Transient climatic change to 3×CO2 conditions

The CSIRO coupled global climatic model has been integrated for transient increasing atmospheric CO2 conditions from 1 to 3×CO2. The basis for this experiment is that there is little expectation that atmospheric CO2 levels will be stabilised at or below 2×CO2 concentrations given projected emission levels. Examination of changes in climatic variables between 1 and 2×CO2 and 2 and 3×CO2 states revealed an expected general diminution in the magnitude of the changes for the latter state. This is attributed to the progressive saturation of the CO2 absorption spectrum at the higher CO2 concentrations. An examination of annual mean surface temperature changes at decadal intervals associated with the increasing CO2 indicated that it takes 60–70 years before temperature fluctuations due to natural climatic variability are finally overwhelmed by the greenhouse effect. Surface temperatures at 3×CO2 under transient conditions were similar to those obtained under 2×CO2 equilibrium conditions with a slab ocean, except for much reduced warmings in high southern latitudes. Time series of climatic variables for selected variables for the control and transient runs revealed noticeable interannual and decadal fluctuations in both runs, and highlighted the potential problems of ‘samplingʹ data at arbitrary timeframes. The model maintained El Niño/Southern Oscillation events throughout the transient run, suggesting that this feature is a robust characteristic of the climatic system. No untoward climatic fluctuations were noted during the transient run, implying that climatic ‘surprisesʹ are unlikely, at least for present model formulations.