Turns out, it is Chinese mine fires.
Via Marginal Revolution:
China alone loses between 100 million and 200 million tons of coal each year to mine fires, as much as 20 percent of their annual production, according to the International Institute for Geo-Information Science and Earth Observation, based in Enschede, Netherlands. The Institute estimates that carbon dioxide emissions from these fires are as high as 1.1 billion metric tons, more than the total carbon dioxide emissions from automobiles in the United States. Second to China is India, where 10 million tons of coal burns annually in mine fires, contributing a further 51 million metric tons of carbon dioxide to the atmosphere.
I guess that means we call all feel better about cruising around with the our windows down and the A/C blasting.