It's good to think about the enviroment, but as the other members posted a dirt bike is not the big issue when you compare it to a vessel, an airplane, a truck or a car as the time of use has to be taken in consideration.
Anyway if you want to ride more enviroment friendly, I recommend enviroment friendly gasoline and two stroke oil,
like the Swedish Aspen gasoline: http://en.aspen.se/Consumer/Motor_sport
I don't know if you can get it in the US but as this gasoline was developed for the forest industry it might be available in the US too.
Just to note: The standard chain saw gasoline have not enough octane, you will need Aspen+ with 95 octane or Aspen racing with 100 octane for avoiding any engine damage. Btw. the Aspen gas is completly ethanol free and can be stored in years! I'am using it now for three years without any problems.