This term I’m teaching The Physics of Energy and the Environment. A few weeks ago, I flew to France for a 3-day meeting. Obviously, there is a tension between these statements. How can I reconcile them, not only to counter the (ostensible) hypocrisy of caring about climate change while contributing more to it than the average American, but also to use this as a tool for students to enhance their understanding of transportation and the environment? Does this have broader implications for the public perception of issues related to climate change?
This is a long post; you can skip the “answers” sections without missing much, and you can go to the “marketing” part for a neat paper that investigates this question.
The course  is for non-science-major undergraduates. I aim to convey, among other things, an understanding of how fairly simple physics determines how much energy we use for various aspects of modern civilization. Transportation is a key part of this; we spend quite a while on the physics of cars , for example.
As my students know, I commute by bike every day, rain or shine. Which is greater, the carbon dioxide emissions from my 10,000 mile flight to and from France, or the CO2 emissions avoided by my biking, rather than driving, 10 miles a day to and from work?
(The reader may wish to pause and answer, before reading on.)
This question is one that everyone should be able to ask, and that, I would argue, a good fraction of people in an “ideal,” scientifically literate society should be able to decompose into the pieces that determine its answer. Of course, one could just look up the CO2 emissions per mile for a car at some speed and for a plane. This is fine, but its limitation is that these numbers appear “by magic,” — we don’ get a sense of why they are what they are, and whether there’s anything we can do about it. Before getting to the consequences of the answer, here are three ways we can tackle it.
Despite my statement above, we’ll first (in this post, not in the class) invoke magic and look up that a 30 mpg car emits about 0.3 kg CO2 per mile. Supposing about 250 travel days per year, this gives 750 kG CO2 for my avoided commute. For a plane, the emissions are 0.2 kG CO2 per person per mile (for a Boeing 737-400) (source), giving 2000 kG CO2.
A better approach is to realize that we can consider the engines of cars and planes. Imagining a 100 horsepower (hp) car, noting that power is the rate at which energy is consumed, and imagining that I’d drive at 30 mph over 10 miles of placid Eugene streets 250 times a year, we find  a total energy consumption for my imaginary year of automobile driving of about 8000 horsepower-hours, or 20 GigaJoules.
A Boeing 747 has four 200 kN engines and carries 400 people, giving a total energy consumption per person  of about 30 GigaJoules.
Both vehicles burn liquid fossils fuels, and so will have similar CO2 emissions per Joule of energy, so we’d again conclude that the plane trip’s emissions “win.”
The best way to think about this question is to figure out why a car or a plane uses the power it does. For a car, this is set by two things: air resistance (essentially pushing a swathe of air set by the size of the car) and thermodynamics (paying the cost of turning high-entropy thermal energy released by burning fuel into low-entropy kinetic energy). The former tells us that any normal-sized car on the freeway will need about 20 horsepower to push air along, and the latter that its combustion engine will need to supply about 5 times that, giving us 100 hp. We go through all this over several class periods with a lot of small-group activities; see  for more. Then, given the simple chemical reaction of combustion, we can calculate the CO2 emissions per mile. The students do this as a homework assignment, treating gasoline as pure nonane (C9H20) to make things simpler; the estimate is within 10% of the “true” answer (see above).
We don’t do the same analysis of the physics of flying — it’s a bit more complicated, and time is short — but I briefly note that it’s possible, and not beyond our abilities.
No matter how you slice it, it’s clear that in terms of carbon dioxide emissions, my trip to France outweighs my biking to work. When I put this question to the class, they easily realized it also. What can I do about this unfortunate conclusion?
1. Nothing. One perfectly good response would be “nothing” — we don’t travel simply to aimlessly wander the skies (or roads), but because someone thinks the benefit of what we’re doing is greater than the cost — in this case, that having intense conversations about science funding with people from twenty countries is worth the expense of getting all of them in a room. As mentioned, this is a reasonable answer, and I think it’s the answer that most people traveling for example to scientific conferences, if they thought about the question at all, would give. (The benefits of science in particular, one might claim, outweigh the harm.) The limitation of this perspective, of course, is that the costs are external, not accounted for in the price of the plane ticket. We don’t tax CO2, or assess its harm to the planet (especially long-term harm). This makes accounting for costs and benefits very difficult, and of course everyone will claim that their benefits to society are greater than their costs!
2. Something. I’ve become increasingly unhappy with the answer above. I’ve therefore decided to buy carbon offsets for the flights I take, paying for an equivalent number of trees to be planted to counter my CO2 emissions. (There are also offset schemes that involve subsidizing clean energy sources; I prefer the trees both because of the additional, and even more important, benefit of wildlife conservation, and because there are issues with distortions of energy costs.) I’ve bought carbon offsets in the past, but not often, and not consistently. I’ve done so for all my 2017 flights, however, and will continue. The offset for the round-trip to France cost about $25 and took the form of conservation efforts in the Alto Mayo Forest in Peru, purchased through United Airlines. (I actually bought $50 worth, but I don’t promise to always pay twice.)
Carbon offsets aren’t without criticism. Some view them as akin to medieval Catholic indulgences, simply money paid to assuage the guilt of harming the environment. This complaint is silly; unlike indulgences, the offset is directly linked to the harm. The atmosphere doesn’t care what the source of CO2 is; adding 1 kg of carbon dioxide by burning a fuel and subtracting 1 kg by locking it into a tree is a net change of zero. A different criticism relates to energy markets (noted above); I won’t go into this. A third criticism is that offset schemes (like saving forests) may be hard to maintain in the long term, and may not hold as much carbon as one would like. I haven’t researched this in detail, and it would be good to read a thorough study. A fourth criticism (that I haven’t seen written down) is that this isn’t scalable. There’s no way everyone could plant enough trees to offset their carbon emissions; there isn’t enough land to make this physically possible. (In fact, about a quarter of the anthropogenic atmospheric CO2 increase is due to de-forestation!) But: I’m not trying to offset everyone’s carbon, just mine, and there’s certainly room for that.
Of course, I’m not the only one to notice the often large carboon footprints of academics who care about climate issues, which brings me to a very interesting recent paper whose abstract starts off with the blunt question: “Would you follow advice about personal energy conservation from a climate specialist with a large carbon footprint?” Continuing:
Many climate researchers report anecdotes in which their sincerity was challenged based on their alleged failure to reduce carbon emissions. Here, we report the results of two large online surveys that measure the perceived credibility of a climate researcher who provides advice on how to reduce energy use … as a function of that researcher’s personal carbon footprint description. … We show that alleged large carbon footprints can greatly reduce the researcher’s credibility compared to low footprints. We also show that these differences in perceived credibility strongly affect participants’ reported intentions to change personal energy consumption. These effects are large, both for participants who believe climate change is important and for those who do not.
S. Z. Attari, D. H. Krantz, E. U. Weber, Statements about climate researchers’ carbon footprints affect their credibility and the impact of their advice. Climatic Change. 138, 325–338 (2016). [Link]
The survey explicitly considers flying and carbon offsets: “Three of the descriptions [of researchers] concerned his/her air travel: flies often, flies little, or flies often but purchases carbon offsets.” The perceived credibility of frequent fliers was, as noted, worse than that of low fliers; the credibility of those with carbon offsets is intermediate. Strikingly, the credibility of researchers who describe “strenuous” efforts to conserve energy at home is best of all. Why? I doubt that typical home energy conservation efforts are enough to outweigh large air travel emissions, and I also doubt that the survey respondents thought about this question quantitatively. As with many aspects of modern life, perceptions matter more than rational analysis, which is fascinating, and which should spur us to develop ways for “marketable” messages to align with effective actions.
Put in the context of my class: did my students trust me more because I religiously bike to work, even though this doesn’t counter my many miles of flying?
I started this post about six weeks ago, and was thinking about it more at this year’s American Physical Society (APS) meeting, to which I and thousands of other physicists flew. (I bought my carbon offsets!)
Would it be worthwhile for the APS to use (and increase) meeting registration fees to buy carbon offsets, or to otherwise neutralize the travel-related carbon footprint of its meetings? It would have a large impact, both materially (in terms of CO2 emissions) and symbolically (in terms of a scientific organization putting actual resources into battling climate change). One could argue against this, with the perspective that such actions should be left to individuals. The APS, however, has recently proclaimed its support for the upcoming March for Science, which I view as merely a cheerleading activity. Carbon-neutral conference travel would be a more concrete action to support a scientific worldview, and would probably have a greater impact on scientists’ credibility to the public.
One could ask this same question of other scientific organizations, and even institutes and universities.
Both for myself and for the broader community, I’m not sure what the optimal approach that combines scientific literacy, personal responsibility, and social awareness is. I do think, however, the topic of our own carbon footprints is one that shouldn’t be avoided.
I rarely paint landscapes, and I thought I’d try to practice by copying some wonderful paintings by Tony Foster, in this book. Attempt #1 was horrible. Today’s illustration at the top of the post is #2. Foster’s original is much better:
 See e.g. Raghuveer Parthasarathy, “Cars and Kinetic Energy – Some Simple Physics with Real-World Relevance,” The Physics Teacher 50: 395-397 (2012). [Link]
 Elaborating a bit: Energy = Power x time = Power x distance / velocity = 100 hp x (10 miles x 250 trips)/ 30 mph = 8000 horsepower-hours. One horsepower equals 750 Watts, there are 3600 seconds in an hour, and a Watt-second is a Joule.
 The “thrust of a cruising 747 is 200 kN” (for each of the 4 engines); from David MacKay’s Sustainable Energy: Without the Hot Air. Link. (The total power (force x velocity) is therefore about 250 MW.) The energy consumed is E = force x distance = 4 x 200 x 10^3 x 16 x 10^6 meters = 12,800 GigaJoules for all 400 passengers.