INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. (2008) Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/joc.1729 Review Possible impacts of anthropogenic and natural aerosols on Australian climate: a review Leon D. Rotstayn, a,e * Melita D. Keywood, a,e Bruce W. Forgan, b Albert J. Gabric, c Ian E. Galbally, a,e John L. Gras, a,e Ashok K. Luhar, a,e Grant H. McTainsh, c Ross M. Mitchell d,e and Stuart A. Young a,e a CSIRO Marine and Atmospheric Research, Aspendale, Vic 3195, Australia b Bureau of Meterology, Melbourne, Vic 3001, Australia c Griffith School of Environment, Griffith University, Brisbane, Qld 4111, Australia d CSIRO Marine and Atmospheric Research, Black Mountain, ACT 2601, Australia e The Centre for Australian Weather and Climate Research, a partnership between CSIRO and the Bureau of Meteorology, Australia ABSTRACT: A review is presented of the aerosol–climate interaction with specific focus on the Australian region. The uncertainties associated with this interaction are much larger than those associated with greenhouse gases or other forcing agents, and are currently a major obstacle in climate-change research. However, new research suggests that aerosol effects are of comparable importance to greenhouse gases as a driver of recent climate trends in the Southern Hemisphere, including Australia. A significant new result from climate modelling is that anthropogenic aerosol over Asia affects meridional temperature gradients and atmospheric circulation, and may have caused an increase in rainfall over north-western Australia. Global ocean circulation provides another mechanism whereby aerosol changes in the Northern Hemisphere can affect climate in the Southern Hemisphere, suggesting an urgent need for further targeted studies using coupled ocean-atmosphere global climate models. To better model climate variability and climate change in the Australian region, more research is needed into the sources of aerosol and their precursors, their atmospheric distributions and transformations, and how to incorporate these processes robustly in global climate models (GCMs). The following priorities are suggested for further research in Australia linking aerosol observations and modelling: natural aerosol over the Southern Ocean, tropical biomass- burning aerosol in Indonesia and Australia, secondary organic aerosol (SOA) from volatile organic compounds (VOCs), wind-blown dust and modulation of rainfall by anthropogenic aerosol. Copyright  2008 Royal Meteorological Society KEY WORDS aerosols; climate change; climate modelling; Australia; Southern Hemisphere Received 21 December 2007; Revised 10 April 2008; Accepted 7 May 2008 1. Introduction Aerosol effects are the most uncertain of the radiative forcing components by which human activity is altering our climate, according to the Intergovernmental Panel on Climate Change (IPCC); Forster et al., 2007). There are three ways in which aerosols are known to affect climate. The direct effect of aerosol on short and long wave radiation comes from the scattering and absorption properties of the aerosol. The direct effect is uncertain, with an estimated net global-mean forcing of −0.1 to −0.9Wm −2 . The indirect effects of aerosol on climate occur when aerosols modify cloud properties by acting as cloud condensation nuclei (CCN). An increase in aerosol concentration causes the liquid water in clouds to be distributed over a larger number of smaller cloud * Correspondence to: Leon D. Rotstayn, CSIRO Marine and Atmos- pheric Research, Private Bag 1, Aspendale, Vic 3195, Australia. E-mail: Leon.Rotstayn@csiro.au droplets, leading to an increase in cloud albedo (Twomey, 1974). This first indirect effect is now considered to be well established, although its magnitude is very uncertain, with a range of −0.3 to −1.8Wm −2 for its global-mean radiative forcing, according to the IPCC. The second indirect effect, whereby smaller cloud droplets cause a decrease in the precipitation efficiency, is even more uncertain, and the IPCC does not give an estimate of its magnitude. These uncertainty ranges are much larger than those associated with greenhouse gases or other forcing agents, and are currently a major obstacle in climate- change research. The effect of atmospheric aerosol on climate has been observed empirically for some time. After the great eruption of Tambora in 1815, the Northern Hemisphere had a ‘year without a summer’ (Stommel and Stommel, 1983) and more recently the effects of major volcanic eruptions on global temperatures have been observed (e.g. Soden et al., 2002). There is also palaeoclimatic evidence Copyright  2008 Royal Meteorological Society