The Canberra fires of 2003: a pine planataion on the edge of the Brindebella Ranges ignites
Credit: SMH
The story by now is a familiar one, but it is no less urgent for that. When certain gases - carbon dioxide, water vapour, methane, chlorofluorocarbons, and others - are released into the atmosphere, they act like the glass panes of a greenhouse. Visible light can pass through them, but radiant heat from the Earth's surface cannot flow back as readily into space.
Since the Industrial Revolution, the greenhouse gases in the atmosphere have had their heat-retaining properties enhanced by a 30 per cent increase in atmospheric carbon dioxide. Carbon dioxide levels are now higher than they have been at any other time in the past 420,000 years. The resultant warming is thought to be a factor in the further desiccation of areas that are already dry.
Australia's normal climate variability makes it difficult to be specific about which effects are attributable to global warming - not that theses are easily itemised even in far less complicated systems. Global warming, despite its name, is not merely a uniform rise in global temperature. The retained heat can lead to quite paradoxical outcomes. Some global-warming models, for instance, predict a massive cooling in northern Europe, as changes in the salinity of Atlantic sea water - resulting from the melting of polar ice - shut down the Gulf Stream.
An additional, complicating factor in the case of Australia is the effect of aerosols - fine droplets of water, or particles of dust, soot, pollen, and the like, which can remain suspended in the atmosphere for weeks at a time or longer. Beate G. Liepert, a climatologist at Columbia University's Lamont-Doherty Earth Observatory in Palisades, New York, studied the effects of atmospheric aerosols in combination with global warming.
With a computer model developed by Johann Feichter and Erich Roeckner at the Max Planck Institute for Meteorology in Hamburg, Germany, and Ulrike Lohmann of the Swiss Federal Institute of Technology in Zurich, Liepert found that atmospheric aerosols make Australia drier. Aerosols, like greenhouse gases, contribute to global warming. The warmer the atmosphere, the more water it can hold. And in general, if water is locked up in the atmosphere, global rainfall is reduced, including the rainfall over Australia. So as El Niño brings dry times, aerosols make Australia's climate even drier.
Intriguingly, Liepert discovered that the effects of aerosols, like those of global warming, are uneven: they do not weaken India's monsoon system, for instance. In fact, she predicts the combined effect of greenhouse gases and more aerosols in the atmosphere would make the Indian monsoon wetter, even as it makes Australia drier.
In spite of such progress in understanding the system, much work remains to be done. "We've only known about El Niño for the past 20 years," says Swetnam, "and only better understood it in the past 10." And, he adds, there are other weather cycles that climatologists are only beginning to recognise.
For example, some researchers believe the frequency and strength of El Niño and La Niña episodes are influenced by something called the Pacific Decadal Oscillation (PDO), which occurs in the North Pacific Ocean. Unlike the tropical Pacific's El Niños and La Niñas, which last for relatively short periods, PDO's two alternatives - 'positive' and 'negative' - each last 20 to 30 years.
During a positive PDO, the waters in the central north Pacific are cool, and the waters along the west coast of North America are warm. The converse is true with the negative phase. During the past century, PDO was in its positive phase from 1925 to 1946 and again from 1977 to 1997. The frequency of El Niño episodes increased during the later period and two of the strongest El Niño episodes on record occurred in 1982-83 and 1997-98.
However David Jones also emphasises the uncertainties. No one knows, he says, whether 90 per cent or 10 per cent of the changes in rainfall are the result of human-induced climate change. Yet he says, "It is now fairly clear that the current large and complex patterns of climate change are the result of aerosol changes, ozone change and greenhouse-gas changes, with a component of natural variability thrown in for good measure".
