  In this commentary we discuss the validity of scaling laws and their
relevance for understanding urban systems and helping policy makers. We show
how the recent controversy about the scaling of CO2 transport-related emissions
with population size, where different authors reach contradictory conclusions,
is symptomatic of the lack of understanding of the underlying mechanisms. In
particular, we highlight different sources of errors, ranging from incorrect
estimate of CO2 to problems related with the definition of cities. We argue
here that while data are necessary to build of a new science of cities, they
are not enough: they have to go hand in hand with a theoretical understanding
of the main processes. This effort of building models whose predictions agree
with data is the prerequisite for a science of cities. In the meantime, policy
advice are, at best, a shot in the dark.
