Papa Don commented a few posts back about the role of bicycling and reduction of CO2 emissions. He brought up an interesting idea: does riding a bike matter in reducing carbon dioxide?
To answer this question, first we need to discuss the Carbon cycle and the methods of carbon release. The thing is, there are two ‘branches’ of the carbon cycle. The Slow (ie, geological times scales) and Fast (ie, Physical/Biological time scales) Cycle. Like any of these cycles, we are assuming it is a closed system, where mass is conserved. Thus, the ‘cycle’ in the phrase Carbon Cycle, is simply the flux of mass between different elements within the system.
In the geologic time scale, carbon in the atmosphere reacts with seawater by means of a buffered equilibrium reaction (g=gas, aq=aqueous, l=liquid):
- CO2(g) <–> CO2(aq)
- CO2(aq) + H2O(l) <–> H2CO3(aq)
- H2CO3(aq) <–> H+(aq) + HCO3–(aq)
- HCO3–(aq) <–> H+(aq) + CO32-
where the available carbonate (end of step 4) likes to react with Mg2+ or Ca2+ producing Dolomite or Limestone over millions of years. This limestone precipitation (ie, carbon storage) occurs in temperate, non turbid waters like the Bahamas. During the geologic past, much greater portions of the worlds oceans were favorable for carbonate precipitation (eg, most of the mid North American continent during the Mesozoic). The massive amounts of carbonate platform are eventually buried by other basin sediments, and/or subducted under continental plates, creating huge stores of underground carbon in the form of fossil fuels and gases.
In addition to the geologic carbon cycle, there is also a physical/biological cycle that reacts much quicker (over 100s-1000s of years) in the form of plant and animal respiration and photosynthesis. During photosynthesis, plants take in CO2 and produce sugars as a form of energy for the plant cells to do work (metabolize). Also, animals respirate CO2 when we break down those sugars (carbohydrates) to make our energy. Generally, the Biological carbon cycle moves 1,000 times more carbon per year than the geological carbon cycle.
At this point, we have two branches of moving carbon, that are interacting, but on a basis which maintains an overall dynamic equilibrium over time.
Now, here is where anthropic impact plays a role. As we consume fossil fuels, we are bridging the gap between the two branches of the Carbon Cycle. By burning gasoline, coal, or natural gasses, which are products of carbon storage over millions of years, we quickly release this otherwise unavailable carbon into the biological/physical branch of the carbon cycle. Though the biological branch of the cycle has some storage capabilities, it is unable to process the extra CO2 flux from the geological cycle we are inputting through use of fossil fuels. It is explicitly for this reason that scientists are stating that we, as a species are increasing the concentration of atmospheric CO2 by our energy consuption practices.
So, coming back to riding a bike. When I ride my bike, I am using energy that has come from eating meats and veggies. All of these things are part of the biological/physical portion of the Carbon Cycle. Thus, any CO2 I contribute to the atmosphere was already part of the normal carbon flux due to photosynthesis/respiration. This is totally different than when I drive my car, cool my house, or use plastic grocery bags, where I am taking carbon that has been stored in the Earth’s crust for millions of years, and releasing it wholesale into the atmosphere. On this basis alone riding a bike is always better than driving a car. Period. Now, there are some important considerations to point out in Don’s favor here. My bike is made up of raw materials, including petroleum based products (ie, plastics, the grease I use for the chain, etc.), thus I do have a ‘carbon footprint’ which include some release of geologic carbon. However, compared to the amount of materials and CO2 used to produce a car, I am still doing better with the bike.
For completeness sake, I will lastly consider just how much more efficient my bike is verses my car.
I looked around a bit, but couldn’t find a stat for how much CO2 a bike puts out, but I did find out that typically an average human outputs about 1kg of CO2 per day. Hypothetically, let’s say I ride for 24 hours, and output 2kg of CO2. I averaged about 12 mph on my bike, so that would mean I would get 288 miles in 24 hours (I could only wish, but it will work for this example). Now my car, a 1997 Honda Accord produces ~8.0 tons of CO2 a year using the default mileage settings (45% highway, 55% city, 15,000 miles/yr) at fueleconomy.gov, which works out to 0.0219 tons/day, or approx. 22kg/day of CO2. Now, to be fair, it will not take all day to drive 288 miles. Assuming a speed of 55mph (as an average), it will take 5.24 hours to go 288 miles in my car, producing 4.8kg of CO2 for the trip plus the 0.22 kg of CO2 I breathed while driving. As a last attempt, I did the same calculation using a 2008 Toyota Prius rather than my faithful Accord. The Pruis would produce 2.45 kg of CO2 plus the 0.22kg from my breathing. This is still more CO2 contribution than for riding my bike, all using conservative figures meant to be in the car’s favor. Oh yeah, and this is all of course moot because I do not contribute to CO2 loading from breathing as discussed above.
So, it is beating a dead horse I know, but it is an important one. Riding a bike is significantly better for the environment than driving a car. End of story. If you hear otherwise, you are unfortunately being misled.