Urban Heat Island warming effects related to population density are quantified for the first time

A new research study from The University of Alabama in Huntsville addresses the question of how much urban areas have warmed from the Urban Heat Island (UHI) effect.

The findings are published in the Journal of Applied Meteorology and Climatology.

UAH Earth System Science Center Research Scientist Dr. Roy Spencer and UAH Earth System Science Center Director and Alabama State Climatologist Dr. John R. Christy have spent several years developing a novel method that quantifies, for the first time, average UHI warming effects related to population density. Their study is titled “Urban Heat Island Effects in U.S. Summer Surface Temperature Data, 1895–2023.” The study was co-authored by UAH researcher Danny Braswell.

Spencer and Christy’s method uses millions of thermometer observations to quantify the well-known tendency for urban areas to be warmer than rural areas. By relating differences in temperature to differences in population density in six classes of population density and in 22 different historical periods between 1880 and 2020, the researchers were able to quantify the average rate of warming as a function of population density.

“While the statistical signal of urban warming was rather weak due to noise in the data from other non-urbanization effects on weather, it was very consistent, showing up in all six classes of population density across all 22 historical periods,” Spencer says.

“One of the interesting results was that the most rapid warming occurs for population growth at the lowest population densities,” Spencer notes. “Then, in heavily urbanized areas, the warming reaches a peak, with little additional warming as the population increases.”

The issue is important because the estimation of the rate of warming in the U.S. due to climate change can be influenced by non-climate processes, such as the Urban Heat Island effect, which would exist even without global warming. For the period 1895 to 2023, it was found that 8% of the rural warming trend was due to the urbanization effect, increasing to about 65% of the observed warming trend for suburban and urban locations.

The thermometer data used was the “raw” data archived by NOAA before any adjustments were made to the data. Official temperature trends the public sees in news reports are based upon adjusted (“homogenized”) data that use comparisons between neighboring stations’ temperature trends to find break points in individual stations’ records, and adjust the measurements based upon a kind of “voting” procedure.

Spencer and Christy—along with a number of other researchers—believe this has not sufficiently removed UHI effects from the data. But it remains to be seen what impact their results will have on the debate over just how much warming has been observed in the U.S. over the last century, and whether current official warming trends have been significantly contaminated by urbanization effects.