The inferno engulfing southern California is a stark reminder that this region is made for fire. The beautiful rolling hillsides are a powderkeg of dead leaves and dry wood, and the Santa Ana winds this week supplied the ignition. Each autumn, these hot, dry winds roar down like blowtorches from the high deserts and squeeze through mountain valleys with gusts and unpredictable eddies that send flames shooting off in all directions. It’s a ludicrously simple equation – explosive fuel and scorching winds equals big fires.

The fires are a violent and highly effective way of recycling dead vegetation into ash that fertilises the soil. They purge the land of pests and diseases, and open up woodlands to sunlight, giving the chance for fresh new vegetation to sprout up. Fire is so natural that many Californian plants rely on a good blaze. The famous sequoias grow their seedlings best on deep-burnt sites free of grasses and other plants; the burnt remains of scrub oak may look like charcoal but they resprout from roots or branches; and lodgepole pine trees seal their cones with a wax that only melts when the fires come, releasing their precious seeds.

As the firefighters play King Canute in the forlorn hope of turning back the tide of flames, the only surprise is why so many people choose to live in this combustible place, in homes mostly built of wood.

Conflagrations on the scale of this week’s disaster are not uncommon.

Only four years ago another inferno sparked by Santa Ana winds destroyed 3,640 homes and killed 14 people. In fact, almost the entire western US is a fire hazard. A firestorm in 1998 in Yellowstone National Park in the northwest of the country destroyed an area roughly half the size of Wales, with flames as tall as tower blocks.

In pioneering days, heat, drought and fire were a grave threat. In October 1871, for instance, an exceptionally dry summer and autumn sent wildfires raging across much of the northern regions, the Rockies and the prairies. On October 8 the forests along the shores of Lake Michigan exploded into flames and wildfire swept the Wisconsin town of Peshtigo. Giant flames created a howling firestorm as air was sucked into the conflagration, with winds that blew at speeds of up to 80mph.

“Wave after wave of flame and masses of fire, with an awful roar,” was how one eyewitness described it. People jumped into the Peshtigo River, but many were suffocated by the lack of oxygen in the burning air. The entire town was destroyed, killing 1,182 people, the greatest death toll by fire in US history. That same week, the desiccating winds also fanned an inferno in Chicago that left 100,000 people homeless and 300 people dead.

But the difference now is that much of the western US has been in drought for almost a decade. That has helped to stoke up the wildfires, and much worse. Forget talk of what global warming might do in 50 years’ time – already large swaths of the West are parched dry as temperatures grow warmer; the mountain snowpack is dwindling and melting earlier; mountain glaciers are disappearing; and drought is killing trees by the millions.

The West depends heavily on snowpack because the melting snow provides three quarters of the water in streams. Over the past 35 years, temperatures across the region have inched up, melting the snow as much as three weeks earlier. Los Angeles has just had its driest 12 months on record, just 8.1cm (3.2in) of rain in the year ending June 30, an alarming statistic when you consider that a desert is defined as having less than 10in of rain on average per year. The Sierra Nevada mountains, which usually provide Los Angeles with half of its water, have supplied just a fifth this year.

But while the rivers dry up, demand for water soars. Not only has southern California seen a growth in its population of more than double the national average in the past 50 years, but neighbouring states such as Nevada and Arizona are also experiencing population booms.

Of course, long droughts have happened before. Tree rings and river sediments reveal that in medieval times droughts lasting decades struck during some 400 years of hot climate. The trouble today is that temperatures seem to be climbing even higher, and the strains on a booming population are reaching crunch point.

Cities such as Tucson, Arizona, are built in a desert, where golf courses, lawns and swimming pools guzzle up precious water supplies, fed largely from the Colorado River. Unfortunately that river also supplies 30 million people across seven states and Mexico, including cities such as Los Angeles and Phoenix, and it is fast running low on water. The Colorado has been in drought for more than a decade and is flowing less than during the Dust Bowl years in the 1930s. The river’s water management now relies mostly on prayer. “Nature never intended to support this many people here,” said David Nahai, president of the Los Angeles water and power commissioners. That shortage of water pits the needs of agriculture against those of city dwellers.

But this week’s fires in southern California are not peculiar to the United States. Identical problems are afflicting Australia, where in December 2002 bushfires raged around the outskirts of Sydney. Southeastern Australia is suffering its worst drought for a thousand years and the Murray-Darling farmbelt, the key agricultural region of Autralia, is in crisis. And closer to home, wildfires have raged out of control through the Mediterranean almost every summer, notably in Greece this year. Fire is now a truly global problem, and a warning of the changing climate.

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