In practice, of course, each additional person contributes a net gain of carbon to the atmosphere, since we combust far more carbon than our bodies sequester. Physiologically speaking, the existence of people and our livestock is removing carbon from the atmosphere, albeit at an incredibly slow rate. Bones decompose very slowly, and some tiny amount of your carbon-how much depends on the conditions of your burial-will ultimately remain sequestered in the ground. Even when a person dies, he takes a little carbon with him. So every time we add a billion people to the planet's population (which we're now doing every 12 years), we end up pulling 10.8 million tons of carbon out of the atmosphere-or enough to offset the annual output of almost 9 million cars. If the average human weight is around 120 pounds-that's the Explainer's very rough estimate, encompassing both children and adults-there are about 21.6 pounds of carbon stored in the average person. How modest? We're each about 18 percent carbon by weight. This means that the human body-like all animals-is a very modest carbon-sequestration device. The amount of carbon that a human breathes out is exactly equal to the amount of carbon he takes in minus the amount of carbon that contributes to the person's body mass. In fact, the loop is not entirely closed. Experts are quick to point out that this figure is meaningless, since human respiration is part of a “closed loop cycle” in which our carbon dioxide output is matched by the carbon dioxide taken in by the wheat, corn, celery, and Ugli fruit that we eat. So the human race breathes out about 8.5 percent as much carbon as we burn. International carbon dioxide emissions from fossil-fuel combustion for 2008 topped 34.7 billion tons. (The exact quantity depends on your activity level-a person engaged in vigorous exercise produces up to eight times as much CO2 as his sedentary brethren.) Take this number and multiply by a population of 7 billion people, breathing away for 365.25 days per year, and you get an annual CO2 output of 2.94 billion tons. The average human exhales about 2.3 pounds of carbon dioxide on an average day. (When we eat meat, we're still eating the same carbon, except that it passes through livestock on its way into our mouths and out into the atmosphere.) The only way to add to the carbon in the atmosphere is to take it from a sequestered source like fossil fuels-where it has been safe from the atmosphere for millions of years-and combust it.

Human beings do exhale almost three billion tons of carbon dioxide annually, but the carbon we exhale is the same carbon that was “inhaled” from the atmosphere by the plants we consume. Does the collective exhalation of carbon dioxide from all those people contribute significantly to global warming? The Population Reference Bureau, a Washington, D.C.–based research institute, released a report Wednesday that projects the world population will reach seven billion at some point in 2011.

Average human breathing zone