Australia: The Land Where Time Began

A biography of the Australian continent 

Anthropogenic Contributions to the Australian Record Summer Temperatures of 2013

In this paper Lewis et al. report investigations of anthropogenic contributions to the record hot summer in Australia in 2013, the hottest summer in the observational record, by the use of a suite of climate model experiments. For the simulations with only natural forcings Australian area-average summer temperatures were compared to simulations using anthropogenic and natural forcings for the period 1976-2005 and the RCP8.5 high-emission simulation of 2006-2020 from 9 Coupled Model Intercomparison Project, phase 5, models. To compare the likelihood of extreme Australian summer temperatures between the experiments, by using fraction of attributable risk, it was very likely, with >90 % confidence, that the odds of extreme heat due to human influences were increased by at least a 2.5 times, by the use of simulations to 2005, and when using the simulations for 2006-2020 there was a 5-fold risk increase. Natural climate variations alone, which included ENSO, are considered by Lewis et al. to be unlikely to explain the record temperatures, but the human contribution to the increased odds for summer extremes in Australia like 2013 was substantial.

In December 2012 to February 2013 the Australian summer was the hottest on record, in which average conditions exceeded the observed 1911-1940 mean by 1.34 K. On daily to seasonal timescales summer temperature records were broken: with the hottest month on record occurring as well as the hottest day for the entire Australian continent (Bureau of Meteorology [BoM] 2013a). Sustained high temperatures were also coincident with bushfires by late summer in southeastern Australia in Victoria and South Australia, and in northeastern Australia in Queensland and New South Wales (Bureau of Meteorology, 2013b) severe flooding. It was dubbed the “angry summer” (Steffen, 2013), because of the severe conditions.

Over land, the intensity, frequency and duration of heat waves increase that was observed is widespread globally (Perkins et al., 2012). On longer timescales, monthly extremes are increasing at a rate that is relatively faster than daily extremes (Coumou & Rahmstorf, 2012), such that many of the large number of the record-breaking heat waves that have occurred recently and summer extremes have been associated with anthropogenic influences (Hansen et al., 2012). Very large percentage changes in the occurrence of extremes (Trenberth, 2012) can result from changes in climate means, the extreme seasonal heat is considered in the context of average temperatures in Australia having increased by 0.9oC since 1910 (BoM, 2012b). The possible anthropogenic contribution to extreme seasonal temperatures have not been considered before, while changes in average temperatures in Australia have been attributed to anthropogenic climate change (Karoly & Braganza, 2005; Stott et al., 2010).

According to Lewis et al. the cause of a particular climatic event cannot be categorically ascribed to anthropogenic climate change; however, the roles of various factors that contribute to the change in odds of an event occurring can be identified. A large ensemble of climate models is used to calculate the probability of an event occurring and then compared to the equivalent probability in a counterfactual experiment, in which only climate forcings are imposed (Allen, 2003). Anthropogenic contributions to specific climate events have been probabilistically estimated using this conceptual framework (Stott et al., 2004; Pall et al., 2011; Christidis et al., 2013). Event attribution and climate prediction services are interrelated; attribution studies are necessary in order to develop meaningful adaptive decisions (Stott et al., 2012).

This study investigated the relative contributions of anthropogenic and natural factors to the record temperatures in the summer of 2013 averaged across Australia. Lewis et al. used a suite of Coupled Model Intercomparison Project phase 5 (CMIP5) detection and attribution experiments (Taylor et al., 2012) to compare the occurrence of extreme summer temperatures in a series of control simulations and simulations with natural forcings only with those occurring  in model experiments including both natural and anthropogenic forcings. If anthropogenic influences contribute to conditions that occurred in the record summer of 2013, then if so, by how much.


This study was motivated by the recent 2012-2013 record temperatures in the Australian summer of 2012-2013. Comparison of the distribution of Australian area-averaged temperature anomalies from various CMIP5 experiments from 9 models shows a significant change in the probability of extreme warm temperatures in summer in model experiments that were forced with increasing human influences, compared with the equivalent naturally forced experiments. Given the La Niña-neutral conditions that prevailed during 2012-2013, and the strong increase in risk that was strongly modelled, associated with this type of seasonal extreme in experiments that were anthropogenically forced, on their own, natural climate variations are not likely to have caused the record Australian “angry summer” of  2013. A clear conclusion that climate change had a substantial influence on the extreme heat over Australia in summer, and that natural climate variations alone are not likely to explain the record summer temperature is supported by these results.

It was conservatively estimated by Lewis et al. that the extreme summer heat was at least 2.5 times more likely (>90% confidence) as a result of anthropogenic influences in the simulations up to 2005. Using simulations centred on 2013, however, it is very likely (>90% confidence) that human influences increased this risk by at least 5-f0ld. The best estimate anthropogenic mean temperature change for 2013 (0.84 K, that was determined from the RCP8.5 ensemble mean for years 2006-2020) was applied to the observational record (for the period 1911-1940 of minimal anthropogenic warming), and the calculated FAR value of 0.90 was found to be close to the equivalent RCP8.5 FAR value of 0.87. I.e., it could be accurately estimated, for the increase in risk for this type of event, simply from the observational record and modelled mean temperature changes. An assessment of risk in this instance could potentially occur in near-real time. It has also been shown by previous studies that precomputing changes in the likelihood of exceeding a temperature threshold of a suite of thresholds enables an assessment that is near real time of the anthropogenic influences on the observed temperatures for some regions (Christidis et al., 2011).

Also, there was a substantial decrease in the return times (increase in the frequency) of extreme summers between the historical (1976-2005) and RCP8.5 (2006-2020) model years. According to Lewis et al. there is likely to be a marked increase beyond the 2020 cutoff that was utilised in this study, in the occurrence of extreme hot seasons under the RCP8.5 scenario. It was shown by other studies that by 2080-2099 in the RCP8.5 simulations, at least 65% of seasons are projected to be extremely hot over all land areas, with frequent heat likely to have severe implications for human and natural systems (Diffenbaugh & Giorgi, 2012). An increase in extreme summer heat in the future is very likely. In eastern Australia, where the highly populated areas that are bushfire prone, understanding the risk of extreme summer temperatures and likely changes to the frequency of summer extremes has implications for making adaptive decisions. According to Lewis et al. it wold be useful to extend this analysis to particular heat waves, such as those that occurred throughout the 2013 Australian summer. There has been a general lengthening of summer-like conditions, with spring and autumn being shortened in many places, beyond the increase in seasonal temperatures that are anthropologically driven (Hansen et al., 2012). It would also be valuable for understanding future changes in heatwave intensity and frequency to investigate human influences of out-of-season heatwaves, such as the record November 2012 and March 2013 heatwaves in southeastern Australia.


Lewis, Sophie C., and David J. Karoly. "Anthropogenic Contributions to Australia's Record Summer Temperatures of 2013." Geophysical Research Letters 40, no. 14 (2013): 3705-09.

Author: M. H. Monroe
Last updated 06/11/2013
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