Government agencies have estimated that there is about 850 billion to 998 billion tons of coal in the ground that can be economically recoverable.

Not so, says David Rutledge, the Kiyo and Eiko Tomiyasu Professor of Engineering at Caltech, who has done his own estimates and showed them off this week at the American Geophysical Union in San Francisco.

Rutledge estimates that the Earth had only 662 billion tons of recoverable coal in the first place and around 59 percent of the total remains. Thus, the real estimate of existing reserves is closer to 400 billion tons. (The earth, he added, contained the equivalent of 1 trillion tons of oil before the industrial age began.) He came to the conclusion by analyzing production data, similar to how M. King Hubbert in 1956 predicted U.S. oil production would peak in 1970. It peaked in 1971.

That number, to some degree, is good news for the renewable power industry. Nearly 400 tons of coal would be enough to provide electricity for decades. It would also add huge amounts of pollution to the atmosphere. Still, it makes a peak for coal more tangible and realistic and thus can prompt policy makers and investors to take even more interest in things like solar thermal power or nuclear.

How come his estimates differ so much from the officially sanctioned ones from government? Secrecy and national pride play a huge part (see Plentiful Coal, Not Peak Oil Is Greatest Global Warming Threat and Global Warming Talks Pose Big Impact on Greentech). Russia analyzes its own coal estimates but the real number is treated as a state secret. Sometimes it slips.

China also has forwarded only two estimates of its reserves to the World Energy Council and the second, later estimate was higher than the first. (The original estimates of China's coal reserves were published by Germans and Europeans working there before the 1949 revolution.)

The U.S. has done a better job of estimating reserves, but slips too. Paul Averitt controlled the process for estimating U.S. reserves from the late 1940s through the early ‘70s. He retired in 1974.

"Present estimates of coal reserves are based upon methods that have not been reviewed or revised since their inception in 1974. Recent programs to assess reserves in limited areas using updated methods indicate that only a small fraction of previously estimated reserves are actually minable reserves," reads a 2007 National Academy of Sciences report.

Interestingly, Wolfgang Zittel and Joerg Schindler of Energy Watch Group have noted that various countries have been lowering their coal estimates in recent years.

Technology has also cut down the ability to get coal out of the ground. Decades ago, miners dug it out of seams. They could get into smaller cracks than machinery can. It's one of the rare examples of technology doing a less thorough job.

Coal, like oil, peaks in production. In Britain, coal production peaked in 1913.

A big problem with containing coal consumption could arrive in the anticipated peak in global oil production, which many believe will occur in a decade or so. If oil peaks before electric cars or biofuels are ready, energy companies might turn toward coal-to-liquids technologies to keep cars and trucks on the road. That would increase emissions because coal-to-liquid fuel is dirtier and some energy gets lots in the conversion, according to Ken Caldeira, from the Carnegie Institution at Stanford University.