TRANSIENT CLIMATIC-CHANGE TO 3XCO2 CONDITIONS

The CSIRO coupled global climatic model has been integrated for transi ent increasing atmospheric CO2, conditions from 1 to 3 x CO2. The basi s for this experiment is that there is little expectation that atmosph eric CO2 levels will be stabilised at or below 2 x CO2, concentrations given projected emission levels. Examination of changes in climatic v ariables between 1 and 2 x CO2, and 2 and 3 x CO2, states revealed an expected general diminution in the magnitude of the changes for the la tter state. This is attributed to the progressive saturation of the CO 2, absorption spectrum at the higher CO2, concentrations. An examinati on of annual mean surface temperature changes at decadal intervals ass ociated with the increasing CO2, indicated that it takes 60-70 years b efore temperature fluctuations due to natural climatic variability are finally overwhelmed by the greenhouse effect. Surface temperatures at 3 x CO2, under transient conditions were similar to those obtained un der 2 x 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 re vealed noticeable interannual and decadal fluctuations in both runs, a nd highlighted the potential problems of 'sampling' data at arbitrary timeframes. The model maintained El Nino/Southern Oscillation events t hroughout the transient run, suggesting that this feature is a robust characteristic of the climatic system. No untoward climatic fluctuatio ns were noted during the transient run, implying that climatic 'surpri ses' are unlikely, at least for present model formulations. (C) 1998 E lsevier Science B.V. All rights reserved.