Are there multiple routes to a zero emissions bus future?

The efficiency benefits of electrifying city buses are well established, with their predictable energy requirements and return-to-base schedules simplifying charging headaches. Given the lifespan of the typical bus—ten to 15 years—and the urgent need to reduce emissions, public transit authorities are unlikely to place orders for pure internal combustion engine (ICE) buses moving forward. Although the vast majority of manufacturers agree that electric is the way to go, might there be different routes to a zero emissions future?