Dr. Dave and Thread Bear (sounds like childrens show characters), they are quite simply using faulty data to produce the result they want! <<They concede that their simulations produced variable outputs depending on the parameters, including a 7.2% increase under some circumstances. 3.9% is the weighted mean.>> If you are going to limit jets to 24000' maximum when they are designed to go at 35,000'+ for efficiency (and the B777 higher still), then it is going to cost a darn sight more than 3.9% or 7.2%! Trouble is, these people look up in the sky and see those contrails and think what a shame they appear to be screwing up the ecology (that is until they want to go somewhere hot and sunny when it's OK for the world to make the sacrifice). So they grab hold of faulty data to prove what they want.
Watch my (virtual) lips- they are wrong. They are using faulty data. Therefore the world will be using up vastly greater resources and producing more pollution to get the job done if these idiots start getting people to believe them! Me- I don't care. If I am made to go below 24,000, suits me- I'll be getting less radiation, flying everywhere lower and vastly slower and not getting so out of breath. But you know it is just a left wing/chemtrailer/Greenpeace creep conspiracy, don't you?