New Climate Modelling Suggests the ?Asian Haze? Has Affected Australian Rainfall
New research suggests that elevated particle emissions resulting from increased economic activity in Asia may have increased Australia?s tropical rainfall. Until now, there has been ample evidence that these particles have important effects on climate in the Northern Hemisphere but little such evidence in the Southern Hemisphere. Because the sky is generally blue over Australia, climate scientists have tended to assume that these particles (called aerosols) are essentially a Northern-Hemisphere problem.
Recent simulations using the CSIRO climate model suggest that the ‘Asian haze’ is having an effect on the Australian hydrological cycle and has generated increasing rainfall and cloudiness since 1950, especially over northwest and central Australia. The effect occurs because the haze cools the Asian continent and nearby oceans, and thereby alters the delicate balance of temperature and winds between Asia and Australia. It has nothing to do with Asian pollution being transported directly over Australia.
This implies that decreasing pollution in Asia later this century could reverse this effect and lead to an increase in Australian drying trends. We are only at the beginning of understanding the trends but sooner or later these emissions will be cleaned up and then a trend of increasing rainfall in the northwest and centre could be reversed. This is potentially serious for Australia, because the northwest and centre are the only parts of Australia where rainfall has been increasing in recent decades.
An aerosol is a haze of small particles or droplets suspended in the atmosphere. Representing aerosols in climate models and understanding their influence on cloud formation and rainfall is one of the biggest challenges facing climate scientists. Climate modelling is a valuable tool for teasing out what is actually causing climatic changes, rather than simply assuming that future trends will be an extrapolation of existing trends.
This research, to be published early in 2007 in the Journal of Geophysical Research, increases confidence in the accuracy of future climate simulations for Australia, because it improves the simulation of 20th century rainfall trends over Australia. Since the cooling effect of aerosol pollution is possibly comparable to the warming effect of increased levels of carbon dioxide, one message from this research is that aerosols are an essential inclusion if we are to accurately describe present and future Australian climate.
The new research is based on simulations performed with a new low-resolution version of CSIRO’s global climate model – including a treatment of aerosols from both natural and human-induced sources. The work was done in collaboration with scientists from the Cooperative Research Centre for Greenhouse Accounting at the Australian National University; the University of Michigan’s Department of Atmospheric, Oceanic and Space Sciences, and the U.S. National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory in Princeton. A copy of the full paper can be downloaded from the following website: http://www.csiro.au/resources/pfoq.html .
Journal of Geophysical Research (to be published)
Science, Vol. 315, p1217, 2 March 2007