Reducing airplane contrails can reduce GHG emissions (but not much))

I never realized it, but apparently airplanes that leave contrails contribute significantly more to greenhouse gases and climate change than those that don't. In fact, according to the UN's Intergovernmental Panel on Climate Change (IPCC), contrails increase aviation's share of global CO2 emissions from 2.5% to 3.5% (although how they can measure that I have no idea).

Contrails, those white vapour trails left in the sky by some planes, are formed in moist, cool atmospheric conditions, when ice and water vapour cling to the sooty emissions of aircraft, and produce a kind of long cloud that trails the aircraft. They are particularly prevalent over the North Atlantic and parts of North America and Europe, where the cool, wet air is most suitable for their formation, and particularly at night.

Not all flights produce contrails, and some routes are bigger offenders than others. Studies have estimate that between 2% and 12% of flights create 80% of all contrails (although that seems a very wide and inconclusive range to me). 

It seems that, in these days of Big Data, contrail creation can be predicted with a reasonable level of accuracy, and that is something that people like Google, the Massachusetts Institute of Technology, and various airlines and plane manufacturers have recently involved themselves with. Airlines in particular are keen to reduce their outsized and rather embarrassing carbon footprints, and until electric planes or some other sustainable aviation fuel (green hydrogen?) become available, this is one of the few things airlines can do right now to mitigate their carbon emissions.

Real world studies are already underway, with some American Airlines flights following routes suggested by Google specifically to reduce their contrail production. This may involve flying at a higher or lower altitude than usual, or taking a different route completely, depending on local atmospheric conditions. Available results thus far from these trials suggest that the re-routing saved 54% of contrails, BUT ... fuel consumption rose by about 2% in doing so. 

American Airlines' chief sustainability officer says that a 2% added fuel-burn is probably too much, and that the trade-off of extra fuel use for fewer contrails is not working out in terms of overall carbon emissions (and certainly not in terms of added costs). Other airlines, including Canadian ones, are eagerly watching the trials, though.