Most experts agree that the age of oil, amid dwindling resources and spiraling prices, will be over soon. There are different ways out of that dependence, one being the process of turning solid coal into liquid fuels.

The United States, thanks to huge domestic coal resources, could satisfy its energy needs for the entire 21st century with liquid fuels derived from coal, at less than $30 a barrel, Klaus Lackner and Jeffrey Sachs, energy experts at the Earth Institute at Columbia University, in New York, have said in their new paper.

"(With widespread use of coal liquefication) the long term price of liquid hydrocarbon fuels may be lower than it is today, even allowing for pessimistic forecasts for oil and gas reserves," the authors write. "Even with the most conservative assumptions about learning curves," they write, it is safe to assume that synthetic fuel derived from coal will cost "below $30 per barrel."

The most common way to convert coal into liquid fuels is the Fischer-Tropsch process, named after two German scientists who developed the technique in 1925.

To create the fuel, coal is mixed with oxygen and steam at high temperature and pressure to produce carbon monoxide and hydrogen. The second step, called Fischer-Tropsch synthesis, uses a catalyst to transform the gas into liquid synthetic crude, which is further refined. Along the way, mercury, sulfur, ammonia and other compounds are extracted and can be sold on the commodities market, according to Scientific American.

The only company in the world to use the coal liquefication process is Sasol in South Africa at a price between $35 and $50 a barrel. It produces a variety of synthetic petroleum products, including most of the country's diesel fuel.

While other techniques, such as coal gasification and gas-to-liquids are still cheaper, Lackner and Sachs believe the process could be employed on a large scale by the world's coal powers, mainly the United States and China, who together own 40 percent of global coal reserves, according to an estimate by British Petroleum.

The scientists argue that one key advantage of the Fischer-Tropsch process is that it also allows for lignites and other low-grade coals, which exist in much greater supply than high-level coals, to be turned into synthetic fuels.

In Germany, where high-grade coal is in short supply but low-grade brown coal is available in vast quantities (about 230 years at current rates of production), the technique could have a real impact, they write.

What gives Lackner and Sachs headaches, however, are the increased emissions of carbon dioxide from the combustion of fossil fuels into the atmosphere.

They believe that human-induced climate change such as global warming can be avoided through a global effort to capture and sequester carbon dioxide below ground. Such a program of geological carbon sequestration, they estimate, would cost less than 1 percent of gross world product by 2050, albeit research and development as well as investments started soon.

Thus, Lackner and Sachs say "the single most urgent step" is for the United States and Europe to work together with China and India (whose energy demands will surge over the next years) to perfect clean coal technologies and reduce emissions to a minimum; they also push for more stringent efficiency standards with automobiles, including introducing hybrid cars.

That is "far beyond what any nation is ready to embrace in the near future," William Pizer, energy expert at Resources for the Future, a Washington-based research organization, writes in his critique of the paper, after praising its more general findings.