Coal has been getting a bad rap recently. And rightfully so. The dense lumps of dead dinos and expired plants belch sooty, carbon dioxide-laden smoke when burned. Low prices keep the world addicted despite the mineral's dirty reputation. But coal's days may be numbered if a new study's projections pan out.

Governments and organizations around the world may be significantly overestimating just how much coal is available, the new study says. Such fallacious reporting is nothing new—the United States government happily overestimated oil reserves in the 1950s and 1960s until peak oil hit the lower 48. David Rutledge, professor of engineering and applied sciences at the California Institute of Technology, claims the same mistakes are being repeated with coal. His results, reported in a panel discussion at this year’s American Geophysical Union meeting, state the world only has 662 billion tons of coal, including reserves already exploited. The estimate is well short of the 1,027 billion tons remaining in proven and projected reserves, according to the World Energy Council.

This new lower number, Rutledge says, also means fewer fossil fuels to burn and less carbon dioxide to pump into the atmosphere. The net result? He predicts atmospheric carbon dioxide will peak at 460 parts per million before 2100, nearly 100ppm lower than the best-case scenario put forth by the Intergovernmental Panel on Climate Change (IPCC).

Rutledge's model puts a new spin on resource estimation. He fits lines to cumulative production reports, or the total amount of coal produced to date. By extrapolating those curves, Rutledge was able to revise estimates for recoverable coal reserves on each continent. In addition to settling well below the World Energy Council’s most recent estimate, his global sum significantly undercuts the largest number used by the IPCC.

Resource estimation is not a new science. The most famous example is Hubbert's peak, the result of a theory developed in 1956 by M. King Hubbert, a Shell Oil geoscientist. At that time, Hubbert correctly forecast peak oil would hit the continental United States in the late 1960s or early 1970s. Scientists and amateurs have spent countless hours since then attempting to predict global peak oil.

Coal has its own set of prognosticators. At the height of the Industrial Revolution, the United Kingdom commissioned a number of estimates on national coal reserves, many of which significantly overestimated the actual extant. The US has done the same, with early estimates claiming nearly unlimited reserves. Those numbers began to fall, though, under the direction of Paul Averitt, the coal man at the US Geological Survey. US government estimates bottomed out in 1974, the year Averitt retired, and have not changed since. The methods used by the government have changed little in the intervening years, according to a study released by the National Academy of Science.

Limited coal resources are certainly good news in the battle against global warming. But Rutledge warns the revised estimate is still far enough above pre-industrial levels to cause a planet-wide warming of 2-3° C. Many climate experts now believe the best way to avoid the worst effects of climate change would be to trim atmospheric carbon dioxide to 350ppm, an eight percent drop below 2007 levels.

Rutledge also has a PowerPoint presentation available on his site with details on the study.