I found this file mouldering in my archived documents, and thought it worth saving.

Apparently, at one point there was some comment that humans produced more CO2 than cars, which was supposed to mean that CO2 could not possibly be a greenhouse gas, by the classical naturalistic fallacy. It’s fun to debunk people within their own world views. And by fun I mean potentially more effective.

The average adult (resting) respiratory rate is 10-14 breaths per minute. [1]
The average adult tidal volume is 0.50 liters, with a total volume of 4-6 liters. [2]
Expired air contains 3.6% CO2 by volume, compared to 0.03% in atmospheric air. [3]
Each breath thus exhales 0.018L of CO2 ((3.6%-0.03%)*(0.50L)). Assuming STP, the weight of CO2 added by each breath is 0.035g. Doing the math, that means each human expires a minimum of 222kg CO2/year (489 lb/year).

The average car has a fuel economy of 17.2mpg. Each gallon of gas produces 8.79kg of CO2. [4]
In the US, the average per-vehicle yearly mileage was 12,016 miles in 2006. [5]
Doing the math, that means every car expires an average of 6130 kg CO2/year (13,500 lb/year).

The average car produces 27.6 times as much CO2 as does a baseline adult human. Furthermore, the carbon for human-sourced CO2 is from crops grown recently. However, the carbon for vehicle-sourced CO2 is from fossil fuels sequestered many millions of years ago. (Or, like most people making the argument believe, 5000 years ago when Noah built a boat to hold a breeding pair of millions of different species during a global flood).

But perhaps, you might say, there’s more humans than cars, so humans really are pumping out more CO2? Okay, let’s test that: There were, in 2002, approximately 590 million cars in the world. That’s one car for every ten people. [6] So if ten people expired more CO2 than one car, then breathing would indeed be more responsible for global CO2 than the cars. But a car expires the CO2 of 27.6 people, meaning that cars produce overall 2.7 times the amount that humans do.