SAN FRANCISCO — A new calculation of the world's coal reserves is much lower than previous estimates. If validated, the new info could have a massive impact on the fate of the planet's climate.

That's because coal is responsible for most of the CO2 emissions that drive climate change. If there were actually less coal available for burning, climate modelers would have to rethink their estimates of the level of emissions that humans will produce.

The new model, created by Dave Rutledge, chair of Caltech's engineering and applied sciences division, suggests that humans will only pull up a total — including all past mining — of 662 billion tons of coal out of the Earth. The best previous estimate, from the World Energy Council, says that the world has almost 850 billion tons of coal still left to be mined.

"Every estimate of the ultimate coal resource has been larger," said ecologist Ken Caldeira of Stanford University, who was not involved with the new study. "But if there's much less coal than we think, that's good news for climate."

The carbon dioxide emitted when humans burn coal to create usable energy is primarily responsible for global warming. Leading scientists think that the stability of Earth's climate will be dictated by how the world uses — or doesn't use — its coal resources. And the thinking has been that the world has more than enough coal to wreak catastrophic damage to the climate system, absent major societal or governmental changes.

So the new estimate, which opens the slim possibility that humankind could do nothing to mitigate carbon dioxide emissions and still escape some of the impacts of climate change, comes as quite a shock.

Rutledge argues that governments are terrible at estimating their own fossil fuel reserves. He developed his new model by looking back at historical examples of fossil fuel exhaustion. For example, British coal production fell precipitously form its 1913 peak. American oil production famously peaked in 1970, as controversially predicted by King Hubbert. Both countries had heartily overestimated their reserves.

It was from manipulating the data from the previous peaks that Rutledge developed his new model, based on fitting curves to the cumulative production of a region. He says that they provide much more stable estimates than other techniques and are much more accurate than those made by individual countries.

"The record of geological estimates made by governments for their fossil fuel estimates is really horrible," Rutledge said during a press conference at the American Geological Union annual meeting. "And the estimates tend to be quite high. They over-predict future coal production."

More specifically, Rutledge says that big surveys of natural resources underestimate the difficulty and expense of getting to the coal reserves of the world. And that's assuming that the countries have at least tried to offer a real estimate to the international community. China, for example, has only submitted two estimates of its coal reserves to the World Energy Council — and they were wildly different.

"The Chinese are interested in producing coal, not figuring out how much they have," Rutledge said. "That much is obvious."

The National Research Council's Committee on Coal Research, Technology, and Resource Assessments to Inform Energy Policy actually agrees with many of Rutledge's criticisms, while continuing to maintain far sunnier estimates of the recoverable stocks of American coal.

"Present estimates of coal reserves are based upon methods that have not been reviewed or revised since their inception in 1974, and much of the input data were compiled in the early 1970’s," the committee wrote in a 2007 report. "Recent programs to assess reserves in limited areas using updated methods indicate that only a small fraction of previously estimated reserves are actually mineable reserves.”

And don't look to technology to bail out coal miners. Mechanization has actually decreased the world's recoverable reserves, because huge mining machines aren't quite as good at digging out coal as human beings are.

With Rutledge's new numbers, the world could burn all the coal (and other fossil fuels) it can get to, and the atmospheric concentration of CO2 would only end up around 460 parts per million, which is predicted to cause a 2-degree-Celsius rise in global temperatures.

For many scientists, that's too much warming. A growing coalition is calling for limiting the CO2 in the atmosphere to 350 parts per million, down from the 380 ppm of today, but it's a far cry from some of the more devastating scenarios devised by the Intergovernmental Panel on Climate Change.

"Coal emissions really need to be phased out proactively — we can't just wait for them to run out — by the year 2030," said Pushker Kharecha, a scientist at NASA's Goddard Institute for Space Studies. "There is more than enough coal to keep CO2 well above 350 ppm well beyond this century."

The Intergovernmental Panel on Climate Change uses economic models that assume that the world will not run out of coal. Some IPCC scenarios show 3.4 billion tons of coal being burned just through 2100. That's more than five times what Rutledge thinks will be possible — and a good deal higher than the WEC's estimate for recoverable coal reserves, too.

On the other hand, if the world were really to encounter a swift and steep decline in accessible coal resources, it's unclear how humans could retain our current levels of transportation, industry and general energy-usage.

So, even if coal were to run out and the most dangerous climate change averted, the imperative to develop non–fossil-fuel energy sources would remain.

"Peak Oil and peak gas and peak coal could really go either way for the climate," Kharecha said. "It all depends on choices for subsequent energy sources."

Image: Mining equipment at Black Thunder Mine near Wright, Wyo. AP Photo/Carson Walker

See Also:

WiSci 2.0: Alexis Madrigal's Twitter , Google Reader feed, and project site, Inventing Green: the lost history of American clean tech; Wired Science on Facebook.