It seems to be related to their "climate action plan".
"In 2006, Berkeley voters overwhelmingly endorsed Measure G, a ballot measure that mandated that Berkeley reduce our entire community’s greenhouse gas emissions by 80% below 2000 levels by 2050. The plan is designed to reduce community wide greenhouse gas emissions 33 percent below 2000 levels by 2020, and 80 percent by 2050."
I say, good for them for trying to stick with their plan.