The diurnal temperature range (DTR) is the difference between the daily low temperature at night and the daily high temperature during each day. In some places this range is decreasing because the nights are getting warmer. Proponents of carbon dioxide induced warming (AGW) tout this as proof that carbon dioxide is indeed warming the planet.
In the following graph, compiled from the weather station at Phoenix Sky Harbor airport, we indeed see that the nightly minimums (red) are rising while the daily highs (blue) are fairly steady. This could imply that carbon dioxide is trapping heat at night and delaying the nightly cooling in absence of sunlight.
Apparently, however, carbon dioxide works only in the cities. The same kind of data collected near the airport in rural Childs, Arizona (near Ajo), show no such rising nighttime minimum temperatures.
Rising levels of atmospheric carbon dioxide fail to explain the different DTR responses between rural and urban stations. However, the urban heat island effect does provide an explanation. During the day, concrete and asphalt in cities absorb energy from the sun and radiate it back during the night, thereby making the nights warmer.
Many researchers have studied the urban heat island effect. For instance, Gallo et al., from the National Climatic Data Center found: “Those stations that were associated with predominantly rural land use land cover usually displayed the greatest observed DTR, whereas those associated with urban related land use or land cover displayed the least observed DTR.” That means the night temperatures in urban areas were warmer than those in rural areas.
The warming effect of urbanization is shown by many examples. For instance, the next graph compares the stations in urban Tucson versus rural Tombstone:
You can see that Tucson temperatures have been rising but there is no trend in the rural Tombstone station. Carbon dioxide works in mysterious ways. This difference between urban and rural stations is not a local phenomenon, but a world-wide one. For instance, the next graph compares the temperature records of urban and rural stations in South Africa:
This dichotomy of temperatures shows that land-based temperature data have an artificial warming bias since most stations are in or near urban areas. Of course, the keepers of the data say they can mathematically “correct” the data to account for the urban heat island effect. But too often, it seems that they are “correcting” away inconvenient records. For instance, read how NASA is “correcting” away the warming of the 1930s here.
Once again, the hypothesis of carbon dioxide caused warming fails in the light of actual data.
Gallo, Kevin P.; Easterling, David R.; and Peterson, Thomas C., “The Influence of Land Use/Land Cover on Climatological Values of the Diurnal Temperature Range” (1996). Papers in Natural Resources. Paper 191, Journal of Climate 9.