Thousands of underground coal fires are burning out of control, oozing greenhouse gases through cracks in the ground.

Right now, thousands of coal fires are burning out of control around the world. The fires are heaving untold amounts of mercury, the greenhouse gas carbon dioxide (CO2) and other pollutants into the air.

The fires are notoriously hard to monitor; they tend to start at the surface but quickly scurry underground, only to ooze gases through soil and cracks in the ground. But an ambitious new study is now taking the first steps toward tallying their contribution to air pollution around the world.

Earlier this year, Allan Kolker of the United States Geological Survey in Reston, Va. and a team of scientists traveled to the Powder River Basin, a coal-rich region straddling the Wyoming-Montana border where dozens of fires are burning.

Using ground-based measurements, the team found that the Welch Ranch Fire emitted about 12 tons of CO2, and about 270 milligrams of mercury per day.

That's not much, but it's just one fire. Measuring the perhaps tens of thousands of coal blazes active in the world will be an arduous process, but the team hopes to speed things up by developing a method to fly over fires in an airplane and measure heat using an infrared camera. Carbon dioxide and mercury emissions should relate to temperature, which would allow researchers to calculate emissions from a fly-by.

For now, the numbers remain shaky.

Team member Leonard Levin of the Electric Power Research Institute in Palo Alto, Calif., said that fires could emit 200 tons of mercury each year, but he called the figure a "wild guess."

A report issued by the USGS earlier this year suggests just 48 tons are emitted annually, but even that is roughly equivalent to all of the mercury generated by coal-fired power plants in the United States.

People living in towns and villages near the blazes could be at higher risk of mercury or arsenic poisoning. Mercury also accumulates in fish and waterways.

Carbon dioxide figures are even more elusive. In the same report, authors (Kolker among them) estimated that China, the world's largest coal-producing country, lost anywhere between 10 an 200 million tons of coal a year to wildfires, or between 0.5 and 10 percent of national production.

Whether or not that's enough to seriously impact climate models of global warming remains to be seen.

"The field is wide open as to how important these emissions are to global climate," team member Mark Engle, also of the USGS said.

"It's certainly an issue," Stefan Schloemer of Germany's Federal Institute for Geosciences and Natural Resources (BGR) in Hannover said. Schloemer is part of a Chinese-German cooperative team attempting to map emissions from coal fires in China. "But there are hardly any real numbers as to how much is emitted."