The Mediterranean bargain (two seasons, not four)

Adelaide, like the shores of the actual Mediterranean, more or less gives up on four seasons and settles for two: a mild, green, wet winter, and a long, brown, rainless summer that can turn lethal. Nearly all the year’s rain falls in the cool half, roughly May to October, and then the tap shuts off. This is one of the rarest climate arrangements on the planet. Only five patches of the Earth run their seasons this way — the Mediterranean basin, coastal California, central Chile, the Cape of South Africa, and southern and south-western Australia — and Adelaide sits at the eastern edge of the Australian one.

The mechanism is a kind of atmospheric traffic jam called the subtropical ridge: a belt of high pressure girdling the globe at around 30° latitude, the same belt that builds most of the world’s deserts. In summer it parks over southern Australia, and descending air makes clear skies rather than rain, shutting the door on any relieving front. In winter it slides north, the door swings open, and Southern Ocean cold fronts march in on the westerly “roaring forties,” bringing the bulk of the city’s rain. Summer under the lid, winter in the draught — that, in a sentence, is a Mediterranean climate.

Those five small regions cover well under two per cent of the world’s land, yet hold something like a sixth of its plant species (Cowling et al. 1996). That is one of nature’s great disproportions, and no accident: months of guaranteed drought followed by recurrent fire is a brutal filter, and brutal filters, given a few million years, breed variety rather than emptiness. Plants that meet the terms converge on the same playbook wherever the climate turns up — small, hard, evergreen leaves that hoard water, and a fire-survival kit of resprouting buds, seed locked in woody fruit, and smoke-cued germination — so the fynbos of the Cape, the chaparral of California, the matorral of Chile and the mallee-heaths of southern Australia are, in their bones, independently arrived-at answers to the same hard question (Rundel et al. 2016; Keeley et al. 2012).

That is why this concept sits at the hinge between the deep-time chapters and everything living. It is not backdrop. Every duplex soil that waterlogs in winter and bakes in summer, every stiff grey-green leaf on the range front, every fire scar on a stringybark trunk, is downstream of this one ridge that comes and goes with the seasons. Read the Mediterranean bargain first, and the gulf-to-range gradient stops looking like a list of facts and starts looking like one argument, restated in rock, leaf and flame.