Winter comes swiftly to China's far west, and the firefighters of Xinjiang are striking camp. For eight months a year, before snow and ice make their work impossible, they battle a deadly menace - raging coal fires, which throw up as much greenhouse gas as all the cars and trucks in the US.
At the UN Climate Change Conference in Bali, starting on 3 December, China's reliance on coal will come under closer examination than ever before. Yet few other than specialists realise that (according to the Netherlands Earth Observation Network), China's coal fires produce between 1 and 3 per cent of global carbon emissions.
China is blessed - or cursed - with abundant coal reserves. Rich seams run for 3,000 miles from the border with Kazakhstan to the East China Sea. Under the right conditions, coal ignites spontaneously, and fires burn downwards, acquiring oxygen through fissures in the rock and tiny spaces in the earth. Some fires have been blazing for centuries; Marco Polo wrote of "burning mountains along the Silk Road".
In shallow fires, flames lick up from caverns and smoke rises from vents. But others burn unseen far below, sometimes reaching temperatures of 1,000°C, travelling several metres a month and consuming millions of tonnes of coal.
The firefighters manoeuvre bulldozers over the burning surface coals, billowing dust mingling with smoke in a haze of methane, sulphur and other toxic gases, the temperature in the cab rising to 50°C in summer. Men in flimsy masks pump water to cool the ground, injecting mud and slurry down cracks in the rock to block the oxygen that feeds the fires. They cover the ground with soil, and measure the underground temperature through a tube until it has cooled to 70°C, when the fire can be deemed extinguished.
"The Chinese government is under pressure to reduce emissions, so we firefighters are also under pressure to speed up our work," said Cao Jianwen, a geologist with the Xinjiang Coal Fire Administration. In the past, the emphasis was on the pollutants produced, the hazards to people who lived in the vicinity and the wasted coal reserves, but now, with climate change in mind, the whole world is watching.
In Beijing, 1,500 miles away, Professor Li Jing stared at his computer screen, in effect looking down on the fires from on high. He is involved in a Sino-German project using remote sensing from satellites and aircraft to pinpoint coal fires. "When coal burns underground, the temperature rises on the surface. By using remote sensing imagery, especially thermal infrared technology, we can clearly locate the rising heat and quickly identify the fire area," he said.
China relies on coal for two-thirds of its energy. Such is the pace of economic growth, that its carbon emissions now exceed those of the US. Professor Li believes that extinguishing coal fires will reduce emissions far more rapidly than trying to save energy. But his images tell him that the fires are spreading. "In the Fifties and Sixties, coal mines were owned and operated by the state. Safety and fire prevention were a big part of a strict management regime, and there weren't many coal fires," he said. "But in the last few decades, many private mine businesses appeared, which ignore safety and fire prevention. As a result, we see fires everywhere now."
In the Wuda coalfield in Inner Mongolia, most fires did not erupt spontaneously but were sparked by careless mining. At a typical open-cast mine, diggers scrape away the soil to expose the seam. In the scooped-out section below, where coal has already been removed, smoke and flames escape from fissures; the soil has not been pushed back to prevent fire.
Government inspectors visit once a year, but reportedly make only half-hearted attempts to close unsafe or illegal mines. Most stop operations for just a few weeks. Mine bosses, in league with local officials, flout environmental laws with impunity. China's demand for coal is so high that power stations may not ask the provenance of supplies. Running a small mine is an easy way to get rich, and private mine owners either do not know or do not care about fires.
In Xinjiang, Cao Jianwen showed off the smooth, strangely whitish surface of Liuhanggou, Sulphur Valley, where the country's biggest fire used to consume an estimated 1.8 million tonnes of coal a year. A plaque mounted on a hill praises the firefighters who battled for four years until it was extinguished in 2004.
But, hidden by hills a few hundred yards away, smoke was pouring from vents in the ground. Tyre marks and hastily raked soil told the story - illegal miners had appeared, probably at night, hacked away a few tonnes, and reignited the coal.
Extinguishing coal fires is a major contribution China could make to cutting carbon emissions, but if the government fails to regulate mining, the work of its firefighters will be in vain.
Lindsey Hilsum is China correspondent for Channel 4 News