The article seems to be looking at marginal CO2 production, so that's what I'll look at. To be precise, let's consider this situation: you want to go from your present position at point A to some other point B. There's a nice road from A to B, which is suitable for walking, biking, and driving. There's also a train track beside the road with stations at A and B. You already own and regularly use a bike, an ICE car, and an EV.

The question then is if you take a trip from A to B, which choice from {walk, bike, train, ICE, EV} will result in the lowest change in total atmospheric CO2 from the time you leave A to the time you arrive in B?

Let's first just look at walking vs biking. Perhaps surprisingly you will actually produce more CO2 walking. Here's a site [1] with some example calculations. Their example is a trip of 3.2 km, with a walking speed of 4.8 km/hr and a biking speed of 12.8 km/hr.

Walking would take 40 minutes and burn 167 kCal, but just being at rest for 40 minutes would burn 56 kCal so we should count walking as costing 111 kCal.

Biking would be 15 minutes and burn 70 kCal, but just being at rest for 15 minutes would burn 21 kCal so we should biking as costing 49 kCal.

We produce about 0.7 kg of CO2 per 2000 kCal, so that gives 0.03885 kg CO2 for walking and 0.01715 kg CO2 for biking.

But wait...should any of that actually be included? The C in the CO2 we exhale comes from the C in the food we eat. For plants we eat they get it from atmospheric CO2. For animals we eat they get it from the food they eat, and so on with it also ultimately coming from plants that get it from CO2 in the atmosphere.

That's just contributing to fluctuations in atmospheric CO2 levels, not to an increase over time in CO2 levels. The question asked above probably should have not been about the lowest change in total atmospheric CO2 over the time of the trip but about the lowest change in the amount of CO2 in the atmosphere that had not recently been in the atmosphere before the trip.

How to count the train is also unclear. The most straightforward way would be to figure the energy the train has to expend for the trip and divide it by the number of passengers, then attribute to each passenger the CO2 from producing that energy.

But the train is still going to make that trip regardless of whether or not you decide to take it. One could argue that for this comparison we should be looking at how much additional energy the train uses if you are on it compared to if you are not. That's going to be very small, and the corresponding CO2 is going to be very small even if the train gets it energy from fossil fuels.

You can get a situation where if you have a dirty activity and a clean activity you only actually come out ahead if enough people switch to the clean activity so that the dirty activity can end.

This is something to watch out for. It can lead to cases where the rational behavior is to advocate for the discontinuation of something on environmental grounds but to personally continue to do/use that thing until regulation or economics make it stop. Some people mistake that for hypocrisy but it is not.

If the train is an electric train and it has a clean source of electricity it might be down near walking and biking.

Similar for the EV. If it has a clean electricity source, it too might be down there near walking and biking. If we are including exhaled CO2 it could actually be lower if it has 100% green electricity because your kCal burn rate in the car should be the same as your at rest burn rate.

The ICE car is going to be way up there.

In summary I think then it would be bike and walk very low or even zero, ICE car very high, EV anywhere between walk/bike and maybe 80% of ICE car depending on its electric source, and train somewhere between EV and walk/bike again depending on electric source (assuming EV train).

[1] https://www.globe.gov/explore-science/scientists-blog/archiv...