According to a study by the USDA, an average person's respiration generates approximately 450 liters (roughly 900 grams) of carbon dioxide per day.
1 pound = 453.59237 grams
Therefore the average person produces 1.984 pounds of carbon dioxide each day.
The current world population is roughly 6,705,320,595.
6,705,320,595 humans * 1.984 lbs/day = 13,303,356,060.48 lbs of carbon dioxide humanity produces every day by breathing.
13,303,356,060.48 lbs / 2000 = 6,651,678 tons of carbon dioxide humanity produces every day by breathing.
6,651,678 tons * 365 = 2,427,862,481 tons of carbon dioxide produced by humanity every year by breathing.
The world total of annual non-natural CO2 emissions is estimated to be 24,126,416,000 tons.
Total CO2 in the atmosphere: 2,700,000,000,000 tons
Total man-made non-natural CO2: 24,126,416,000 tons annually
Total man-made natural CO2: 2,427,862,481 tons annually
Now consider all the bears, monkeys, turtles, salmon, spiders, elephants, chickens, squirrels, whales, deer, snakes, trout, antelope, penguins, frogs, lions, donkeys, woodchucks, beetles, baboons, dolphins, anteaters, etc etc etc. all producing carbon dioxide.
If humans produce 2.5 billion tons of CO2 every year simply by breathing, how much does the rest of the animal kingdom produce?
In conclusion I don't feel that we should ignore the effects of our non-natural CO2 production. However in my own personal opinion I feel that the "humanity is causing global warming" zealots needs to take this CO2 source into greater consideration.
Just my $0.02
Please don't use this information as an excuse to reduce the number of CO2 producing creatures.
Sources:
Natural Human CO2 emissions: http://en.wikipedia.org/wiki/Carbon_dioxide
CO2 emission figures: http://en.wikipedia.org/wiki/List_of_countries_by_carbon_dioxide_emissions
World population figures: http://www.ibiblio.org/lunarbin/worldpop