BEIJING (REUTERS) - Chinese leaders dazzled the world by clearing the skies as if by edict before the Beijing Summer Olympics in 2008. Fast forward to January 2013, and the government seems powerless against those same skies, tarnished by an opaque, toxic cloud that has smothered the city for nearly a week.

The number-two leader in the country's Communist Party hierarchy, Mr Li Keqiang, appealed this week for Beijing's 20 million residents to show patience during what he said would be a "long-term" clean-up.

Lower-level officials took emergency steps to cut traffic and factory emissions to clear the worst outbreak of smog on record, but the moves are likely to bring only temporary relief from a chronic problem that has been years in the making.

Why have conditions deteriorated so drastically?

Environmentalists and analysts suggest a complex mix of causes, from an over-reliance on heavy industry and an addiction to dirty coal, to poor enforcement of pollution laws, hundreds of thousands of new cars on the roads and incentives for local officials to promote economic growth at all costs.

China's capital has been enveloped in smog since Friday, and the municipal government warned residents to stay indoors after pollution readings hit record rates late on Saturday.

In the face of widespread public anger and rare media criticism, the government said it would force some vehicles off the road and temporarily close dozens of factories. But environmentalists say more comprehensive solutions are required.

"It is really just a temporary measure, and in the longer term you really have to get at the root causes like coal-burning factories," said Mr Ming Sung, chief Asia-Pacific representative with the United States (US)-based Clean Air Task Force.

KING COAL, DIRTY COAL

Coal production and consumption have tripled since 2000, while steel output is expected to have reached around 720 million tonnes in 2012, over five times the 2000 rate.

Such breakneck expansion has come at a huge price.

A sustained effort to reduce dependence on heavy industry, and on the fossil fuels that sustain it, is still necessary, said Yang Fuqiang, a former government energy policy researcher and now senior adviser at the Natural Resources Defence Council.

China plans to cap total coal consumption at around 3.9 billion tonnes by 2015, but the government, environmental critics say, needs eventually to force through a cut.

"Enough is enough - last year we burned more than 3.6 billion tonnes of coal and some people expect that to increase to 4 or 5 billion tonnes, but that is impossible," Mr Yang said.

"We have to say when coal consumption will start going down.

This is not just for Beijing but the whole of China."

NOT JUST BEIJING

In the run-up to the Olympic Games in 2008, authorities in Beijing replaced coal with natural gas in some power stations and for indoor heating in parts of the capital. The city also temporarily shut factories and forced the biggest industrial firms to depart from heavily populated central districts.

One of the city's flagship enterprises, the Shougang Group, relocated all its steelmaking facilities to the new port of Caofeidian in neighbouring Hebei province 200 km away.

But such moves simply shifted rather than eliminated pollution sources, and air quality has continued to deteriorate.

"You can't tell pollution not to come over to my territory - this really needs to be nationwide," said Mr Sung.

The China Academy of Sciences estimates a quarter of the lethal PM2.5 emissions drifting across Beijing originate from beyond its borders, with the industrial heartland of Hebei province, which surrounds the capital, a major culprit.

Hebei is responsible for more than a quarter of national steel output - an industry that accounts for a quarter of the nation's total coal consumption - and it is also a major cement producer. Its cities, including Shijiazhuang, were at least as badly hit as Beijing over the weekend.

GROWTH AT ALL COSTS

China's efforts to tackle pollution have not been successful because most local governments, including Beijing, still identify industrial growth as their main priority, said Mr Yang.

"The Beijing Development and Reform Commission asked me to give them ideas how to make Beijing much cleaner while still maintaining economic growth. I told them they have to change their ideas." Much of Beijing's industry has been shifted out of the city centre, but with most still in the suburbs, the move did little to improve overall air quality, especially when the pollution is trapped by weather conditions - as it was over the weekend.

Coal-fired power stations remain in operation in the heart of the capital's business districts and Beijing continues to vie with rivals like Tianjin for heavy industrial projects.

Although Beijing could develop cleaner, high-tech options, such a transformation would be even harder in the mining belts of Shanxi, Shaanxi and Inner Mongolia, regions that share the notoriously polluted coal region known as the Black Triangle.

Mr Sung said the technologies already exist to clean up coal and power production in the region, but more incentives were needed to implement them. China has already made it compulsory to install desulphurising facilities at power plants, but regulation has been weak and some plants have been accused of disabling equipment in order to cut costs.

Beijing could take other steps relatively quickly, including mandating use of effective catalytic convertors and filters in vehicles, and implementing long-term guidelines to reduce traffic congestion. Despite current restrictions, about a quarter-million new cars are added to city roads each year.

The government also needs to tackle the problem of heating, with most residential buildings outside Beijing's second ring road still reliant on coal during the freezing winters.

Premier-designate Li said on Tuesday that tackling pollution was a "long-term process", but Yang said it needed to be treated as a national emergency akin to London's "killer smog" of 1952, which led to new laws that transformed Britain's skies.

"If the Chinese still say this is long-term, I don't think they will clean up. Ordinary measures no longer help and I think Beijing has to adopt a more emergency response."