Cars are bad but they're a pretty small problem compared to airplanes. A single trip across the Atlantic and back is about equivalent to commuting in a SUV for a year. Per passenger-mile cars and planes use about the same amount of carbon but people tend to take far longer plane trips.
But it seems to me that the marginal cost in terms of CO2 emissions is not exactly clear for flying. Whether I fly or not, the planes will fly. My addition to a single plane in terms of the weight is not that significant. It becomes a more murky economic argument. I agree people should not fly, but this is more of a collective action type approach in my view. It takes roughly a plane load of people to make a difference.
In contrast, a driver has much more direct control over the CO2 emissions. The marginal addition of CO2 is much clearer.
If people stop or reduce flying one by one there's clearly some point at which a stepwise reduction in service occurs. Your are unlikely to be the person who triggers this but the reduction will also be proportionately larger than your contribution too. So you should think of yourself as reducing carbon emissions by the person-mile amount in expectation even if it will never actually be that amount.
Or to frame it another way, when you fly you're playing Russian roulette with adding an amount of CO2 to the atmosphere dwarfing anything else you might be causing.
