Is biking green?

Nowadays, you hear a lot about "being green". And certainly biking is one of the most energy-efficient transportation [1]. But being energy efficient does not mean to be environmental-friendly, does it?

Well, there is this big problem with greenhouse gasses, like CO₂. Hmm, CO₂? The thing we produce during respiration? So let us estimate how greenhouse-friendly is actually biking to the work.

We need to estimate CO₂ production during the biking, i.e. we need the difference between CO₂ production during the biking and the usual activity. Unfortunately, CO₂ production of humans is not very advertised topic. But what comes out must come in. In fact, the CO₂ exhalation is preceded by O₂ inhalation with the same (molar) amount. Thus we just need to figure out molar amount of O₂.

Now, O₂ is more studied. There is a lot of sport/fitness research for maximum oxygen consumption (VO2 max). So let us do the estimate.
According to [2] the maximal VO2 for an average person is roughly  35 ml*kg^-1*min^-1. From [3] we get that the actual difference between rest and biking at 70% of max heart rate corresponds roughly to 25% of VO2 max.

Putting this together for my weight 70kg and time 60 minutes, it is 35 * 70 * 60 * 0.25 = cca 37 litres of oxygen. Moving back to molar amounts - 1 mol of ideal gas under room temperature corresponds to roughly 24 litres [4]. Note that oxygen is not so far from ideal gas in this scenario. This gives us roughly 1.5mol of oxygen.

According to [5] the molar mass of CO₂ is 44g/mol. This yelds 66 grams of per CO₂ the one-hour long trip. Now, what is the comparison to other means of transportation? SBB claims [6] that for the same trip, the car would produce 4.2kg of CO₂ which is quite a lot. But the train under average utilization will produce only 0.18kg -- only three times as much as bike! (I guess this also includes CO₂ production during electricity generation).

So what is the bottomline? Yes, the biking is green. But japanese trains are greener :-)

P.S. As a good scientist, one should always cross-check his results. Especially if the sources are just a random collection from the internet. So let us do the task. According to [1], on the flat ground a normal person expends 1.62 kJ*kg^-1* km^-1. For my trip of roughly 25 kilometers this is 1.62 * 70 * 25 = 2 835 kJ.
Putting 2800kJ energy inbalance into [7] gives roughly 0.63 kg per week, or roughly 90 grams per day. So, the workout should reduce weight 90 grams, we have 66 grams from CO₂, where is the rest? The answer can be found in [8] -- according to the respiration equation with each mol of CO₂ we also produce a mol of H₂O and some energy. And using [5] again, we obtain that this corresponds to the missing 20 grams. So our results are (quite surprisingly) reliable.

[1] http://en.wikipedia.org/wiki/Bicycle_performance#Energy_efficiency].

[2] http://www.topendsports.com/testing/vo2norms.htm
[3] http://www.rice.edu/~jenky/sports/hr.html
[4] http://en.wikipedia.org/wiki/Molar_volume#Ideal_gases
[5] http://www.webqc.org/mmcalc.php
[6] sbb.ch ecocalculator for route Chavornay->Renens (direct, train S1)
[7] http://www.health-calc.com/diet/weight-loss-calculator
[8] http://respirationequation.com/