The impact of flying has been very nicely estimated in the freely available book "Sustainable Energy - without the hot air" by the late Prof. David MacKay FRS: http://withouthotair.com/c5/page_35.shtml and http://withouthotair.com/cC/page_269.shtml if you are interested in how he arrives at the estimate.

The book also remarks upon how a (theoretical) revival of ocean liners would actually be worse.