2. Energy source matters

While the IPCC figure suggests that trains aren’t much better than planes, many Europeans responded to our tweets with figures showing a big difference (see below). What’s going on here? Turns out that trains in Europe are mainly electrified, with a recent push to supply them with energy from renewable sources, while those in North America use fossil fuels (often diesel). In Europe, trains are by far the best choice in terms of climate benefits, even if that’s not as true elsewhere.

3. Passenger number matters, too

All of the above estimates of emissions are reported per passenger. However, the data don’t indicate the assumptions made about how many people are in the car, train or plane. But other graphs are explicit, and show how emissions vary along those assumptions. The figure below clearly shows the assumption that on average a train will have 156 people; a plane, 88; and a bus, 12.7. It also shows how car emissions vary by the size of the car and the number of people in the car. In North America, given the similarity of emissions per kilometer among car, train and plane, it might be more important to carpool than to find another mode of transportation.

4. Marginal effects need to be considered Public transit runs based on expected average number of passengers, whether you personally use it or not. It will require large shifts in numbers of passengers to change the numbers of buses, trains or planes running. An individual’s decision to take or not take public transit does not affect the emissions of that trip. But a decision to drive does. So, if you choose to drive because it is more climate friendly than flying short-haul, you are adding an extra car on the road while the plane would have flown anyway. However, in the long run, if many people choose to drive (hopefully in a full car), it is likely there will be fewer short-haul flights.

5. Short-term effects differ from long-term effects Trends suggest that ground transportation is increasingly being electrified (with the potential for using renewable sources). However, there is likely no such technological breakthrough on the horizon for planes. Thus, flying less is an important long-term commitment because it helps to make sure there are more alternative transportation options, and shows where we want government and industry to prioritize efforts toward efficiency and transit.

6. The bigger picture is important While we want to reduce the climate impacts of aviation, we need to remember that flying produces only 2% of total emissions today. Even if everyone were to stop flying, the total climate mitigation impact would be negligible. While flying is the biggest culprit in terms of climate impacts for those who can afford to fly (including most climate scientists), the majority of the world’s people do not fly and road transportation remains the largest share of transportation emissions. While refusing to fly does send an important message, it’s important to make sure a narrow focus on flight emissions doesn’t cause us to lose sight of the need for impactful climate action in multiple sectors.

7. Our decisions are not all just about climate change Navin’s mother lives in India. Elena has family in the United States. Lior has family in Israel. We all have friends all over the planet. Science is also a social endeavor, and it is hard to replace face-to-face gatherings with telepresence. Feeding our souls with family and friends and collaborating on our work requires travel, and sometimes the only way to get to these people or events is on a plane.