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Do Urban CO2 Domes Contribute to Urban Heat Islands?
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.

Past research in Phoenix, Arizona has demonstrated the existence of a well-developed urban heat island and a well-developed urban CO2 dome that are spatially and temporally coherent. Given the great interest in the climate-alarmist claim that the historical increase in the air's CO2 content is responsible for the global warming of the past century or so, the authors thought it would be advisable to see if the Phoenix CO2 dome is significantly contributing to the Phoenix heat island.

What was done
Over a 14-day period in January 2000, the authors obtained vertical profiles of atmospheric CO2 concentration, temperature and humidity over Phoenix, Arizona 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 times of its maximum manifestation. 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.

What was learned
The CO2 concentration in the air over Phoenix was found to drop off rapidly with altitude, returning to a normal non-urban background value of approximately 378 ppm at an air pressure of 800 hPa. Hence, Phoenix's urban CO2 dome did not have a great impact on its near-surface air temperature, creating a calculated warming of but 0.12C at the time of maximum CO2-induced warming potential.

What it means
Since the small CO2-dome-induced warming (0.12C) is to be compared to a similarly calculated warming of only 0.46C for a doubling of the entire atmospheric CO2 concentration, it could be construed to imply a much larger "real-city" 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 doings of man are indeed responsible for high urban air temperatures (which can sometimes rise 10C above those of surrounding rural areas), as well as high urban CO2 concentrations (which can sometimes top 600 ppm), high urban temperatures are not the result of a local CO2-enhanced greenhouse effect.

Reviewed 6 February 2002