Suppose someone told you that they were deeply concerned about environmental issues, the consequences of fossil fuel use in particular, and that they would therefore take concrete action to reduce their energy footprint. The action: driving one mile per hour more slowly than usual — 54 rather than 55 mph. You would probably be unimpressed. I was at a presentation recently, at a university but not mine, and from a staff member but not a scientist, at which the speaker told us that to reduce her environmental impact, she intends to grow more food in her garden. I was floored. Growing food is wonderful — healthy, relaxing, interesting, and perhaps even economical. But does growing one’s own food matter from the perspective of fossil fuel use? How much energy is one saving by growing food in one’s backyard garden?
Let’s estimate this, giving the greatest possible benefit of the doubt to the backyard energetics by comparing home-grown to greenhouse-grown tomatoes. (The cultivation and sale of greenhouse tomatoes requires a notoriously large amount of fuel.) Greenhouse-grown tomatoes require about 66 MegaJoules per kilogram (from Table 1 of Ref. [1], a study in New York) — vastly more than field-grown tomatoes. The average American consumes about 10 kg of tomatoes per year (source), and therefore a greenhouse-tomato-buying American has a tomato-associated energy footprint of about 660 MegaJoules. Imagine — being generous and neglecting, for example, the energy demands of watering one’s garden — that backyard tomatoes require zero energy. Gardening saves 660 MegaJoules each year; is this a lot?
Let’s compare it to the superficially silly example above, of driving at 54 rather than 55 miles per hour. At these speeds, air resistance determines the energetic cost of driving [2]. To travel a given distance, the energy required scales as the square of the car’s speed, and so the lower speed requires (54/55)2 = 0.964 times as much energy as the higher speed. The average American drives 14,000 miles per year (source) of which about 9000 are on highways (source); the average car gets about 25 miles per gallon (source); and a gallon of gasoline contains about 130 x MegaJoules. Combining these, 9000 miles / 25 miles per gallon x 130 MegaJoules/gallon = 47,000 MegaJoules. Driving one mile per hour slower on the highway therefore saves, per person, 47,000 x (1-0.964) = 1700 MegaJoules each year.
660 is considerably less than 1700.
Even our egregiously charitable assessment of home gardening (comparing to commercial greenhouses, neglecting home water use, etc.) shows a far smaller benefit for energy consumption than a tiny change in driving habits. Field-grown tomatoes require only 0.8 MJ/kg (from an article by the eminent Vaclav Smil), against which the backyard garden saves only 8 MJ per year, 200 times less significant than driving 1 mph slower!
Another comparison: The total annual energy consumption in the United States is about 42,000 MJ per person. The 660 MJ of tomato-growing-savings are about 1.6% of 42,000 MJ. The more realistic 8 MJ from bypassing field-grown tomatoes corresponds to an energy savings of 0.02%, which hardly seems worth bragging about.
The deeper question, which gets us to the title of the post: What does it mean to care about the environment? If one is a child, it’s sufficient to say that this or that makes us happy or sad. At some point, though, one should realize that challenging problems are challenging because they are complex, and actions that may address them are not simply binary (good or bad) but have gradations of importance and impact. What’s more, focusing on minor, insignificant actions takes time and resources away from effective actions. Time and resources are always in short supply. As David McKay notes in “Sustainable Energy: Without the Hot Air” “Every little bit” does not help. Big things help. Small things are a counter-productive distraction.
Of course, this doesn’t mean we must all constantly quantify every aspect of our lives, ranking their merits on scales like energy consumption. That would be exhausting. But if we do decide to embark on a path, deciding for example to reduce energy consumption, we should use actual numbers to assess what’s worth doing, what isn’t worth doing, and whether we’re giving sensible advice to others. One might do this and conclude that one’s actions are inconsequential, which is perhaps not very uplifting, but which is honest. Or one might not reach any conclusion at all, as was the case for me in this 2017 post on travel-related CO2 emissions. (I should update the post; I haven’t bought carbon offsets in years, and the questions raised in that post call for revised answers.) But, again, without numbers one cannot be effective or honest. With them, one has a chance.
When I teach my Energy and the Environment for non-science-majors class, I try to convey the importance of numbers, that all of us have the ability to make rough calculations and estimates, and that this is an almost magical skill that lets us make sense of the world. The students seem to like it, at least those who show up, which is encouraging. It would be better still if numerical literacy wasn’t such a novelty.
Today’s illustration
I drew this succulent — I don’t know what species it is — at Stanford’s cactus garden last week. I painted it yesterday.
— Raghuveer Parthasarathy. June 9, 2023
[1] de Villiers, D.S., Wien, H.C., Reid, J.E. and Albright, L.D., “Energy Use and Yields in Tomato Production: Field, High Tunnel and Greenhouse Compared for the Northern Tier of the USA (Upstate New York)” . Acta Hortic. 893, 373-380 (2011). DOI: 10.17660/ActaHortic.2011.893.34 https://doi.org/10.17660/ActaHortic.2011.893.34 ; available at link
[2] Raghuveer Parthasarathy, “Cars and Kinetic Energy – Some Simple Physics with Real-World Relevance,” The Physics Teacher 50: 395-397 (2012). link
The Eighteenth Elephant 