AUSTRALIA has long been a land of droughts and flooding rains, and the past two years have been a good example of that well-known climatic pattern.
When the dams are full, the paddocks are green and the dry soil has turned soggy, we tend to forget about drought and its risks for our health and well-being.
However, it has always been wise to take a long-term perspective and to acknowledge that a few wet years don't mean we'll never see a severe drought again. Furthermore, with climate change now in the mix, it is even more important to take a broader, long-term perspective.
Climate change is influencing more than just droughts, as the recent CSIRO-Bureau of Meteorology State of the Climate 2012 report clearly outlines.
Temperatures over land and in the oceans continue to increase rapidly, sea levels are rising and extremely hot days have become more common. But it is the recent period of very wet, cool weather bringing floods to many parts of Australia that has grabbed the most attention in the past few months.
The Climate Commission's report on the science behind southeast Australia's wet, cool summer provides the broader, long-term perspective needed to understand the significance of the big wet.
The 2010-2011 period was the wettest two-year period on record in Australia. The exceptionally high rainfall was driven by back-to-back La Nina events, the phase of natural climate variability that periodically brings heavy rainfall to eastern Australia.
La Nina events are associated with higher-than-average sea surface temperatures across Australia, which lead to higher rates of evaporation from the surface ocean and more water in the air for rainstorms. In fact, the sea surface temperatures to the north of Australia during the spring and early summer of 2010-2011 were the highest on record, very likely contributing to the amount and intensity of the rainfall.
The very high sea surface temperatures were also, in part, a result of the underlying global warming trend. We are not only seeing increasing temperatures over land, but the surface waters of the ocean are also heating up. For this reason, many scientists are concerned global warming may have contributed to the strength of the La Nina event and thus to the heavy rainfall and flooding.
In the longer term, it is likely that the wet years of 2010 and 2011 were only a short interruption in the drier conditions we've observed in southeast Australia during the past 40 years.
The bulk of the rain in the past two years fell in spring and summer, not in the normal autumn-winter period of wetter conditions that typifies the southeast's climate. Changes in the timing of rainfall, not just the amount, have important consequences for farming and water resources.
The drop in autumn-winter rainfall, primarily since the mid-1990s, is associated with a southwards shift of the rain systems from the Southern Ocean that normally provided the rain for this season. This same shift is largely responsible for the reduction in autumn-winter rainfall in southwest Western Australia.
This emerging pattern of long-term drying across southern Australia, exacerbated by hot days and weeks and periodically interrupted by very intense rainfall and flooding, comes as no surprise to climate scientists. It is entirely consistent with what we expect from a changing climate.
How will droughts and floods change in the future?
Again, a long-term perspective is essential. Extended dry periods are expected to increase in southwest and southeast Australia by the end of this century, increasing the risk of drought.
On the other hand, it is more likely than not that heavy rainfall events will also become more frequent across much of Australia. So when long dry periods are interrupted by welcome periods of wet weather, the rain is more likely to fall as heavy downpours than as extended drizzle.
It is virtually certain the global average temperature will continue to rise through the 21st century. This is likely to increase the number, length and intensity of heat waves across many regions of Australia. Very hot days coupled with extended dry periods create significant stress for plants and animals, and pose serious risks for human health and well-being.
The quintessential Australian climatic pattern of intense droughts and flooding rains will still be with us in the future. But the added risks associated with climate change make it even more important that we plan and act on a careful analysis of the risks that climate variability and climate change together bring.
The magnitude of these risks ultimately depends on the effectiveness of global emission reduction efforts, including by Australia. The transition to a clean energy economy, which is gathering speed in many parts of the world, gives us great hope that we can minimise these potential risks.
Will Steffen is Climate Commissioner and executive director of the Australian National University's Climate Change Institute.