Air traffic provides significant emissions of greenhouse gases. Because the aircraft is outstanding over long distances, I don´t think the traffic will slow down unless... we get another Eyjafjallajökull.
However, the change has already begun. Experiments with biofuels and solar airplanes like the solar airplane "Impulse" (photos) are some examples. I think we will witness an environmental revolution in the air traffic within a few decades!
I imagine that future aircraft fuel will be a hybrid of solar power and energy stored in batteries. As of today, solar cells and batteries are neither light nor efficient enough. The challenge is mainly to make batteries lightweight, with an excellent capacity for storing energy over time. The "fuel" will be both the excess energy from the solar cells and energy that is stored in the batteries before the start.
In the transition from gasoline to electricity, planes can be built as hybrid models in the same way as the hybrid cars are made today.
If enough resources and effort being put in the work of environmental technology in aircrafts, we could witness giant leaps forward in a short span of time. Just think about how fast the reduction of the computer's size and capacity developed!