  The question of whether urbanization contributes to increasing carbon dioxide
emissions has been mainly investigated via scaling relationships with
population or population density. However, these approaches overlook the
correlations between population and area, and ignore possible interactions
between these quantities. Here, we propose a generalized framework that
simultaneously considers the effects of population and area along with possible
interactions between these urban metrics. Our results significantly improve the
description of emissions and reveal the coupled role between population and
density on emissions. These models show that variations in emissions associated
with proportionate changes in population or density may not only depend on the
magnitude of these changes but also on the initial values of these quantities.
For US areas, the larger the city, the higher is the impact of changing its
population or density on its emissions; but population changes always have a
greater effect on emissions than population density.
