How do we define drought? What causes them? And are they getting worse?
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| Animal tracks criss-cross the Southern Macquarie Marshes Nature Reserve in August. Credit: Wolter Peeters |
How severe is this drought? Is this the new normal? And what is the role of climate change?
What is drought?
Drought is about what's missing – rain. Or, as the Bureau of Meteorology defines it, an "acute water shortage".
Unlike other extreme weather events such as heavy rain or heatwaves, though, droughts are tricky to measure.
Queensland's latest government estimate is that two-thirds of the state is affected by drought. In NSW, it is just about all of the state – 98.4 per cent, according to the government. Victoria hasn't declared drought, but says central and east Gippsland and Millewa in the north-west are its main dry regions. Western Australia has had its third-driest start to any year – and the driest since 1936 – and northern South Australia has not had a drier January-October on record.
The weather bureau uses "rainfall deficiencies" as its measure. It looks at where rainfall is less than 10 per cent of historical averages and deems them to have "serious" deficiency. Those at less than five per cent are rated as "severe".
Another kind of measure is a so-called hydrological drought, which measures periods of very low river flow.
Rainfall is below average or less in large parts of Australia January 2017 to October 31, 2019  |
| Source: Bureau of Meteorology
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Australia's biggest dry spells include the Federation Drought (1895-1902), World War II Drought (1937-1945) and the Millennium Drought (1997-2009).
Measured on a range of time scales, the current drought is extreme. Some areas report record poor rains.
For Australia's food bowl, the Murray-Darling Basin, rainfall has averaged 887 millimetres over the 34 months to the end of October. That's "clearly the lowest on record", says David Jones, manager of climate services at the weather bureau.
Record heat has compounded the stress. The basin's mean temperatures for those 34 months is running at 1.65 degrees above the bureau's 1961-90 baseline, easily beating previous record highs, says Dr Jones.
Nationally, daytime temperatures for January to October are also at records highs, the bureau says. In the basin alone, mean temperatures were the hottest on record for that period too, for the third year in a row, says Dr Jones.
Perhaps the most unusual feature of the current drought, though, is the absence of cool season rainfall for three years running for much of the Murray-Darling Basin.
"That’s never happened in the instrumental record," says Michael Roderick, a climate researcher at the Australian National University. "They’ve never really had two failed winters in a row."
For the basin, just under 50 millimetres fell last winter, or less than half the 1961-90 average of 111 millimetres, weather bureau data reveals.
Is climate change playing a role?
If droughts can be hard to pin down, explaining their connection to climate change adds to the complexity.
The facts are that scientists cannot say definitively that a specific drought is caused by climate change, but they can say definitively that climate change makes the effects of droughts stronger and more damaging.
Andy Pitman, director of the ARC Centre of Excellence for Climate Extremes, recently ignited a brief firestorm over his comment to a business forum that "there is no link between climate change and drought".
Some media jumped on his views, prompting his centre to issue a belated correction saying he erred by leaving out one word, as in "there is no direct link between climate change and drought".
Indirect links should be cause enough for concern in a country with Australia's variable rainfall.The indirect links, though, should be cause enough for concern in a country with Australia's variable rainfall.
The weather bureau and CSIRO are very confident that rainfall during the so-called cool season from April to October is trending lower for both the south-west and south-east of Australia, as noted in last year's State of the Climate report.
Farmers rely on that rain to grow the winter crops that make up the bulk of the nation's output. Where winter rain is on the rise – in parts of the north and interior – the extra moisture is typically on top of a low base and in sparsely populated regions.
Climate change is blamed for accelerating the winds that circle around Antarctica, drawing storm tracks further south so some miss the mainland.
By contrast, for some areas in southern Australia, rainfall is increasing during the warmer months. That shift, though, comes as little consolation for farmers now reliant on winter harvests.
In 20 years, cool season rainfall has been the lowest on record in some places and above average in others. April to October rainfall 1999-2018  |
| Source: Bureau of Meteorology |
More summer rain "should green up the landscape because plants have more water when they need it most but it will dry up our rivers", Professor Roderick says.
While it's not clear how annual rainfall totals will change in a warming world, future droughts will be hotter when they do arrive, says Ben Henley, a climate researcher at the University of Melbourne.
"We're really quite concerned in southern Australia," he says. "Even if we get the same degree of annual rainfall, if that’s falling in the hot time of the year, that’s more likely to be evaporated off."
Is this the new normal?
Cutting-edge research includes work to investigate whether droughts such as the current one are likely to become more prolonged and more frequent.
One area of study is looking at flash droughts, the unusually rapid intensification of some dry spells.
"This event is shorter at the moment [compared with some droughts in the past] but very sharp," Dr Jones says.
A paper out this year by researchers, including the weather bureau's Hanh Nguyen, has found most of eastern Australia "suddenly changed from wet conditions in December 2017 to dry conditions in January 2018". It cites sheep farmer Kym Thomas, from Cunnamulla in the northern Murray Darling Basin in Queensland, who was forced to sell all her livestock in early 2018. Local sheep numbers in her region dived to their lowest in 100 years.
One smoking gun is that rainforests are now burning."By June 2018, they reported that all types of trees were dying, leaving a desert-like landscape of sand dunes replacing the normally vegetated scene," the paper says.
As plants dry or die,the risk of major bushfires increases. And, as plants also help moderate the local climate through a process called evapotranspiration, when they die another hand brake on the heat is removed.
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| The dried-out Southern Macquarie Marshes nature reserve in August. Credit: Wolter Peeters |
"It's quite possible ... we never came out of it," he says, adding a couple of wet years in 2010 and 2011 may have been "a bit of an hiatus in the middle".
Indeed, while drought is measured against historical averages, it may be time to redefine what we considered as normal. "One smoking gun is that rainforests are now burning," he says.
How much longer will this drought last?
Weather in Australia is being influenced by a pattern known as a positive Indian Ocean Dipole (IOD), which reduces the chances for convection and the formation of north-west cloud bands that typically bring good rains to inland Australia, especially the Murray-Darling Basin.
This IOD pattern typically starts to break down by November's end as the northern monsoon arrives. While this year's seasonal cycle may be later than usual, at least one curb on rainfall should soon be wound back.
The weather bureau's three-monthly rainfall outlook indeed starts to shift the odds in favour of closer-to-average rains over much of the country by the tail end of summer. Before then, though, most of Australia is highly likely to have a drier and hotter than average November-February.
And it will probably take a lot more than near-normal rain to have this drought declared broken. As Professor Roderick says, the Millennium Drought had really only two very dry years – 2002 and 2006 – but is considered to have lasted about a decade.
While places such as Sydney have seen a steep fall in dam levels, 50 per cent faster over the past two years than during the Millennium Drought, its ability to tap a desalination plant for 15 per cent of its needs means it is "effectively immortal for water", says Professor Roderick.
Not so for towns such as Guyra in northern NSW, which has just eight months' supply, even with full dams. Many other towns had just one to two years worth of water, a problem that this drought has exposed, he says.
Links
- The Science Of Drought Is Complex But The Message On Climate Change Is Clear
- Thirst Turns To Anger As Australia's Mighty River Runs Dry
- Drought Plan Must Factor In Climate Change
- More Than Climate Change Driving Queensland Fires, Explain Climatologists
- ‘Dead Things Everywhere’: Is Australia Facing The Summer From Hell?
- Farmers Call For National Strategy On Climate Change And Agriculture
- Extreme Weather Caused By Climate Change Has Damaged 45% Of Australia’s Coastal Habitat
- Drought Now Officially Our Worst On Record
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