(this item first appeared in Physiology News, the magazine of the Physiological Society - Spring 2008 No 70, p53 - see www.physoc.org/site/cms/contentChapterView.asp?chapter=151)
Have you ever wondered how much CO2 all those frantic joggers add to the global warming problem? And how does it compare with travel by the vilified car? Given the result of some simple physiological calculations, I suspect some will be surprised.
A car running at about 40 mpg and 100 km per hour – very achievable for modern vehicles – will generate about 150g CO2 per km (according to published data that you can check in any recent car advert). For an exercising, efficient human we can assume a near-maximum, steady-state, oxygen consumption of about 50ml O2 per kg body weight per minute. Let’s take the conventional ‘textbook’ 70kg man; that equate with 3.5 litres of O2 consumed per minute. Assume, at this stage, that he’s running his metabolism on carbohydrate; the respiratory quotient (RQ) for this substrate is about 1.0. This means his oxygen consumption will result in about 3.5 litres of CO2 breathed out per minute. The density of CO2 is nearly 2g per litre (under standard conditions) which means a CO2 production of 7g per minute. He will also be running at close to his maximum, sustainable, aerobic speed. If our runner is pretty fit ( … very fit!), he might be running at 5 minutes per mile rate, or approximately 3 minutes per kilometre. Thus, to run a kilometre, he will generate (3 x 7) = 21g of CO2. Of course, if he was less fit or efficient, he’d be producing more CO2 to cover each kilometre, and also taking longer over it.
So, if our car was carrying 4 people, over a 100 km trip it would generate some (150 x 100)/1000 = 15kg of CO2. If those same 4 passengers were to run the 100 km instead of riding, they would generate about (4 x 21 x 100)/1000 = 8.4kg between them, a little over half as much as taking the car. But we have overlooked something. After about an hour, when the car arrives and stops, it ceases to produce any more CO2. However, our passengers will still be respiring at their resting rate as will our runners too (once they have stopped puffing after their 100 km ‘jog’). Thus, they carry on producing at least 30g of CO2 per hour, day and night. Indeed they were still producing that much, even when being driven in the car. In 24 hours, after just one 100km trip, our 4 travellers by car will generate 2.9kg themselves and their car 15kg; a total of just under 18kg for the day. The runners, who spent 5 hours running and 19 at rest, will produce (8.4 + (4 x 19 x 0.03)) = 10.7kg for the day. So, the car journey only produced an ‘extra’ CO2 output comparable with 4 or 5 people sitting around doing nothing for a few days. If the journey were shorter, the CO2 ‘gap’ would obviously be less remarkable too. And furthermore, unlike the car, in order to run at this speed, our joggers would have to be jogging almost daily to keep fit between trips too; ‘training’ generates still more CO2 .
What’s the alternative? If our travellers continued sitting around doing nothing, thereby becoming couch potatoes, they would not travel or run as far, but they would reduce their global CO2 ‘footprint’ by 75% for the day. But there is a further bonus. The term ‘couch potato’ is misleading because our idlers would lay down fat, not starch. Adipose tissue is surely an excellent way of sequestering carbon. The metabolic rate of adipose tissue, once built, is pretty close to zero. And when they do eventually come to burn it off, either in exercise or, I suppose, eventually in a crematorium, remember that the RQ for fat is only 0.7. Less CO2 production in the end too!
More cynically still, has Gaia perhaps found the right strategy to save the planet by changing the shape and reducing the life expectancy of the average American (because they can’t be persuaded out of their SUVs)? This is not for the squeamish: achieving ideal carbon sequestering requires an early death for our tubbies, ideally from an abrupt cause since wasting diseases rather defeat the objective (I said this was cynical). Obesity–driven CVS problems fit the remit. And finally, disposal of the body demands an hermetically-sealed mode of burial to avoid carbon recycling.
The arithmetic implies we should eat more and exercise less to help save the planet. I rather suspect a better answer lies elsewhere!
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