Phoenix, Arizona has both a strong urban heat island and a large near-surface atmospheric CO2 dome during periods of atmospheric stability. In this investigation, we use a detailed one-dimensional infrared radiation simulation model to determine the thermal impact of the elevated CO2 levels in the urban environment. We find that the increased CO2 concentrations below the inversion layer contribute only slightly to the observed heat island, which suggests that other factors, such as absorption of solar energy by urban surface materials and lower soil moisture levels, are largely responsible for the observed increase in urban temperatures. ¿ 2001 American Geophysical Union