Answer:
According to other Wikipedia answers, breathing by humans produces approximately 2.3 pounds (1 kg) of carbon dioxide per day per person (http:/en.wikipedia.org/wiki/Carbon_dioxide#Human_physiology).
If there are approximately 6.8 billion humans on the earth, collectively they would emit 6.8 billion kg per day, or 6.8 million metric tons per day. In a 365 day year, humans would produce 2.482 billion tons from breathing.
The Energy Information Administration of the US government said in their May 27, 2009 International Energy Outlook 2009 that world carbon dioxide emissions were 29.0 billion metric tons in 2006 (http:/www.eia.doe.gov/oiaf/ieo/highlights.html).
Human breathing would then be contributing approximately 8.5% of all CO2 emissions.