One such solution I ran across while doing some research is AGE-85 -- short for Aviation Grade Ethanol. Ethanol as an internal combustion engine fuel has been around nearly as long as the engines themselves. In modern times it was studied in earnest in the 70s, and is enjoying interest -- and development -- once again.
The beauty of AGE85 is that rather than requiring a switch to diesel engines -- no small feat -- AGE-85 can be used as a replacement for traditional avgas.
According to AGE-85.org:
"AGE-85 (Aviation Grade Ethanol) is a high-performance fuel that may be used in any piston engine aircraft. It contains approximately 85% ethanol, along with light hydrocarbons and biodiesel fuel. AGE-85 is specifically blended for cold starting and good mixture balance. AGE is unleaded, burns cleaner, has lower exhaust emissions and is more environmentally friendly than traditional aviation fuels. The ethanol in AGE-85 prevents carburetor and fuel line icing, and provides excellent detonation margins."
Sounds promising, if not yet a reality.