Flying is contributing ~2.5% to the global CO2 emissions, due to the emission in the stratosphere the climate impact is substantially higher, currently estimated at around 5% in the EU. At this level its impact is about a third of all emissions from transport, or 1/5 of that of global energy production.  And it typically dominates the emission budget of scientists.
Currently the emissions from flying is increasing yearly, e.g. in the EU by 87% between 1990 and 2006  continuing until now . This is due to a highly competitive market, global marketing, and a substantial public subsidy by e.g. missing taxes on kerosene.
In comparison to other means of transportation, flying has a very high CO2 emission per trip and also per person-kilometer. Per person-km it does consume similar to a (single or double occupancy) car, but trips are of course substantially longer, typically. Train travel consumes less per km, substantially, with details depending on electricity source, hence country, train type, etc. But in any case per trip a flight generally emits substantially more CO2 than any other means of transportation. In addition, the emission at altitude of airplanes has an extra climate impact due to water vapour reflecting infrared heat radiation back down. The climate impact of flying is hence twice as high as for the same amount of CO2 emitted at sea-level.
So, what to do?
Do these 5% contributions matter? Yes, they do: (a) Because humanity needs to become carbon neutral asap, this means reduction of emissions at any source. (b) The emissions from transportation are rising continuously, also for flying. There is a major boom of vacation, business trips, trade, and academic travels. More destinations, more often, more distant. At an affordable, highly publicly subsidised price. This trend needs to be reversed.
What we as academics need to do make sure that we emit CO2 wisely. Meaning: To assess (a) whether a certain trip is necessary, (b) if there are lower carbon-footprint means of transportation, (c) if we can change in the future what constitutes a “necessary trip”, by change of our metrics and by means of communication technology. We don’t have clear solutions for (c) and need to develop these, but we have (b) often available and can evaluate (a). What does this entail? We will come to this in other posts.