The way I look at it is this...
For environmental issues; burning fuel the way we do (not just pilots) cannot be solved by individuals. Big corporations and governments must deal with it.
We can of course all do our part in adding less to it, but as a job, it is a no brainer. As a pilot, I may be adding to the slowly deteriorating health of the world, but look at other professions...some cause immediate death, worsening health and so on.
As a pilot, I feel I'm adding more to the world's economy and bringing the world closer than, say, a lumberjack (just an example, please don't take offence) who is cutting down a tree to only produce maybe a few tables and chairs; and at the end probably contributed to as much demage as I did. OR, bringing food to those who need it via the fastest way instead of letting them rot in a ship that may take a few weeks or months (who incidentally also burn fuel)...
It's cost vs effect here and unless I'm flying purely for the heck of burning fuel, I don't think there is a hugh burden on my conscience as long as I know I am doing more good than bad.
For ethical issues; as long as aeroplanes are flying, and airlines (or any airceft operating company) are hiring pilots, then it is not up to pilots to say they are doing a disservice to the world.
Someone has to do the job and unless there are alternatives, pilots are using the only means there are in this world to move people and goods around the globe in the quickest and most effcient way possible.
How sure are you, if you don't become a pilot, that you will add little to the world's woe and instead help lessen it? Unless you become a hard-core environmentalist (and they are not always in the right, or use ethical solutions), there is just no way to be sure. A wall-street exec uses paper, drives to work, heats his apartment and may drive up the cost of food through other means....
So, do a job you know how to and do it well. That is all anyone could and should strive towards. On your spare time, show the world you care through other means.
Richard Branson pledged almost all of Virgin Airlines' profit for the next 10 years to solving environmental woes instead of stopping his aeroplanes from taking off. He understands that stopping flying is not a solution, it will only add to the woe in other ways.
For economical issues, there is nothing to worry about. The laws of economics will always dictate that only the more efficient methods will survive in the long run. So unless a new and better way can be found, flying aeroplane is it.
I understand that you may worry that one day fuelled-aeroplanes will not be around any more, but surely you will agree that another means of transport will have to replace it?! It will most likely be similar in nature (nuclear cessna 172? or something like that?)...and guess what...they'll require you to have some form of licence to operate it...and the transition will also likely be that pilots can convert to it first...would it then be easier or harder (costlier?) for you to get this new license? You'll be much older, with no experience but plenty of regret...
I guess the questions you have to ask yourself are: do you enjoy flying? Do you want to work as a pilot?
If your answer to both is 'yes'...then anything else is a distraction...we cannot all be hippies (I'd really want to be one, but I'll have to wait till my children are done with university...) and there is no guaranteed way to save the world without causing other problems...just live today the best way you know how...do something you enjoy and is useful to others.
Merry Christmas...