Last night the mercury fell to –2°C in the mallee. Cold on the extremities, but not cold in the extreme. Cold in the extreme? Last seen, winter ’82.
Every winter, the small town of Ouyen – a grain silo, roadhouse, general store and little more – gets about 18 frosts. Most years, the coldest night is a chilly –1°C. Thirty two years ago, the mercury plummeted.
In the early 1980s, Australia was crippled by drought. Between dust storms and bush fires, El Nino brought clear days and cold nights. Friday morning, June 4th 1982 was colder than usual: –2.2°C. The next five nights were colder again. Saturday hit a record low: –4.8°C. Pity the publican hosing the footpath outside the Victoria Hotel.
A month later, the frosts returned: –2.2°, –5.8°, –3.2°, –2.4°, –5.0° and –2.0°C, night after night. In Ouyen, –5.8°C isn’t just cold on the extremities. It’s cold in the extreme and extremely rare. Since records began, it has happened just once, on 17th July 1982.
Ouyen isn’t the coldest place in the mallee. In June ’82, the weather station outside the old ranger’s house at Wyperfeld National Park hit –5°, –7°, –8°, –7°, –6° and –5°C. In the cold snap of July, the mercury sunk to a record low, –8°C. The next day, sky clear once more, things mostly went back to normal.
‘Back to normal’ and ‘same as it ever was’ are two very different things. Exposed to extreme cold, something snapped in the mallee. One observer wrote,
The impact on the vegetation was dramatic. Thousands of hectares of mallee scrub and heathland were devastated, especially in the interdune swales.
The warm thermometer
The standard technique for measuring temperature is to hang a minimum-maximum thermometer in a white slatted box, 1 to 2 m above the ground. The box is called a Stevenson Screen; named after Thomas Stevenson, a Scottish lighthouse designer (that’s an exclusive niche, I hear you say) who designed the prototype in 1864.
On winter nights it’s warmer inside the box than out. Cold air rolls past the screen to the lowest point in the landscape. Down past the ankles, to the frost hollows, the swales, between the dunes of mallee sand.
Nobody knows how cold it got in the mallee swales in winter ‘82. One researcher estimated –13°C. Whatever the number, the drought-stricken plants hadn’t felt that cold for a long, long time.
The iceberg in the ice-chest
Have you ever put a lettuce in the freezer? Gourmet salad begone. As the water in a lettuce leaf freezes and expands, the cell walls break, the cells die, and your defrosted iceberg turns to sludge; to something you could drizzle across a salad. Cold-dwelling plants can’t be so hyper-sensitive. They sit in the freezer every winter.
Desert Banksia, Banksia ornata, is a short stocky shrub, 2-3 m tall. It grows across the southern mallee from the low swales to high dune crests. The fat yellow honeysuckle flowers feed pygmy possums, honey-eaters, moths and bees.
Curious scientists once put Banksia ornata seedlings in the deep freeze, at –20°C every night for a week. On the seventh day, they thawed and sliced each leaf, stained the cells with Evans Blue, and inspected the sections under a microscope. Less than 5% of the cells showed any damage. The researchers reported calmly:
… foliage of this species appears to be remarkably resistant to repeated freezing and thawing…
Yet the cold snap of ‘82 broke the banksias. Desert Banksias and other species died across huge areas.
With the exception of sporadic survivors, all adult [Banksia] ornata in frost-affected swales were eliminated (O’Brien et al. 1986).
And they never came back
Fire, not frost, fuels the life cycle of Banksia ornata. Hot fires kill plants and warm the resin that binds closed their seed cones. The heated resin melts, the cones open, seeds fall to the ground, and new seedlings grow.
The frosts of ‘82 failed to melt that resinous glue. Plants died but the cones stayed closed. Few seeds fell to the ground and fewer seedlings regenerated. The Banksias disappeared from the swales; their evidence consumed by fires that later burnt the dead, frost-bitten plants.
In many parts of the Park, the only surviving adult [Banksia] specimens are now located on dune crests and upper dune slopes above the level affected by the frosts (O’Brien et al. 1986).
The white witch stays
Could that extreme event, that super-cold frost, occur again in a world of global warming? As temperatures rise, frosts will surely fall. Logical as that may seem, it isn’t happening. At least, not yet.
Steven Crimp and colleagues from CSIRO studied weather records from 1960 to 2011 from across south-east Australia. In a new paper, they discovered that frosts are becoming more common, not less common, in inland areas below 100 m altitude; an enormous region that includes the Mallee and much of north-west Victoria and south-west NSW.
Surprisingly, average temperatures are rising and frosts are getting more common. More surprising still, the frost season is getting longer.
Consistent across southern Australia is a later cessation of frosts, with some areas of southeastern Australia experiencing the last frost an average 4 weeks later than in the 1960s. (Crimp et al. 2014).
Crimp and colleagues weren’t concerned about Banksias. They worry about wheat. Compared to a Banksia seedling, wheat is a wimp. Like a lettuce in a freezer, a young ear of grain can be ruined by a single night of wild frost. Not surprisingly:
This apparent ‘paradox’ of minimum temperature warming but increasing frost extremes is of significant concern to various agricultural industries. (Crimp et al. 2014).
The alarming paradox has a cause. Atmospheric circulation systems are moving. As high pressure systems move south, they create clear, cold and frosty nights in the mallee. Which is why it’s so cold on the extremities in our thin nylon tent.
Climate change provides no respite from cold nights in the mallee. But what of nights ‘cold in the extreme’? Will global warming mean the end of –13°C in a low mallee swale? Truth is, we don’t know.
The story of climate change is partly about changes in average temperatures. But it’s equally a story about changes in extremes. About the increasing frequency of unexpectedly severe events. The rare events that change ecosystems. Events that kill desert shrubs and make bats and birds drop from trees. Events that are increasing not declining.
There’s a strange disconnect in an altered landscape. Perched on a mallee dune, resting from our walk, we gaze upon one of the most beautiful, natural landscapes I know. A landscape my brain compulsively re-populates with banksias. A species decimated not by persecution, human folly or greed, but by a change in the weather. What could be more natural? A short bout of extreme temperature, a few cold nights in a thin nylon tent.
As we wander, I contemplate the future. How will naturalists and ecologists in the year 2100 think about ecosystems altered by a ‘change in the weather’? Will they, like me, repopulate the view with vanished species? And what new words will they use to describe a world transformed by climate change? Will old terms like balance, equilibrium, intact, local provenance and climax be abandoned as archaic, redundant, quaint?
Or, accustomed to constant change, will they behold the most beautiful landscapes they know? And sit and rest, and enjoy the view that we left behind?
I have posted a series of photos that show the aftermath of the 1982 frosts in a later post, Bringing back the dead with old ecology photos.
All of the photos in the post are from the web. The top photo of the tent in frost is from the Camp in Mudgee website.
Crimp, S., Bakar, K.S., Kokic, P., Jin, H., Nicholls, N. & Howden, M. (2014). Bayesian space–time model to analyse frost risk for agriculture in Southeast Australia. International Journal of Climatology, early view. GRDC (2014). Frost risk on the rise despite warmer climate [web page].
O’Brien, T., Forbes, C. & Jobe, L. (1986). The impact of severe frost on Mallee eucalypts, Banksia ornata, and Leptospermum coriaceum at Wyperfeld National Park. Proceedings of the Royal Society of Victoria 98, 121-31. [pdf of full article]
O’Brien, T. (1989). The impact of severe frost. In: Mediterranean Landscapes in Australia, Mallee Ecosystems and Their Management, pp. 181-188. (Edited by J.C. Noble & R.A. Bradstock). CSIRO, East Melbourne. [pdf of full article]
- Bringing back the dead with old ecology photos
- Forgotten woodlands, future landscapes
- Ageing the mallee: a history in burnt trees