Monday, October 6, 2008

Our hot, dry future

  • David Jones
  • The Age, October 6, 2008

THESE days, it can be hard to imagine how Melbourne ever earned a reputation as the gloomy, rain-filled capital of the south. But, growing up in the 1970s, my memories are full of muddy ovals, local creeks in flood and catching tadpoles in puddles that lasted for months on end. How things have changed.

Since 1996, each successive calendar year has brought the city below-average rainfall. With 299 millimetres recorded so far this year, and with just three months to go, it seems virtually certain that this year will become the 12th in a row that has failed to get to the average of 650 millimetres. September 2008 was the driest on record in Melbourne, and the outlook for the remainder of the year suggests that below-average rainfall will continue.

So why has it been so dry? The drought started in late 1996, and the subsequent El Nino years of 1997, 2002 and 2006 have each been particularly dry. Ordinarily, these events would have been interspersed with wetter years, but since 1996 the intervening periods have only approached average at best, with the deepening drought particularly evident in our reservoirs and stream-flows.

The overall impression is that the past cycle of droughts and flooding rains, which once characterised our climate in Melbourne, has been replaced by one of droughts followed by almost "normal" rains. Of course, the drought has not been helped by rising temperatures, which have increased losses through evaporation. The Bureau of Meteorology and CSIRO, in collaboration with researchers across government and industry, have been working to understand the drought.

There is much this research shows us, but there are still questions that remain unresolved. We do know that in Melbourne the 11 consecutive years of below-average rainfall is close to double the previous record run, which was set from 1979 to 1984, and almost triple the previous record of four years before that. Observations for Melbourne go back to 1855, meaning that this new record is highly significant.

We also know that over the past 11 years Melbourne's rainfall has been about 20% below the long-term average, and that south-east Australia as a whole has now missed out on more than a year's worth of its normal rainfall over the duration of the event. The run-off into Melbourne's dams has been 40% below average over this drought period compared with the longer term, while regional areas have fared even worse. And the drought hasn't ended.

A notable feature of this drought is the long series of failed autumn rains. Victoria, and indeed most of southern Australia, has experienced a substantial decline in its autumn rainfall since the 1970s. This is important because autumn rainfall "wets up" the soil in catchments, allowing winter rain and snow to flow into rivers. Decent autumn rain also gives crops and pastures a start after the heat and dry of summer. Victoria has now experienced eight dry autumns in a row, and indeed 16 of the past 19 autumns have received below-average rainfall.

The autumn drying trend was first noticed in south-west Western Australia in the mid-1970s, and has subsequently spread to the south-eastern states. This rainfall decline is driven by a rise in atmospheric pressures, and a weakening of cold fronts and low-pressure systems that once reliably brought rainfall to southern Australia. This shift in weather systems and rainfall has been linked by scientists to human-induced climate change, be it through greenhouse gases or changes in the ozone layer over Antarctica.

While we will continue to see the occasional wet autumn in the future, the chances are that we will not see a return to the wet autumns that were once commonplace. Over the past few years we have also seen a repeated failure of spring rains, which is a projected response to global warming across southern Australia.

The clearest signature of climate change in this drought is that of rising temperatures. Victoria has experienced substantial warming over recent decades as part of global warming. Last year was by far Victoria's hottest year on record, and Victoria has now warmed by about 1 degree since the middle of last century. Recent years have been up to 2 degrees warmer than those in the first half of the last century, a difference the equivalent of moving Victoria about 400 kilometres closer to the equator.

It is now 12 years since we experienced a year that was cooler than average for Victoria. Analysis from a recent CSIRO study using observations from the Murray-Darling Basin has shown that stream-flows may decline by 15% for each 1 degree of warming. With a further warming of about 1.5 degrees projected for Victoria by 2050, and further declines in our winter and spring rainfall, we could experience increasingly low stream-flows throughout this century.

Should Victorians view this drought as climate change? This drought is now far beyond our historical experience. It is very difficult to make a case that this is just simply a run of bad luck driven by a natural cycle and that a return to more normal rainfall is inevitable, as some would hope.

Climate change caused by humans is now acting to make droughts more severe and increasingly likely, while substantial warming of our climate is inevitable as a result of greenhouse gases already emitted and those that will be emitted in the decades to come.

Regardless of the underlying cause, the drought provides Victorians with a snapshot of a hot and dry future that we all will collectively face.

Dr David Jones is head of climate analysis at the Bureau of Meteorology.

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