[Hankenstein2]

I don't know 3% seems small but significant...

"Across the world, thousands of underground coal fires are burning at any given moment. The problem is most acute in industrializing, coal-rich nations such as China.[7] Global coal fire emissions are estimated to cause 40 tons of mercury to enter the atmosphere annually, and to represent three percent of the world's annual CO2 emissions.[8]"

From the wikipedia article above