Flying is ok because aviation only produces a few percent of global CO2 emissions.

Truth: Globally, it is true that aviation does not produce more than a few percent of CO2. However if you take a single intercontinental flight every year, this is likely to dominate your own personal CO2 emissions.
More importantly, emitting the gas high up in the atmosphere means that the global warming effect of every tonne of CO2 emitted by a plane is 2-4 times worse than that of the same tonne emitted on the ground!
Another problem is the speed of growth of the aviation industry: between 1990 and 2001, UK passenger numbers increased by 73%. Over the same period, average fuel consumption per passenger per kilometre travelled actually increased by 12%!  In 2008, aviation was responsible for 23% of transport fuel use in the UK.