The authors suggest AGW may have played a minor role in the decade-long drought based on climate model simulations, while admitting that "[the models] severely underestimate the observed poleward expansion of the subtropical dry-zone and associated impacts," which would be essential in order to determine anthropogenic impacts.
Furthermore, the authors find the primary reason for the Millennium drought was an upward trend in the Southern Annular Mode [SAM], which prior papers have linked to solar activity.
Journal of Climate 2014 ; e-View
Wenju Cai,* Ariaan Purich, Tim Cowan, Peter van Rensch, and Evan Weller
The Australian decade-long “Millennium drought” broke in the summer of 2010/11, and was considered the most severe drought in many measures since instrumental records began in the 1900s. A crucial question is whether climate change played a role in inducing the rainfall deficit. The climate modes in question include the Indian Ocean Dipole (IOD) affecting southern Australia in winter and spring; the Southern Annular Mode (SAM) with an opposing influence on southern Australia in winter to that in spring; and El Niño-Southern Oscillation affecting northern and eastern Australia in most seasons, and southeastern Australia in spring through its coherence with the IOD. Furthermore, the poleward edge of the Southern Hemisphere Hadley cell, which indicates the position of the subtropical dry-zone, has possible implications for recent rainfall changes in autumn. Using observations and Coupled Model Intercomparison Project phase 5 (CMIP5) simulations, it is shown that the drought over southwest Western Australia is partly attributable to a long-term upward SAM trend, which contributed to half of the winter rainfall reduction in this region. For southeast Australia, models simulate weak trends in the pertinent climate modes. In particular, they [the models] severely underestimate the observed poleward expansion of the subtropical dry-zone and associated impacts. Thus, although climate models generally suggest that Australia’s Millennium drought was mostly due to [natural] multi-decadal variability, some late-twentieth-century changes in climate modes that influence regional rainfall are partially attributable to anthropogenic greenhouse warming.