Climate Central claims in “Summer Warming (1970–2025) Driven By Climate Change” that summers have warmed in 97 percent of 243 U.S. cities and that human-caused climate change is the leading driver in 91 percent of them. This is misleading at best. The analysis relies on biased average temperature trends and model-based attribution while downplaying one of the most important and well-documented drivers of urban warming: the Urban Heat Island (UHI) effect.
Climate Central claims:
Summers have warmed since 1970 in 97% of 243 U.S. cities analyzed. A new Climate Central analysis shows that human-caused climate change is the leading driver of summer warming trends in 221 (91%) of these cities.
Digging deeper, we find that Climate Central’s findings are based on changes in average summer temperature, calculated from daily highs and lows. That matters. Because when you look at how UHI typically manifests, the strongest signal is not in daytime highs, it is in nighttime minimum temperatures.
The Urban Heat Island is not a fringe concept. It is a well-documented physical process, and it has been observed and mapped worldwide. Even the often-biased website Wikipedia gets it, saying, “[t]he urban heat island (UHI) effect is a meteorological and climatological phenomenon in which urban areas experience significantly warmer temperatures than surrounding rural areas. The temperature difference is usually larger at night than during the day.”
Roy Spencer Ph.D., a principal research scientist at the University of Alabama in Huntsville, and the U.S. Science Team leader for the Advanced Microwave Scanning Radiometer on NASA’s Aqua satellite, has globally mapped UHI, including the United States and published a peer reviewed paper on the subject. Just one look at the U.S. UHI map produced from that paper is all you need to understand the concept.
You can clearly see the UHI in major cities in the United States, Canada, and Mexico.
As summarized in Climate at a Glance: Urban Heat Islands, built-up city environments absorb solar energy during the day and release it slowly at night, suppressing natural nighttime cooling. The result is elevated overnight minimum temperatures, especially in cities with extensive pavement, buildings, and infrastructure.
This is precisely the pattern seen in the real-world measurement comparisons done by Climate Central.
In The Heartland Institute’s Global Open Atmospheric Temperature System project in Reno, Nevada, a properly sited reference station was installed just 1.1 miles from the official airport ASOS station. Both stations experienced the same regional weather. The only meaningful difference was microsite exposure. The official station sits amid runways and pavement. The GOATS station was placed over natural ground, more than 100 feet from artificial heat sources, consistent with National Oceanic and Atmospheric Administration guidance.
The results are telling.
Across two full years of side-by-side data, the official Reno station measured warmer than the reference station on the vast majority of days. The biggest difference was not in daytime highs. It was in nighttime lows. In 2024, the official station averaged nearly 3°F warmer in nighttime readings than the properly sited reference station. In 2025, the nighttime difference was still more than 2°F.
That is classic Urban Heat Island behavior.
It is also important to note that Urban Heat Island effects are most pronounced during summer, precisely when Climate Central is measuring its “summer warming” signal. UHI is fundamentally driven by solar insolation. Asphalt, concrete, rooftops, and infrastructure absorb the greatest amount of solar energy during the long, high-sun days of summer. That stored heat is then released slowly after sunset, elevating overnight minimum temperatures and suppressing natural cooling. The combination of peak solar loading and dense built environments means that summer is when UHI influence is strongest and most persistent. Therefore, analyzing summer averages in urban stations without fully isolating UHI amplification risks attributing a seasonally maximized urban signal to broader atmospheric change.
When Climate Central reports that summers have warmed by 2.6°F on average across 236 cities since 1970, and then attributes at least half of that warming to human-caused climate change in 91 percent of cities, it ignores entirely the UHI. It would be a safe bet to believe that most of those 236 cities fall within the cities on the UHI map produced by Spencer that have experience between 0.2 and 2.0 UHI measured warming. Also undercutting Climate Central’s claim is the fact that it uses a statistical attribution model that compares ERA5-based modeled climate with the built in assumption that human-emissions drive heating with counterfactual ERA5-based model with no anthropogenic forcing. In other words, it is model against model with a big dose of UHI thrown in. That’s not science, it’s pre-determining a result.
Although, the document admits that “secondary drivers” include natural climate variability and the urban heat island effect influence measured temperatures, it treats UHI, the prime factor in measured warming, as residual noise.
Using the daily average temperature as a metric amplifies this problem. Because the average is calculated as the mean of daily high temperature and nighttime low temperature, any systematic upward bias in nighttime lows directly inflates the average. The Reno case study demonstrates that station placement alone can introduce 1 to 3°F of warming into reported records, particularly via elevated overnight minimum temperatures. That is comparable to the multi-decade summer warming Climate Central is highlighting.
This is not a theoretical quibble, it is a measurement integrity issue.
Climate Central emphasizes that 22 more summer days are now “hotter-than-normal” compared to the early 1970s. But “normal” is defined relative to 1991–2020 baselines, and hotter-than-normal days are again based on average temperature metrics. Urban growth and densification has intensified around many of these stations over the past half century, thus part of what is being counted as climate signal is localized microsite warming or UHI.
This is especially relevant for airports, which host many official climate measurement stations. Airports are highly built-up environments dominated by concrete, asphalt, jet exhaust, and infrastructure. The Reno comparison is not an anomaly. It is a case study illustrating the systemic vulnerability in the U.S. surface temperature network as documented in the 2022 nationwide project, Corrupted Climate Stations.
None of this proves that all warming is artificial, the Reno report itself makes that clear. But it does demonstrate that before declaring that 91 percent of urban summer warming is primarily human-caused, one must rigorously quantify and account for station siting bias and UHI amplification of average temperatures. In fact, 91 percent or some similar number may be human caused, but caused by the UHI a result of human development and poor measuring site placement, not “global” climate change.
The Urban Heat Island is a primary factor in urban warming trends, not the footnote that Climate Central treats it as. Before attributing summer warming in 221 cities to human fossil fuel use, Climate Central should first ensure that what they are measuring is a long-term atmospheric temperature trend, not the bias from urban asphalt and artificial heat sources.
Anthony Watts is a senior fellow for environment and climate at The Heartland Institute. Watts has been in the weather business both in front of, and behind the camera as an on-air television meteorologist since 1978, and currently does daily radio forecasts. He has created weather graphics presentation systems for television, specialized weather instrumentation, as well as co-authored peer-reviewed papers on climate issues. He operates the most viewed website in the world on climate, the award-winning website wattsupwiththat.com.
Originally posted at ClimateREALISM
