Central to the CO2-induced global warming hypothesis is the assumption that humans are responsible for nearly all of the increase in the air's CO2 content that has occurred since the inception of the Industrial Revolution, which increase is presumed to have resulted from the burning of fossil fuels that accompanied the rise of mechanized industry and the concomitant explosive growth of the planet's human population. This being the case, one might expect large population centers to exhibit elevated near-surface atmospheric CO2 concentrations; and a number of studies substantiate this expectation, as we indicate here in our brief review of papers pertaining to Phoenix, Arizona, USA, where the concept was first elucidated in detail.
Idso et al. (1998a) measured air temperature, relative humidity and atmospheric CO2 concentration at a height of two meters above the ground at approximate 1.6-km (1.0-mile) intervals prior to sunrise and in the middle of the afternoon across four transect routes through the metropolitan area of Phoenix, Arizona, during a five-day period in January of 1998. These data revealed the presence of what they called an "urban CO2 dome," the two-meter-height atmospheric CO2 concentrations of which were found to be as high as 555 ppm at the center of the city. Concentrations diminished from the city center towards the outlying rural areas, where they were around 370 ppm. Pre-dawn CO2 values inside the dome were found to be considerably higher than mid-afternoon values; and temperature and relative humidity were found to have little influence on either the magnitude or location of the CO2 dome.
In a follow-up study, Idso et al. (1998b) repeated the measurement program described above during the hottest period of the year in an effort to determine whether or not the winter relationship between urban and rural near-surface atmospheric CO2 concentrations would apply to Phoenix in the summer. 
Although maximum and minimum daily air temperatures were 24°C warmer, and wind speeds were nearly twice as great as they were in the winter, near-surface atmospheric CO2 concentrations varied but little between the two times of year. Fine-scale measurements of atmospheric CO2 concentration measured every three seconds while traveling from the eastern edge of the city through the center of the metropolitan area to the western edge of the city (see accompanying figure) reveal the complexity of the urban CO2 dome with its several localized peaks and valleys.
These two studies were the first to document a large and substantial enhancement of near-surface atmospheric CO2 concentration in the urban environment; and they spawned a number of subsequent studies of the characteristics, sources and potential ramifications of this phenomenon.
Much more, for example, was learned from the study of Idso et al. (2002), in which atmospheric CO2 concentration, temperature and wind speed were measured two meters above the ground every minute of almost an entire year in a residential neighborhood of a suburb of Phoenix. Extrapolating their results to central Phoenix, based on data obtained from prior studies, Idso et al. derived the average value for the cold-season maximum near-surface atmospheric CO2 concentration in the vicinity of the city's center: 620 ppm, which is 67% greater than the rural background mean for that time of year. At the residential site, however, the maximum value of the near-surface atmospheric CO2 concentration was only 33% greater than that of the surrounding rural area; and averaged over the entire night, this enhancement dropped to 25% in the cold season of the year and 11% in the warm season, while over the complete daylight period it averaged just over 10% in both seasons. In addition, the authors noted that CO2 concentrations were greater on weekdays than on weekends from 0415 to 0830 in the warm season and from 0445 to 1045 in the cold season. The maximum weekday-weekend CO2 differential was 40 ppm in the cold season and 22 ppm in the warm season.
As to why low-level atmospheric CO2 concentrations behave as they do in the Phoenix metro area, Idso et al. note that "the primary controlling factors of the strength of the CO2 dome are (1) the presence of air temperature inversions at night and in the early morning, which inversions trap vehicular-generated CO2 near the ground … and (2) solar-induced convective mixing during the mid-day period, which greatly dilutes the air's CO2 content near the ground." Secondary controlling factors identified in the study were wind speed and direction, particularly with respect to the locations of primary sources of CO2 (freeways and major roads) and areas of pristine rural air.
Additional insight into the sources of the urban CO2 dome of Phoenix is provided by Koerner and Klopatek (2002). In descending order of importance, and including the percentage of the total CO2 emitted to the atmosphere within the confines of the Phoenix metropolitan area, they list the following contributors: vehicles (79.9%), soil respiration (15.8%), power plants (2.2%), human respiration (1.6%), landfills (0.5%), airplanes (less than 0.1%). Hence, the character of the city's urban CO2 dome is almost exclusively a product of vehicular emissions and the region's distinctive meteorology, as noted in the papers of Idso et al. (1998a, 2001, 2002).
What are some of the consequences of the high near-surface atmospheric CO2 concentrations of urban areas such as Phoenix? And do they significantly impact local temperatures and/or vegetative productivity?
In addressing the first of these issues, Balling et al. (2002) obtained vertical profiles of atmospheric CO2 concentration, temperature and humidity over Phoenix from measurements made in association with once-daily aircraft flights that extended through, and far above, the top of the city's urban CO2 dome during the time of its maximum manifestation over a 14-day period in January 2000. They then employed a detailed one-dimensional infrared radiation simulation model to determine the thermal impact of the urban CO2 dome on the near-surface temperature of the city.
According to Balling et al.'s results, the CO2 concentration of the air over Phoenix dropped off rapidly with altitude, returning to a normal non-urban background value of approximately 378 ppm at an air pressure of 800 hPa. Consequently, Phoenix's urban CO2 dome did not have much of an impact on its near-surface air temperature, creating a calculated warming of but 0.12°C at the time of maximum CO2-induced warming potential. Since this small CO2-dome-induced warming is to be compared to a similarly calculated warming of only 0.46°C for a doubling of the entire atmospheric CO2 concentration, however, it could be construed to imply a much larger warming, perhaps as much as one-fourth of that typically predicted for the globe as a whole by most GCMs for a doubling of the air's CO2 content. Since the larger warmings of GCM predictions are primarily due to water vapor and other large-scale feedbacks that are obviously not operative in a region as small as an urban complex and over timescales defined by mere hours, however, the result derived from the simplified model is more realistic for the case to which the authors applied it. Their final conclusion, therefore, is that the warming induced by the urban CO2 dome of Phoenix is possibly two orders of magnitude smaller than that produced by other sources of the city's urban heat island (lower soil moisture levels and enhanced absorption of solar energy by urban surface materials, for example). Hence, although the presence of man and his alteration of the local environment are indeed responsible for high urban air temperatures (which can sometimes rise as much as 10°C above those of surrounding rural areas) and high urban near-surface atmospheric CO2 concentrations (which can sometimes top 600 ppm), the high urban near-surface air temperatures are not the result of a local CO2-enhanced greenhouse effect.
With respect to its influence on vegetation, the urban CO2 dome likely enhances the robustness of urban vegetation, given the well-documented fact that atmospheric CO2 enrichment tends to enhance plant growth rates and increase the efficiencies with which plants utilize water to produce organic matter. In addition, elevated urban CO2 concentrations should reduce the deleterious effects of airborne pollutants on plant health (Allen, 1990) by reducing the apertures of the stomatal openings by which pollutants gain entry into plant leaves (Pallas, 1965; Kimball and Idso, 1983). Within an urban CO2 dome, therefore, some of the positive effects of one of the major end products (CO2) of urban combustion processes tend to counteract one of the negative effects of some of the minor by-products (air pollutants) of those same processes.
References
Allen, L.H. Jr. 1990. Plant responses to rising carbon dioxide and potential interactions with air pollutants. Journal of Environmental Quality 19: 15-34.
Balling Jr., R.C., Cerveny, R.S. and Idso, C.D. 2002. Does the urban CO2 dome of Phoenix, Arizona contribute to its heat island? Geophysical Research Letters 28: 4599-4601.
Idso, C.D., Idso, S.B. and Balling Jr., R.C. 1998a. The urban CO2 dome of Phoenix, Arizona. Physical Geography 19: 95-108.
Idso, C.D., Idso, S.B., Idso, K.E., Brooks, T. and Balling Jr., R.C. 1998b. Spatial and temporal characteristics of the urban CO2 dome over Phoenix, Arizona. Preprint volume of the 23rd Conference on Agricultural & Forest Meteorology, 13th Conference on Biometeorology and Aerobiology, and 2nd Urban Environment Symposium, pp. 46-48. American Meteorological Society, Boston, MA.
Idso, C.D., Idso, S.B. and Balling Jr., R.C. 2001. An intensive two-week study of an urban CO2 dome. Atmospheric Environment 35: 995-1000.
Idso, S.B., Idso, C.D. and Balling Jr., R.C. 2002. Seasonal and diurnal variations of near-surface atmospheric CO2 concentrations within a residential sector of the urban CO2 dome of Phoenix, AZ, USA. Atmospheric Environment 36: 1655-1660.
Kimball, B.A. and Idso, S.B. 1983. Increasing atmospheric CO2: Effects on crop yield, water use and climate. Agricultural Water Management 7: 55-72.
Koerner, B. and Klopatek, J. 2002. Anthropogenic and natural CO2 emission sources in an arid urban environment. Environmental Pollution 116, Supplement 1: S45-S51.
Pallas, J.E. 1965. Transpiration and stomatal opening with changes in carbon dioxide content of the air. Science 147: 171-173.