Since it will occur in my lifetime all I have to do is sit back, relax, and watch it happen. When the oil runs out and prices run high, nothing will take to the air. Airlines will sell fewer tickets, lay off thousands of workers, go bankrupt, get bought out. The usual. Eventually no airline will exist, just like supersonic transport. Gas guzzling unessential industries will be abandoned. The only aircraft flying will be military and scientific (perhaps jointly) powered by who knows what kind of powerplant. I doubt the military cares about increased NOx emissions. If hydrogen-burning jet fighters are the only thing in the air, then we will retain air supremacy. I personally don't care either if the military uses hydrogen - go for it. Even a major armed conflict won't pollute as much as general usage. The rest of us will use something else.