Is Australia ’ s continued warming caused by drought ?

Nicholls and Larsen (2011) demonstrated that daily maximum temperatures at Melbourne, Australia, tended to be higher during and following droughts than otherwise. Studies in Europe and elsewhere also have demonstrated that low soil moisture associated with a drought is a likely contributor to high temperatures (eg. Durre et al. 2000; Seneviratne at al. 2006, 2010; Fischer et al. 2007a, b; Dole et al. 2011). Temperatures averaged over Australia have been warming since the middle of the twentieth century (Fig. 1; CSIRO & Bureau of Meteorology 2012). The question arises, therefore, as to whether or not changes in drought and rainfall could be the explanation for this warming. Since droughts exacerbate daily maximum temperatures (e.g. Nicholls and Larsen 2011), does this mean that a trend towards lower rainfall might be the cause of the warming observed across Australia? Coughlan (1979) and Nicholls et al. (1996) demonstrated that Australian annual mean temperature was negatively correlated with annual mean rainfall. Power et al. (1998) demonstrated that this out-of-phase relationship occurred throughout the country, and not just in Australia-wide averages. Nicholls et al (1996) identified, using data up to 1992, that the Australia-wide average of mean daily maximum temperature appeared to be increasing relative to the temperatures expected from the rainfall–temperature relationship. Nicholls (2003) extended the Nicholls et al. (1996) study by including data up to 2002 and demonstrated that this ‘anomalous’ warming had continued up to that year. Others (eg. Cai and Cowan 2008; Karoly and Braganza 2005) have used the changing relationship between temperature and rainfall to maximise the signal-to-noise ratio by removing the fraction of the temperature variability associated with rainfall variations and changes. The residual was then the basis for an attribution study aimed at identifying the physical cause of the warming. Cai et al. (2010) also demonstrated that the warming trend (at least in the Murray–Darling Basin) could not be explained by changes in cloudiness. In this note, data up to 2011 have now been used to determine whether changes in rainfall averaged across Australia can account for the continued warming of Australian average temperature. This period includes the very long drought that affected much of southern Australia over the twelve years up to 2010. If droughts are a factor causing warming, then this extended drought may have at least contributed to the long-term warming across the country.