Remember when I measured the UHI in Reno, NV? It seems the normally alarmist “Climate Central” is just now getting around to recognizing UHI, but of course, they have to put in the obligatory disclaimer that it cannot possibly contribute to the global warming signal. Well, they are just flat wrong about that, but that’s what they are paid to say.
Since 1970, summer temperatures have been rising. While exact rates of warming differ between regions, most cities have been heating up faster than adjacent rural areas all across the United States. The concrete and asphalt surfaces in city buildings, roads, and infrastructure hold more heat and release that heat more slowly than vegetation and organic surfaces. This is known as the Urban Heat Island (UHI) effect. Climate change then takes that heat and makes cities even hotter.
In a new report, Climate Central analyzes how UHI and climate change have affected 60 of the biggest American cities since 1970. The study examines the difference between average summer temperatures in urban areas and nearby rural areas. Some cities had much higher temperature differences: 23 different cities experienced single days that were an astonishing 20°F warmer than the rural areas around them.
With more than 80% of Americans living in cities, these urban heat islands, combined with rising temperatures caused by increasing heat-trapping greenhouse gas emissions, can cause serious health effects for hundreds of millions of people during the hottest months of the year. Heat is the number one weather-related killer in the United States, and the hottest days, particularly days over 90°F, are associated with dangerous ozone pollution levels that can trigger asthma attacks, heart attacks and other serious health impacts.
The study ranks the 60 cities by the intensity of their heat island effect, shows how heat drives air pollution (ozone levels) in nearly every city analyzed, lists cities that have far more days over 90°F than adjacent rural areas, and shows how most cities are warming faster than the surrounding rural areas.
analysis of summer temperatures in 60 of the largest U.S. cities found that:
- 57 cities had measurable urban heat island effects over the past 10 years. Single-day urban temperatures in some metro areas were as much as 27°F higher than the surrounding rural areas, and on average across all 60 cities, the maximum single-day temperature difference was 17.5°F.
- Cities have many more searing hot days each year. Since 2004, 12 cities averaged at least 20 more days a year above 90°F than nearby rural areas. The 60 cities analyzed averaged at least 8 more days over 90°F each summer compared to adjacent rural areas.
- More heat can increase ozone air pollution. All 51 cities with adequate data showed a statistically significant correlation between higher daily summer temperatures and bad air quality (as measured by ground-level ozone concentrations). Temperatures are being forced higher by increasing urbanization and manmade global warming, which could undermine the hard-won improvements in air quality and public health made over the past few decades.
- In two thirds of the cities analyzed (41 of 60), urbanization and climate change appear to be combining to increase summer heat faster than climate change alone is raising regional temperatures. In three quarters (45 of 60) of cities examined, urbanized areas are warming faster than adjacent rural locations.
- The top 10 cities with the most intense summer urban heat islands (average daily urban-rural temperature differences) over the past 10 years are:
- Las Vegas (7.3°F)
- Albuquerque (5.9°F
- Denver (4.9°F)
- Portland (4.8°F)
- Louisville (4.8°F)
- Washington, D.C. (4.7°F)
- Kansas City (4.6°F)
- Columbus (4.4°F)
- Minneapolis (4.3°F)
- Seattle (4.1°F)
- On average across all 60 cities, urban summer temperatures were 2.4°F hotter than rural temperatures.
Urban heat islands are even more intense at night. Over the past 10 years, average summer overnight temperatures were more than 4°F hotter in cities than surrounding rural areas.
Urban heat measured by satellite in Louisville, Ky. Click image to enlarge.
Several independent studies have shown that urban heat islands (in the U.S., and around the world) do not bias global warming measurements, ruling out the possibility that rising global temperatures have been caused by urbanization alone.
Research suggests that urban planning and design that incorporates more trees and parks, white roofs, and alternative materials for urban infrastructure can help reduce the effects of urban heat islands.
But rising greenhouse gas emissions are projected to drive average U.S summer temperatures even higher in the coming decades, exacerbating urban heat islands and their associated health risks.