Understanding the drivers of change

Ballarat Courier
Friday 16/5/2008 Page: 26

0NE of the best natural events that a climate change sceptic may draw on to highlight their cause is a heavy snowfall or good ski season. How can the planet be warning if the ski season at Mt Buller is the best in decades one may ask? To address this question we need to understand the difference between variability and change, and, in many parts of the world, including south-eastern Australia, to separate these two we need more than a few decades record of climatic conditions.

So what drives variability and what drives change? Since the 1960s we have accepted that the continents move on large plates and the suture lines between these plates are zones of earthquakes, volcanoes and the building of mountains. Over tine scales of hundreds of millions of years, continents have coalesced and split apart impacting upon sea and air currents and continentality-the effect of distance from the sea on climates.

Over this time period, geological change has brought about climate change. Certainly since the breaking up of Gondwana, Australia's climate has been affected by our northward, snail's pace, migration. The drying up of Australia 30 million years ago was due, at least in part, to the opening up of the Southern Ocean that incrementally reduced the passage of tropical air to the pole, leading to the refrigeration of Antarctica and the capturing of much surface water as ice.

But even back then the region we know as Ballarat would have experienced cold wet days, and warm dry ones-we just can't see it because the geological record rarely captures evidence at such fine time fractions. Of course the reverse is true for the present day. We have at our fingertips high resolution records of multiple climate variables from a vast network of meteorological stations. So we know, or at least have the capacity to know, much about our present weather, which, integrated across months, years and decades, describes our present climate.

So we can regularly experience the kind of variability that is invisible to the geologist. What the contemporary "cloud spotter" has trouble with, however, is time depth. Can you remember the big drought before this one? What about the one before that? The last big flood was in 1974-76, and before that, the "biggest of all", was 1956-58. Others occurred in 1917 and 1870 and there was a large drought in 1862-65. But what detail can we access of the climate extremes of the 19th century.

Were they unusual or representative? Can we compare the written experiences of that time with ours today when so much else has changed in the meantime? Essentially, what we are witnessing is variability, and, given the prevailing influence of El Nino- La Nina cycles on our climate over annual to decadal cycles, our personal experiences are too short to assess whether we have actually experienced change.

The conclusion of the Intergovernmental Panel on Climate Change - that we are already experiencing climate change and that dangerous climate change is likely if we do not act radically - are based on the best available data and modelling. But how do they deal with the problems of the short-term bias in climate data and experience? Their understanding comes from the inter-relationship between past and future climates that are both underpinned by our knowledge of contemporary climate processes.

Hidden away, archived as if an historical text book, are records of past climate. Better known are the ice cores that contain evidence of past CO2 concentrations but, closer to hand in Tasmania, and near Terang and Camperdown, are sites where cores of different types have revealed much about recent change - and a little about the variability too.

These enable its to place the recently collected instrumental data into a longer-term context. The instrumental data also, for many years, have enabled us to understand climatic processes. These have been built into climatic models that enable us to predict climatic responses to changes in the drivers of climate. The complexity of climate systems is great so you can imagine the computer power needed to run the models.

Confidence in the models can be enhanced if they can be run on the conditions of the past, and indeed, if they then can recreate the climates experienced under those conditions. So the models that are generating scenarios of our climate future draw heavily, not only on the present, but also the deeper past. We can be confident then, that the models, which are clearly rich in resolution, are also time rich, being built up from many records of the biggest snowfalls, floods and droughts of the past, and even more of the conditions in between.

* Peter Gell is Professor of Environmental Science and director of the Centre for Environmental Management, School of Science and Engineering at the University of Ballarat.