During a recent conference on urban sustainability, a city planner asserted, "Implementing green roofs on more buildings will significantly improve city air quality and reduce the urban heat island effect. Therefore, we need to incentivize building owners to install green roofs."
This claim is based on the assumption that there are no other significant factors affecting air quality and the urban heat island effect beyond the presence of green roofs. If this assumption is invalid, then the planner's conclusion might not hold.