GREENHOUSE WARMING AND THRESHOLD TEMPERATURE EVENTS IN VICTORIA, AUSTRALIA

Small changes in the mean climate can produce relatively large changes in the frequency of extreme events. Two experiments were performed: ( i) a sensitivity study where observed temperatures at climate stations were increased by either 1 degrees, 2 degrees or 3 degrees C, and (ii ) a scenario study for the year 2030 where temperatures were increased according to warming scenarios which vary spatially and seasonally. T he sensitivity study shows that at some sites, up to four more summer days over 35 degrees C occur for a 1 degrees C warming and the frequen cy is more than doubled for a 3 degrees C warming. The frequency of su bzero winter days is halved for a 1 degrees C warming. At selected sit es, the current probability of a run of at least five summer days over 35 degrees C is doubled for a 1 degrees C warming and increases by a factor of five for a 3 degrees C warming. Where the current probabilit y of a run of at least five winter days below 0 degrees C is less than 50 per cent, a 1 degrees C warming reduces the probability by a facto r of about three. A 10 per cent change in temperature variance is unli kely to significantly alter these results. A low warming scenario for 2030 gives at least 25 per cent more days over 35 degrees C in summer and spring, and at least 25 per cent fewer winter days below 0 degrees C. A high warming scenario produces 50-100 per cent more extremely ho t summer and spring days and 5 degrees-100 per cent fewer extremely co ld winter days. Larger percentage changes occur in regions where absol ute changes are smaller. The high scenario also gives a 20 per cent in crease in the probability of a run of five summer days over 35 degrees C in the northern half of the State, and the risk of a run of five wi nter days below 0 degrees C is reduced by less than 20 per cent in the western and coastal areas and by 25-40 per cent in the north-east hig hlands. Although a reduction in frost frequency may benefit agricultur e, more extremely hot temperatures may increase bushfire potential, hu man mortality, and heat stress to livestock and crops.