Over the past five years, measurements show that atmospheric carbon dioxide (CO2) concentrations in downtown Phoenix, Arizona can be more than double ambient levels in nearby rural areas. The high CO2 levels in Phoenix are related to (a) high reliance on passenger vehicles, (b) a calm, relatively stable desert climate, and (c) the local topography. Principal components and canonical correlation analyses revealed an “urbanization” component in the data that dominated explained spatial variance in CO2 concentrations. A vegetation component also contributed to explained spatial variance with vegetation being positively related to CO2 in the mornings and negatively related in the afternoons. Due largely to atmospheric stability, CO2 concentrations were greatest in the morning hours during the winter months and least in summer afternoons. Throughout the year, the CO2 levels were highest on weekdays and least on weekends. We used a detailed one-dimensional infrared radiation simulation model and determined that the higher CO2 concentrations in the winter morning periods create a warming of approximately 0.1°C, which is a full order of magnitude less than the observed increase in urban temperatures.