Active modes of travel are not “zero emitters” when it comes to greenhouse gases, according to a new study. Researchers conclude that while cycling is better than walking, active travel may result in people eating more. This could mean that switching from driving to car-sharing could reduce emissions more than switching from driving to walking. (But it probably doesn’t.)
The study, published in the Scientific Reports journal, aimed to demonstrate that assessments of emissions associated with different forms of travel should take into account emissions associated with that person possibly eating more in response to their increased physical activity.
While conceding that, “studies of active transport do not currently provide us with definitive information on the extent or nature of compensatory food intake in response to increased walking and cycling,” researchers assume that a person who shifts from a passive mode of transport (e.g. driving) will eat a little more.
They base their calculations about the emissions cost of this on Mike Berners-Lee’s 2010 book, The Carbon Footprint of Everything. This estimated that a mile cycled in the United Kingdom generates emissions of between 65 gCO2e and 2,800 gCO2e, depending on what the journey was powered by (bananas or air-freighted asparagus).
Clarifying this detail, they write: “The notion that energy expended from a cycle ride may be substituted directly by air-freighted asparagus is far-fetched, but underlines the point that there is a high carbon cost of modern food systems.”
The study estimates that in the UK, emissions resulting from walking could range from 0.05 to 0.25 kgCO2e/km and from 0.03 to 0.13 kgCO2e/km for cycling.
(It’s worth pointing out that they found wide variability between countries for this, representing nearly a five-fold difference between the most and least economically developed countries.)
The Intergovernmental Panel on Climate Change estimates that emissions from cars range from 0.15 kg/km to 0.26 kg/km based on a ‘well-to-wheel’ life cycle assessment.
Based on the extreme ends of these estimations, the researchers at one point write that, “Taking account of walking and cycling emissions may suggest that car share schemes could have a bigger positive emissions impact than increasing walking.”
This – we are astonished to report – is the detail the Daily Mail has picked up on, resulting in its headline, “Walking to work is WORSE for environment than car sharing because it makes you eat more leading to higher greenhouse emissions, new study finds.”
In reality, the study only really concludes that emissions from food required for walking and cycling are “not negligible” in economically developed countries and that they should therefore be considered when estimating net-emissions impacts from transport interventions.