At a time when the world's top climate experts agree that carbon emissions must be rapidly reduced to hold down global warming, Italy's major electricity producer, Enel, is converting its huge power plant here from oil to coal, generally the dirtiest fuel on Earth.

Over the next five years, Italy will increase its reliance on coal from 14 to 33 percent. Power generated by Enel from coal will rise to 50 percent.

Capture and storage vital

And Italy is not alone in its return to coal. Driven by rising demand, record high oil and natural gas prices, concerns over energy security and an aversion to nuclear energy, European countries are scheduled to build about 50 coal-fired plants over the next five years, plants that will be in use for the next five decades.

In the United States, fewer new coal plants are scheduled to go online, in part because it is becoming harder to get regulatory permits and in part because nuclear power remains an alternative. Of 151 proposals in early 2007, more than 60 had been dropped, many blocked by state governments. Dozens of others are stuck in court challenges.

The fast-expanding developing economies of India and China, where coal remains a major fuel source for more than 2 billion people, have long been regarded as among the biggest challenges to reducing carbon emissions. But the return now to coal even in eco-conscious Europe is sowing real alarm among environmentalists who warn that it is setting the world on a disastrous trajectory.

Environmentalists aghast

They are aghast at the renaissance of coal, a fuel more commonly associated with the sooty factories of Charles Dickens' novels, and one that was on its way out just a decade ago. There have been protests here in Civitavecchia, at a new coal plant in Germany and at one in the Czech Republic, as well as at the Kingsnorth power station in Kent, which is scheduled to become Britain's first new coal-fired plant in more than a decade.

Europe's power station owners emphasize that they are making the new coal plants as clean as possible. But critics say that "clean coal" is a pipe dream, an oxymoron in terms of the carbon emissions that count most toward climate change.

"Building new coal-fired power plants is ill conceived," said James E. Hansen, a leading climatologist at the NASA Goddard Institute for Space Studies. "Given our knowledge about what needs to be done to stabilize climate, this plan is like barging into a war without having a plan for how it should be conducted, even though information is available."

Enel, like many electricity companies, says it has little choice but to build coal plants to replace aging infrastructure, particularly in countries like Italy and Germany that have banned the building of nuclear power plants. Fuel costs have risen 151 percent since 1996, and Italians pay the highest electricity costs in Europe.

In terms of cost and energy security, coal has all the advantages, its proponents argue. Coal reserves will last for 200 years, rather than 50 years for gas and oil. Coal is relatively cheap compared with oil and natural gas, although coal prices have tripled in the past few years. More important, hundreds of countries export coal - there is not a coal cartel - so there is more room to negotiate prices.

"In order to get over oil, which is getting more and more expensive, our plan is to convert all oil plants to coal using clean-coal technologies," said Gianfilippo Mancini, Enel's chief of generation and energy management. "This will be the cleanest coal plant in Europe. We are hoping to prove that it will be possible to make sustainable and environmentally friendly use of coal."

"Clean coal" is a term coined by the industry decades ago, referring to its efforts to reduce local pollution. Using new technology, clean coal plants sharply reduced the number of sooty particles spewed into the air, as well as gases like sulfur dioxide and nitrous oxide. The technology has no effect on carbon.

In fact, the technology that the industry is counting on to reduce the carbon-dioxide emissions that add to global warning - carbon capture and storage - is not now commercially available. No one knows if it is feasible on a large, cost-effective scale.

Green challenge

The task - in which carbon emissions are pumped into underground reservoirs rather than released - is challenging for any fuel source, but particularly so for coal, which produces more carbon dioxide than oil or natural gas.

Under optimal current conditions, coal produces more than twice as much carbon dioxide per unit of electricity as natural gas, the second-most-common fuel used for electricity generation, according to the Electric Power Research Institute. In the developing world, where even new coal plants use lower grade coal and less efficient machinery, the equation is even worse.

Without carbon capture and storage, coal cannot be green. But solving that problem will take global coordination and billions of dollars in investment, which no one country or company seems inclined to spend, said Jeffrey D. Sachs, director of the Earth Institute at Columbia University.

There are a few dozen small demonstration projects in Europe and in the United States, most in the early stages. But progress has not been promising.

Many have likened carbon capture's road from the demonstration lab to a safe, cheap, available reality as a challenge equivalent to putting a man on the moon.

It may be even harder than that. It's like a moon landing that must be replicated daily at thousands of coal plants in hundreds of countries - many of them poor. There is a new coal-fire plant going up in India and China every week, and most of those are not constructed in a way that it amenable to carbon capture, even if it were developed.

Plants that could someday be adapted to carbon capture cost 10 to 20 percent more to build, and only a handful exist today. For most coal power plants the costs of converting would be "phenomenal," concluded a report by the U.S. Environmental Protection Agency.

Then there is the problem of storing the carbon, which is at some level an inherently local issue. Geologists have to determine if there is a suitable underground site, calculate how much carbon it can hold and then equip it in a way that prevent leaks and ensures safety. A large leak of underground carbon could be as dangerous as a leak of nuclear fuel, critics say.