26/08/2018

Is Climate Change Supercharging Drought In Australia?

University of Melbourne - Cameron Tampion

Australia, along with much of the planet, is baking under prolonged drought and record heatwaves. Crops are failing and stock are starving as the rains stay away. Wildfires are burning in every hemisphere. Even native animals are feeling the heat despite eons of adaptation to this wide brown land.
Climate scientists have long warned that climate change would make future droughts stronger and more frequent. But is human-caused climate change already to blame, or is the current drought just part of a natural cycle?
Sign rendered pointless by the 2007/2008 Australian drought. Rawnsley Park Station, South Australia. Image credit: Peripitus via Wikimedia
Rainfall is going south for winter
Australia is a continent of droughts and flooding rains. In other words, rainfall totals can see-saw from year to year while averages give an illusion of reliable weather. In reality devastating droughts litter Australia’s colonial period, and the stories of the First Nations’ peoples reveal their long history of survival through climate extremes. So what’s new?
Southern Australia depends upon autumn and winter rains, and these have declined over the period of European settlement. At least in part this is because westerly winds that bring winter cold fronts and storm systems have shifted south, dumping their rain on the southern ocean instead of parched paddocks.
The movement of these westerlies towards or away from the south pole is measured by the Southern Annular Mode (SAM). During positive SAM phases the westerlies are pushed further south than normal by a ridge of high pressure that descends from the subtropics, and southern Australia misses out on winter rains.
Positive SAM in winter is when high pressure systems move further south, pushing rain-bearing westerlies away from Australia. In a negative phase the highs are further north and fronts bring rain to southern Austalia. (Credit: Kate Doyle, ABC Weather. Used with permission.)
The SAM can switch quickly, and Victoria went from a dry early winter in 2018, to a cold and wet late winter. Nevertheless, the increased southward shifts in these weather patterns due to human influences means less moisture on land and greater vulnerability to the next factor.

The boy child grows up
In Spanish the phrase El Niño means ‘boy child’, but for Australian farmers it usually means above average temperatures and below average rainfall. The name El Niño is given to periods when trade winds that usually blow from east to west across the Pacific Ocean weaken, and warm surface water stays closer to South America instead of building up to Australia’s north.

Video: Bureau of Meteorology

 In an El Niño less rain forms near Australia and less falls on eastern Australia. About two thirds of El Niño events since 1900 have brought drought to at least parts of Australia. Some of the worst droughts in Australia’s colonial history coincided with intense El Niños, including the Federation drought of the early 1900s, the World War II drought, and the Millennium drought of the late 1990s and early 2000s.
While we’re not in an El Niño at the moment, there’s a real chance of one forming by the end of the year. This would turn up the dial on the current drought even further.
An El Niño brings a double whammy of desiccation to a landscape already denied rain by migrating westerly winds. Higher temperatures quickly evaporate what little rain falls, leaving less runoff for dams and rivers. And with little moisture left to evaporate, temperatures climb higher still.
Although the inner workings of the El Niño cycle are still being studied, it does look like they are strengthening with climate change. So recent droughts are probably worse than they would have been without climate change, and El Niño events will be even more intense in future. The boy child is growing up.

Attribution science – or pointing the finger
Attribution science is a new field that works out how much more likely an extreme weather event was due to climate change. But with no end in sight to the current Australian drought, it’s too soon for a final measurement of this event.
That’s not to say there wouldn’t also have been a drought without climate change. But climate change isn’t just making droughts more likely, it’s making droughts more likely to be extreme.
What we do know is the 2016-17 heatwave that seared new temperature records into the record books was at least 10 times more likely due to global warming. So when we look back on the length and strength of this Big Dry as a whole, climate change will have to be the prime suspect.

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