An issue likely of little overall impact of itself but could be fairly significant if applied to all future travel situations like this
I've committed to not flying unless completely necessary for environmental reasons (see https://flightfree.co.uk) as flying can easily overwhelm any other measures to reduce personal carbon footprint
I'm travelling from London to Belfast soon and the non-flying routes are extremely long and will take up a whole day each way (too late to book sleepers which would be more time effective)
Flights cost the same and would free up 2 days. On one of these I could work freelance and earn about £300 after tax.
I'm wondering if I should bite the bullet and fly, work on this day and donate the money to a climate related charity. This generally isn't my giving philosophy but it would assuage the guilt from flying somewhat in a way that another donation to AMF wouldn't.
Return flights would release around 0.26tonnes CO2 equivalent, land travel ~25% of this
Issues:
Lack of reliable info on carbon impact of donations unless I just cannot find this - offsetting appears to be very ineffective
Impact of optics ie loss of influence on others to reduce flights if seen to take them
Loss of personal satisfaction at avoiding flying
Any thoughts / experience of similar?
This isn't exactly a comprehensive answer to your question about what's morally permissible and what isn't, but my view is that if it's going to be a huge hassle and expense for you to avoid flying, you shouldn't make yourself feel awful about the fact that you've done something you regard as less than ideal. I would just donate to Clean Air Task Force (probably an amount that will more than cover the impact of the flights in expectation) and continue trying to avoid flights in future when feasible if you think that's something you want to do.
I highly recommend the Founder's Pledge Climate and Lifestyle report.* Take a look specifically at Figure 5 and you will see that donating a small amount to one of their highly effective charities dwarfs the emissions from a single flight like the one you're proposing here.
Zooming out a bit, in my opinion these kinds of decisions shouldn't about guilt (see replacing guilt series) but rather about taking the action that on net is best for others. In that framework working and donating to an effective charity that combats climate change--one of the world's most pressing problems--is likely better than wasting days in a less efficient form of transit and not making such a donation.
*A caveat not relevant here is I think their approach in the report to the impacts of having a child are mistaken but otherwise its a great report.