Carbon Footprints of Cars vs Bikes (vs Ebikes)

Sorry for the lapse between posts, we’ve been a bit busy on boarding a new Jr. Mechanic, whom we’ll introduce in an upcoming employee spotlight.

A couple of weeks ago, I was checking out the Mercedes AMG site for ideas on re-skinning the Electric Elk website and noticed that all of their cars list g/km of carbon footprint.  First of all- well done, this is an awesome step in the right direction for auto manufacturers, and second it got me thinking.  And when I get to thinking, I get to Googling, and then I found this little gem of an article.

The article attempts to estimate g/km for bikes and e-bikes AND take into consideration production costs AND unlike AMG, includes carbon footprint for power generation (excess food for calories in the case of biking, and power to charge batteries in the case of ebikes).   The methodology is far from perfect, and since the AMG site disregards carbon footprint from power consumption (and production) you end up with a bit of an apples to oranges comparison. It is still interesting and worth the read.

Now, we can’t do an apples to apples on the Mercedes since they’re calling their electric vehicles 0g/km, but we know they do use electric, the production of which emits carbon, but we can directionally compare to petrol-powered autos.

Bike Emissions

Seb Scott (the article’s author) finds that, when you factor in the carbon footprint of bicycling is around 21g/km (33 g/mile). He figures this by looking at the excess calories a cyclist uses, then he figures the carbon footprint of consuming those additional calories. Obviously there is a fair bit of room for variability here. For instance, a vegan cyclist vs a vegetarian cyclist vs a meat-eating cyclist (and all through the range of how much meat they eat.) The point being you can smudge the numbers this way or that a bit, but in general he finds 21g/km.

Src: Pixabay

But if you want another dimension to smudge across, he also finds that e-biking actually has less of a carbon footprint- about 16g/km (26 g/mile). It’s lower in short because an electric motor is a more efficient machine than human legs. But obviously this introduces even more variables: pedal assist or electric motor cycle mode? Motor efficiency? If your electric from a relatively clean source like nuclear or wind/solar or a dirty source like coal/oil?  How efficient are the manufacturing of the batteries / actual bike? If pedal assist, how much assist are you getting? But for the sake of argument, let’s just take Scott’s word on this at ~16g/km.

Another apples to oranges problem, Scott figures the carbon footprint of manufacturing e-bikes and batteries, Mercedes does not.  

Auto Emissions

From the EPA we see that burning a gallon of gasoline emits around 8,887 grams of CO2. So let’s figure your car gets 20mpg. Twenty miles is about 32.1869 kilometers, so using the formula 8,887 divided by 32.1869 (or however far your car goes on a gallon of gas), we get about 276 g CO2 per mile. Let’s call this the variable footprint of a car.

Src: Pixabay

An article from the 8th international conference of applied energy gives us some back-of-the-envelope numbers for the production of petrol and electric vehicles (again, a lot of hand waiving, sorry). Production of an electric vehicle is around 14,200 kg CO2 and an internal combustion vehicle is around 9200 kg CO2. Electric vehicles have higher proudction footprint due to the ‘cost’ (in g CO2) of the batteries.

So for a petrol car, let’s figure the ‘fixed cost’ is 9,200 kg of CO2 or 9,200,000 g CO2.

Then, let’s figure (again, via arbitrary hand waving) the life of a petrol car is 100,000 miles, which having worked on some junky old cars I think is pretty fair. Over 100,000 miles, a car that gets 20 miles per gallon (I know no car is going to be driven city for 100k miles, but bear with me), is going to go through 5,000 gallons of gas, which in turn will create about 44,435,000 grams of CO2 from usage plus ~9,200,000 from production, giving us around 53,635,000 grams of CO2 over the working life of the car (admittedly with a great deal of variability), OR an average of around 536g per mile.  

Savings

For this section, let us consider a few characters- Cathy, Sally, and Dave.

Cathy- is super eco conscious. Like that obnoxious kind of eco conscious person who corners you at parties to drone on and on about whatever article she recently read on treehugger.com and cruelly berates people for not being vegan. Obviously she rides a legacy bike, and doesn’t own a car. We will calculate how much good Sally is actually doing by riding her bike everywhere calculated in grams of CO2 per year; but we won’t be giving her full credit since she’s also probably eating vegan and therefore her footprint is less than the 21g CO2 per mile on a bike.

Sally has a petrol powered car, but cares about the environment so she made a New Years resolution to commute to her office via bike one day a week. We will once again make some wild assumptions, but figure how many grams of CO2 Sally’s New Year resolution adds up to over the course of the year.

And finally we will discuss dirty Dave, who just drives to work. The point of Dave’s tale will mainly be a device to show the effect of Cathy and Sally’s efforts. Explicitly, how many trees Dave will have to plant each year to be as clean as Cathy and Sally.

So first some (wild) assumptions: Cathy, Sally, and Dave are all next door neighbors and all work at the same place which is precisely 20 miles door to door from their house. They also all have the same kind of car (except Cathy, who has no car).

Dave and Sally both drive to and from work each day, two gallons a day total. Five days a week for 50 weeks a year, about 250 days a year or 500 gallons a year just commuting to work. Which in turn is 4,443,450 g/CO2 per year just commuting. Note a working year is generally considered 261 days, so I’m also giving them some work remote days, but in general I’m trying to round (in favor of petrol) to numbers that make the math easy.

Dave and Sally also have internal combustion engine cars, figure 9,200,000 g/CO2 from vehicle production. They also drive other places besides work, a total of 12,000 miles per year 10,000 of which is commuting to work (250 x ( 20 x 2) - two hundred and fifty days, 20 miles each way), the other 2,000 miles going to get groceries and such.

So if our cars have an expected life of 100,000 miles, 12,000 miles represents 12% of their life usage per year- 12% of the production footprint of 9,200,000 is 1,104,000 g/CO2, and thus the production ‘cost’ of 1,104,000 g/CO2 + the usage of 4,445,450 g/CO2 commuting 10,000 miles and (2000 miles / 20 miles a gallon = 100 gallons doing other stuff at 8,887 g/CO2 per gallon) and 888,700 g/CO2 from other errands and etc. Gives us an annual total of 6,436,150 grams of CO2 per year.  We will call this dirty Dave’s carbon footprint and use it to measure Sally and Cathy’s foot prints.

Dirty Dave’s Commuting Footprint: 6,436,150 g/CO2

Sally has a New Years resolution to commute via bike 50 times a year. That is on average of once a week- but sometimes its super cold or rains all week so she doesn’t bike at all, sometimes the weather is really nice so she’ll ride twice or even three times that week. Also, for the sake of simple math we assume she sticks to this- so her commuting footprint will be 80% (4 out of 5 days in a car) of what dirty Dave’s was i.e. 4,443,450g/CO2 x 80% ( 3,554,760 g/CO2 ) but also her car will last longer since she’s not putting as many miles on it, so we also only apply 80% of the annual production allotment of 1,104,000 g/CO2 which is 883,200 g/CO2 per year. Also- Sally does now commute ~2000 miles per year on her e-bike which from Scott’s article comes out to (2000 miles x 26g/mile) 52,000 g/CO2.

So Sally, who is sympathetic to environmental concerns, has a footprints of 3,554,760 grams of CO2 from commuting in her car 4 days a week, 883,200 amortized footprint from the production of her car, 52,000 grams from riding her e-bike, and 888,700 from doing ‘other’ things with her car. This gives us a back of the envelope estimate of her annual footprint being 5,378,660.

Sympathetic Sally’s Footprint: 5,378,660 g/CO2

And finally we have cantankerous Cathy, the eco-warrior tree hugger who berates you at parties.  For the sake of argument- Cathy also drives 12,000 miles a year but on a human powered pedicycle- rain or shine- she is hard core, there is no doubt. Based on Scott’s findings- the extra food she has to eat to give her calories to cycle and the footprint of producing and maintaining the bike comes out to 33 grams of CO2 per mile, then figure 12,000 miles- gives her an annual footprint of 396,000 grams of CO2 per mile.

Conscious Cathy’s Footprint: 396,000 g/CO2 per year.

So DIrty Dave produces 1,057,490 g/CO2 more than Sally, and 6,140,150 more than Cathy- but, what does that equate to in an eco warrior favorite measure- trees? Well a tree absorbs around 21 kilograms (21,000 g) of CO2 a year, which again is a sort of hand wavy number- based on what kind of tree, how old is the tree and other variables, but it will give us something to go off of. To put in perspective what Dirty Dave would have to do to shield himself from the smug ire of Sally and Cathy, he would need to plant around 51 trees a year to be on par with Sally and about 293 to be on par(ish) with Cathy. These aren’t unreasonable numbers, and Dirty Dave may in fact be quite the arborer and do just that. 

Src: Pixabay

Conclusion

So to wrap this up- we’ve stood on the shoulders of giants so to speak and looked at how doing nothing, taking a modest step, and going to the extreme to reduce carbon footprints could benefit the environment, and then translated those results into a more digestible statistic: trees planted.

There was a lot of hand waiving and ignoring things like vegetarian / vegan diets have significantly less footprint and Cathy probably is on those diets as well, but the call to action of this tale is to ride your bike more often, even doing so occasionally can have big impacts.