It’s Not Easy Being Green

Hey you.  You organic-eating, fair-trade consuming, straw-abandoning hyper-privileged human with a heart of gold.  You care about appearing, and maybe even being, ‘green’.  How certain are you that your choices made in the context of being environmentally conscious are having the impact you hope they are?

The Question

Here’s a simple quiz.  Imagine, in your ongoing attempt to improve yourself, you’ve decided to pick up Vaclav Smil’s Energy and Civilization: A History.  You live a mile from your local bookstore.  From the perspective of minimizing greenhouse gas emissions, what’s the best method for you to acquire this book?

1. Drive to the store
2. Walk to the store
3. Order the book online

If you sense a trap, good for you – your innocence has been sufficiently tarnished through your daily dose of contact with humanity. 

The answer is, of course, more complicated than you want it to be.  Let’s take a look at a high-level analysis of the greenhouse gas (GHG) impact of each of the three options.  First, a brief biography that captures just how much of a unique snowflake you are.

You’re a middle-aged male that lives in an urban community in the United States.  You weigh the average 191 pounds, you own an average gasoline-fueled sedan, and your home is an average distance away from average things.  Basically, you’re pretty average. 

Drive to the Store

This one’s the easiest.  Your average sedan gets around 25 miles per gallon of gasoline.  The 2 mile roundtrip drive to the store consumes 0.08 gallons of gasoline.  Gasoline generates 8.89 kg of CO₂ per gallon consumed.  That totals up to just over 0.7 kg of CO₂ to pick up the book from the store.  I hope you’re proud.  We’ll blatantly ignore the lifecycle assessment of the carbon footprint of car ownership (tire use, the steel used in the manufacturing of your vehicle, the energy you spend to keep your garage cool in the winter so that you can admire your symbol of freedom in the form of four wheels).

Walk to the Store

Obviously the right answer, right?  Now we start getting to the juicy stuff.  An average male of 191 pounds burns just over 100 kilocalories (kcal) walking a mile, so that 2 mile roundtrip, assuming you walk in a reasonably straight line, takes about 200 kcal of energy.  Compare that to the almost 2300 kcal of energy your car burned to go that same two miles – what a deal!  So what’s the GHG footprint of that energy you just burned? 

Well… it depends what you eat.

The GHG footprint of foods vary widely.  Today’s food supply chain is complex, and an extraordinary number of factors go into calculating the amount of CO₂ equivalent (CO₂e) GHG emissions that a calorie of food comprises (shipping, land-use, food waste, energy inputs to food production, cold-supply chain, fertilizer, etc.).  All caveats about variability aside, let’s use some average numbers. 

An average human in the United States eats a mixed diet with a net GHG footprint of about 2.5 kg CO₂e per 1000 kcal of food consumed.  If you get that extra 200 kcal of energy it took to walk from the store from an average US diet, then it would produce approximately 0.6 kg CO₂e.  So, on average, and ignoring all the complexity associated with other externalities of walking to the store (e.g. health, that extra bottle of water you brought with you, the special walking shoes you bought because they promised to plant a tree in the Amazon for every mile you used them), it’s ever-so-slightly preferable to walk to the store than to drive.

But the story changes dramatically depending on how you get that extra 200 kcal.  For example, if you get it by eating an extra ¼ lb beef patty then that 200 kcal amounts to approximately 5 kg of CO₂e!  200 kcal of dairy amounts to approximately 1.6 kg of CO₂e.  That’s substantially higher GHG impact than driving to the store.  On the other hand, if you’re an average vegetarian, then that 200 kcal only accounts for approximately 0.4 kg of CO₂e, and if you’re an average vegan, only about 0.3 kg of CO₂e.   

Order the Book Online

The real complexity begins.  Calculating the GHG footprint of a delivered package is complicated.  Today’s delivery systems leverage extremely optimized logistics and complex distribution networks to get items from source to destination.  To simplify the analysis, here are some assumptions that we’ll make in our calculation:

1. Your book is delivered to you on a standard P70D van (think UPS truck) after being picked up from a local distribution center
2. The GHG footprint of getting the book to your local store and to your local distribution center are equivalent
3. The only net differences to CO₂e will come from the last-mile delivery from distribution center to your home and the packaging that your book is delivered in

We’re going to ignore the differences in supply chain efficiencies, warehouse electricity use, the possibility that you aren’t home when the delivery is attempted, your particular community’s packaging recycling program, the possibility that your package gets shipped by air because you absolutely-must-have-it-today, and all the other nuances that complicate the story. 

The average UPS route for an example urban environment (in this case, Phoenix, AZ) is approximately 84 miles and 191 drops.  The P70D van gets about 10.6 miles per gallon of diesel, and diesel has an emission factor of about 10.2 kg CO₂e.  That means that an average package contributes 1/191 of the total 81 kg of CO₂ generated from using 7.9 gallons of diesel, or about 0.42 kg.

But you didn’t just receive the book, you also received the cardboard box it came in.  That cardboard took energy to make.  When you throw it away or recycle it, energy is used to transport it again.  If it ends up in a landfill, some of the carbon in that cardboard ends up turning into methane (a greenhouse gas at least 25 times as potent as CO₂).  Depending on your personal recycling practices, the county you live in, the landfilling practices used, and the source of the cardboard (was it itself recycled?) the GHG footprint of that box can vary widely.  On average, at an emissions factor of 1.45 kg CO₂e per kg of cardboard, that 0.1 kg 8”x6”x4” box has a lifecycle CO₂e footprint of about 0.15 kg.

Together, that means your book took about 0.57 kg of CO₂e to get from the distribution center to you.

If you recycle your cardboard, given standard recycling efficiencies of 93%, the total CO₂e footprint of that delivered package is 0.43 kg.

So What Should I Do?

First, you should congratulate yourself for caring enough to try and find out.  You’re a GOOD person.  To answer the question at hand, if the above discussion hasn’t made it clear, the answer is: it depends. 

The answer to “what’s the green choice?”, even from the specific perspective of minimizing GHG footprint, is not always obvious.  The complexity of today’s supply chains make even seemingly simple decisions hard to analyze.  For the highly idealized cases that was discussed above the calculations fall out like so:

Of course, you’re anything but average.  Here’s a short list of things that would dramatically impact the calculation for your own personal decision around how to buy a book:

• Your vehicle’s fuel efficiency (is it electric?)
• The carbon intensity of your local electric grid
• The population density of your community (higher density means more efficient package delivery logistics)
• Your shopping patterns – do you shop in bulk or run multiple errands on the same trip?
• Your recycling behavior
• The efficiency of your bookstore’s supply chain
• …and 1000 other considerations

You can find the calculations used to inform the above analysis here.

References