Carbon Footprints (A Short Story)

Big Feet

The other day, after a strenuous morning discussing global warming and devising draconian penalties to impose on people with oversized carbon footprints, a group of government ministers and their officials decided to continue their debate over a pub lunch at a popular bar about a mile away. Having only one hour for lunch, four of them went together by car, an eco-friendly Toyota hybrid, to save time. After a lively discussion, fuelled by a few beers, they dutifully left the car behind and set off on foot, back to work to find more innovative ways of saving the planet. Being already late they took a brisk walk, interspersed by jogging, just making it back to the office in fifteen minutes.

Big Mouth

First on the agenda was their own carbon footprint. Sheepishly the four admiited to the unnecessary car trip they had just made. During their one mile journey to the pub in the car they had dumped 142.4 grams of CO₂ into the atmosphere. (Based on the manufacturer's perfromance data provided in the sales brochure for the car). They were however happier to talk about their environmentally friendly return journey and to prove their green credentials they set about calculating how much CO₂ they had saved by walking back, using published data they found on the internet.

Big Surprise

This is what they discovered:
The volume of air breathed in and out by an adult male walking or jogging at 4 miles per hour is about 35 litres per minute, compared with less than 10 litres per minute while at rest. (Source - California Environmental Protection Agency. Research Note 94-11)
During their 15 minute journey back to the office each of the group had therefore breathed in and out 525 litres of air.
But this is not all carbon dioxide.
30% of the air we inhale is exhaled again unchanged as "atmospheric air" . This volume is called the "residual volume"and contains about 0.04% of carbon dioxide by volume.
The other 70% of the air intake, the so called "tidal volume", passes through the alveoli of the lungs where the exchange of oxygen and carbon dioxide takes place during respiration. The expelled air is known as "alveolar air" and, of this, 5.7% by volume is CO_2. The rest is mostly nitrogen, oxygen and water vapour. (Source "Biology" by Claude Alvin Villee, ISBN 072169022X )

Ignoring the CO₂ exhaled in the residual volume of atmospheric air contained in the 525 litres of air exhaled by each person during the one mile return journey, the exhaled alveola air alone contributed an increase of 21 litres of CO₂ to the atmosphere and since the density of CO₂ is around 1.98 grams per litre at standard atmospheric temperature and pressure, (Source - Wikipedia), this increase weighs in at 41,5 grams.
With the respiratory system of each individual generating 41.5 grams of CO₂ per mile, the group added 166 grams of CO₂ to the atmosphere on the way back to work. Just over 16% more than the footprint left by the car over the same distance.

But that's not the whole story:
To be strictly comparable, only the incremental CO₂ content of the alveolar air exhaled by the four walkers specifically due to walking should be considered and not the basic CO₂ generated by the body just to stay alive which should therefore be subtracted from their total CO₂ output.
At rest or seated the volume of alveolar air exhaled by each person will be 70% of 10 litres per minute or 105 litres for the 15 minute period. The CO₂ content of this air will be 11.9 grams so that the incremental CO₂ resulting from the body's production of the extra energy needed for walking will be just 30 grams per person or 120 grams for the group of four.
Thus the four walkers together would actually generate 15.9% less CO₂ than the 142.4 grams generated by the car.
But with five passengers in the car compared with five walkers, the result would have come out in favour of the car with same 142.4 grams emitted by the car compared with 150 grams emitted by the five walkers.

A close run thing!!

Big Relief

At least the victuals they consumed at lunch came from renewable sources and did not contain any fossils.

Note: The characters mentioned above are ficticious, the technology and the numbers however are factual.

Carbon Footprints - A Definition

There are many definitions of Carbon Footprints. Over time they have come to incorporate so called greenhouse gases (GHG) of all types, not just carbon dioxide (CO₂), generated by the consumption or burning of fossil fuels. A typical definition is - "The total amount of greenhouse gases produced to directly and indirectly support human activities, usually expressed in equivalent tons of carbon dioxide."
Cost-benefit studies should however be based on net benefits taking into account the energy consumed in manufacturing and maintaining the alternative energy saving equipment used to avoid the consumption of fossil fuels, a consideration which is often overlooked.
The short story above shows the danger of jumping to "obvious" conclusions.


The notion of carbon footprints is often used to justify the installation of "sustainable or renewable energy" alternatives many of which can not be justified on purely economic grounds. See wind power and solar power generators. (Hydro power is a notable exception)