It is not exactly clear whether passenger flights are less environmentally friendly than various other options. Driving for long distances can be worse for the environment than flying commercially[0]. The article claims that Greta saved energy by travelling by train, but it is unclear how the method of calculation (ecopassenger.org) works, whether it considers that rail and road infrastructure requires far more energy and fuel for maintenance than the air and runway does and even the data entry is flawed, since you cannot change the load of plane and train separately, even though it is clear that flight operators often overbook planes, while trains often run with pretty low loads.
[0] http://www.umtri.umich.edu/our-results/publications/making-d...