The question was to what degree are summer minimum temperature climate trends in the latter half of the 20th and early part of the 21st century attributed to local urban development as opposed to global climate change? The approach was to select a range of towns/cities in CA, NV, and AZ for which a pairing of sites from a town/city and a site outside that town/city was possible. Climate records for the period 1948 to 2007 were accessed, and statistical time trends determined for the urban vs. rural locations for towns/cities over a considerable range of population (i.e., from 3.5K to 3.2M).

The urban heat island effect increased with the natural log of the population, ranging from a total change in minimum monthly temperatures of ca. 1.5F to over 12F over the population range of 3.5K to 3.2M. These rates of change in the 1948-2007 period overwhelm any background global climate change, with the exception of the rural sites and smaller towns.

This study for the first time identified the temperature trends of a range of towns and cities in the Sonoran and Mojave deserts to unravel the impact of urban warming from that of global warming in the contemporary global warming era sometimes called the Anthropocene era. Previous literature investigatin these sites were only up to 1984 or did not address the urban warming contribution. The impact depends on land cover and extent of population development over time.