What is the most 'environmentally friendly' way to learn to fly?
Here are some options for flying in an environmentally friendly way:


Use an electric trainer: Since last year the all electric trainer aircraft Pipistrel Alpha Electro has FAA certification. Assuming the batteries are charged with renewable energy, this would mean no greenhouse gas emissions at all (excluding manufacturing). It will probably still take some time until larger all electric aircraft with higher ranges are available, but Airbus has already started research.
Use a glider with a winch: Assuming the winch is powered by renewable energy, this would also result in no greenhouse gas emissions. While winch launches are less common in the US, this might change in the future.


If you use a glider with tow plane, it will depend on how long you can fly with the glider. Assuming good thermals you could stay in the air for quite a while, which would easily be better for the environment than a single engine prop plane. But for training (where you would presumably want many takeoffs and landings), I agree with you that the tow plane would now use more fuel because it has to drag the glider along.