I think the question is the proportionality, i.e., is that 3.5 or 5% impact a disproportionate one. The absolute percentages of even just 3.5% means nothing without seeing it in relative terms. If that 3.5% is caused by 0.0001% of activity then that seems rather significant. I don't see air travel being anything even remotely close to 3.5% of all human climate affecting activity so it strikes me that it really is actually a rather significant matter.
