Urban heat island intensity decreased as average temperatures rose over the last 15 years in a large ensemble of cities, according to researchers from the US. The finding contradicts a number of other studies.
“Urban heat island research is important for the billions of people who live in cities who may be potentially more at risk for heat-related illnesses,” says Anna Scott of John Hopkins University, US. “In a warming world, heat represents a unique but silent risk that urban planners and disaster management officials need to pay attention to. Research into the urban heat islands provides the information that allows decision makers to protect their residents.”
Urban heat islands have been well-documented for several decades. The phenomenon arises as the high concentrations of roads and buildings in cities trap heat more efficiently than the surrounding rural and suburban land, raising temperatures. In recent years, several studies have concluded that as the climate warms, the intensity of urban heat islands – the temperature difference between urban and rural areas – tends to increase.
The need for further research into urban heat island intensity has become critical, according to Scott. Scott and her colleagues recognised that previous studies on urban heat island intensity only considered the effect on individual cities over shorter timescales. To gather more comprehensive data, the researchers used weather data from 54 cities across the US between 2000 and 2015. They calculated the difference between the temperature values recorded at urban and rural weather stations at each location, for both maximum and minimum daily temperatures.
In 38 cities the temperature difference between the weather stations tended to be lower for higher background temperatures, the team found – especially in moister climates. The result held even accounting for extreme heat and variations between different climates.
“Our research shows that during many warmer conditions, temperature differences between cities and rural areas actually decrease because of temperature sensitivity in rural areas,” says Scott. “Many people think the opposite is true, so this has potentially important implications for how governments think about heat in rural areas, which we find can sometimes get left out of the heat-related health discussions because we often focus on urban areas.”
The researchers believe that large-scale trends in weather conditions are responsible for their result, suggesting for the first time that heat mitigation efforts may need to be increasingly focused outside cities.
Scott acknowledges that urban heat island intensity is not the only important parameter when considering the effects of climate change on cities. “Many factors affect thermal comfort, including humidity, which we didn’t consider in this study,” she says. “So, this doesn’t say that climate change won’t affect cities but rather, suggests that rural areas may be more sensitive to warming than previously thought.”
In the future, the insight could be important to consider when making economic projections of climate change and designing methods for relieving and mitigating the effects of heat.
Scott and colleagues reported their findings in Environmental Research Letters (ERL).