This effect is particularly marked for the Atlantic storm track. These patterns correspond closely to changes in relevant diagnostics of the time mean, such as the Eady growth rate and the diabatic heating. In the latter, the downstream intensification is effected by a decrease in the sensible heating and an increase in the latent heating in a warmer, moister atmosphere. Changes in the jet structure are consistent with changes in baroclinicity and eddy forcing. The total zonal-mean poleward energy transport is only slightly different between the two experiments. However, the component of this transport due to transient eddies has a different character when the carbon dioxide is doubled. In particular, the moist contribution to the transient energy flux is enhanced.