We do have to keep this in perspective. I don't currently have access to the figures but air transport is currently a very small contributor to greenhouse emissions. For the UK, transport as a whole is responsible (from the top of my head) for around 20% of our emissions but air transport is only a small proportion of that. Its significance is that it is the fastest growing source of CO2 emissions because we're slowly starting to control our other emissions sources.
An example of the wildly inaccurate statements made by environmental protestors can be found in today's BBC news. There are environmental protestors currently at East Mids Airport. Their spokesman claimed; "Aviation is the greatest contributor to climate change". This is absolutely untrue.
see the article below;
BBC NEWS | England | Nottinghamshire | Green protesters in airport demo
[edit: I've just found some firmer stats. In the EU, aviation is currently responsible for 4% of total CO2 emissions]
Matt