The drought engine

Adelaide, like the real Mediterranean, gives up on four seasons and settles for two: a mild, wet winter, and a long, rainless summer that can turn lethal. On 24 January 2019 West Terrace hit 46.6°C, Adelaide’s hottest day on record — then the wind swung off the sea and the temperature fell fifteen degrees within the hour. Furnace one hour, cardigan the next: not a freak, but this landscape’s engine running in the open.

The mechanism is an atmospheric traffic jam called the subtropical ridge, a belt of high pressure girdling the planet at around 30° latitude and building most of the world’s deserts. In summer it parks over southern Australia; descending air makes clear skies, barring the door to any relieving front. In winter it slides north, the door opens, and Southern Ocean fronts march in on the “roaring forties,” bringing Adelaide’s rain with them. That is a Mediterranean climate.

Only five small patches of Earth run their seasons this way: the Mediterranean basin, coastal California, central Chile, the Cape of South Africa, and southern/south-western Australia, of which Adelaide sits at the eastern edge. Together they cover under two per cent of the planet’s land, yet hold something like a sixth of its plant species (Cowling et al. 1996). No accident: guaranteed drought, then fire, is a brutal filter that, given deep time, breeds invention rather than bare ground. Unrelated floras converged on one toolkit — stiff, evergreen leaves that hoard water; buds under bark; seed locked in fruit only flame cracks; germination held until the seed tastes smoke (Keeley et al. 2012) — each assembling its version separately (Rundel et al. 2016), so the grey-green heath on the range front sat the same exam as the fynbos and chaparral.

The bargain has a hard edge. In 1865, after a ruinous drought, Surveyor-General George Goyder drew a line between reliable cropping country and country fit only to graze — reading the vegetation, not a rain gauge, marking where good grass gave out and saltbush took over (Goyder 1865). Goyder’s Line is the drought engine made visible on a map, and it was ignored within a decade: wet years lured wheat farmers north on “rain follows the plough,” and the returning dry collected what it was owed, leaving roofless stone farmhouses standing north of the Line to this day.

Everything downstream — hard leaves, fire scars on a stringybark trunk, the saltbush boundary itself — replies to this one belt of high pressure. Read the drought engine first, and the gradient stops looking like a list of facts and starts looking like one argument, restated in leaf, flame and line.