It seems like cigarette smokeing is not a significant cause of global warming (cant find a %) but look at these links
Cigarette Smoking Causes Global Warming « Brent Roos
But another site states
Smoking worldwide releases about 2.6 billion kilograms of carbon dioxide in the air every year. It also releases about 5.2 billion kilograms of methane every year. Tobacco growing, curing, and smoking all add to the greenhouse effect and global warming.
ScienceU: Tobacco & You (Global Warming)