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. The 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 frequency is more than doubled for a 3 degrees C warming. The frequency of subzero winter days is halved for a 1 degrees C warming. At selected sites, 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 probability 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 factor of about three. A 10 per cent change in temperature variance is unlikely 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 hot summer and spring days and 5 degrees-100 per cent fewer extremely cold winter days. Larger percentage changes occur in regions where absolute changes are smaller. The high scenario also gives a 20 per cent increase 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 winter 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 highlands. Although a reduction in frost frequency may benefit agriculture, more extremely hot temperatures may increase bushfire potential, human mortality, and heat stress to livestock and crops.