Book Description

Changes in the frequency and/or severity of extreme climate events have the potential to have profound societal and ecological impacts and observations suggest that in some parts of the world such changes are already occurring. The primary objective of this thesis was to identify and analyse the mechanisms which are driving changes in climate extremes in Australia in order to be better prepared for possible future changes. Multiple research tools, methods and data were employed including station observations, reanalyses data, satellite data and model output to address fundamental questions about how climate extremes have changed in Australia over the observational period, whether interactions between and changes in large-scale mechanisms are driving observed trends, and if changes are related to anthropogenic factors. Major results were:1. Across Australia, trends in extremes of both temperature and precipitation were very highly correlated with mean trends indicating an over-arching mechanism driving both. Analysis of the rate of change of extremes and means across Australia as a whole showed most stations have greater absolute trends in extremes than means. There was also some evidence that the trends of the most extreme events of both temperature and precipitation are changing more rapidly in relation to corresponding mean trends than are the trends for more moderate extreme events. The relationships between means and extremes of precipitation on an annual basis in Australia were consistent with all other global regions studied. 2. There have been significant reductions in the frequency of rain bearing synoptic systems affecting southern Australia over the past century, associated with significant decreases in the frequency of severe storms in south-east Australia. 3. Changes in climate extremes are affected to a large extent by variations in global sea surface temperatures. This is particularly true of maximum temperature extremes over Australia which showed significantly different responses to opposite phases of the El Niño-Southern Oscillation i.e. strong La Niña events compared to strong El Niño events. A global climate model forced with observed SSTs was unable to reproduce these observed responses. 4. Multiple simulations from nine global coupled climate models showed that when averaged across Australia the magnitude of trends and interannual variability of temperature extremes were well simulated by most models particularly for the warm nights index. The majority of models also reproduced the correct sign of trend for precipitation extremes although there was much more variation between the individual model runs. However, very few model runs showed significant skill at reproducing the observed spatial pattern of trends in temperature and precipitation extremes, although a pattern correlation measure showed that spatial noise could not be ruled out as dominating these patterns. 5. Trends in warm nights in Australia were consistent with an anthropogenic response but inconsistent with natural-only forcings. This indicates that there is a discernable human signature on the observed warming of minimum temperature extremes across Australia.