This is a point most media conveniently forget. Recently there was one that explained a train from Amsterdam to Barcelona costs € 350, takes 2 days and generates 45kg CO2. A plane costs € 100 (or less), takes 2 hours, and generates 200kg CO2.

The option they forgot was a CO2 compensated flight, it costs € 110 (still 240 less than the train), takes only 2 hours and results in net 0kg CO2.

How do you compensate for the CO2 released? Do you contribute to some kind of CO2 removal project? Not being sarcastic or anything I just genuinely don't know how to compensate for CO2.

Yes, most airlines offer this option. There are two ways, one is buying the CO2 rights from someone else (that doesn't have a net positive effect on the world, don't do this if you really want to contribute something) the other is donating money to projects specifically to reduce CO2. Mostly that's tree planting projects, in areas that have had deforestation in the past.

There are a few organizations that audit these projects to guarantee they're actually planting trees and planting the right amount etc.

When I pay to compensate my CO2 emission, it goes into tree planting project.