What is "necessary"? I, too, regard global warming as the biggest threat to our species. I still don't think that makes someone a hypocrite for flying.
I think maintaining a quality of life for humanity is necessary. Sacrificing everything in the name of the environment is paperclip maximizing - what we really want is to find the optimal level of attention to the environment that gives us the best value over time.
I personally don't have a strong opinion here but do feel like most people lose track of the global picture yet get fixated on individual problems. I think this happens because we don't have a measure of the impact of our activity, we just vaguely feel like some actions are "good" and some "bad".
Corny example, recycling is considered "good", yet still has a net negative impact, but for most people it's enough to feel good and stop worrying.
Very few things are strictly necessary, but many are definitely unnecessary: perverted maximisation of profits, planned obsolescence, avoidable food waste etc. Tech example: avoidable software bloat causing global increase in energy consumption.
What if you are flying an endangered animal some where it can reproduce? Is that ok?