Featured article: Ziter, C.D., E.J. Peterson, C.J. Kucharik and M.G. Turner. 2019. Scale-dependent interactions between tree canopy cover and impervious surfaces reduces daytime urban heat during summer. PNAS 116 (15): 7575–7580. DOI: https://doi.org/10.1073/pnas.1817561116
If you have ever hiked city streets on a sweltering summer day, you know something about the science of urban climate: It’s hot out there. A new study by Concordia University’s Dr. Carly Ziter and her colleagues used a bicycle-mounted sensor array to measure temperatures across urban Madison, Wisconsin, adding vital detail to our understanding of when and where extreme heat occurs in the city and what we humans can do about it.
Since the 1800’s scientists have recognized that cities were often warmer than the nearby countryside. Dubbed the “urban heat island”, this effect is mainly a product of the loss of shading and cooling by natural vegetation and its replacement with so much impervious material like bricks and pavement that soak up solar heat. The urban heat island already causes summertime discomfort along with heat-related deaths and illnesses in cities worldwide (Harlan and Ruddell, 2011). Combined with the impact of climate change and the booming population of city centers, keeping our cities cooler poses one of the most significant health challenges for the 21st century.
Though we know cities are generally hotter, measuring exactly when, where, and how temperature varies across the urban landscape is more difficult than it might seem.
Automated weather stations record temperature at fixed locations, and regular satellite overflights can detect infrared-based surface temperatures more broadly. But none of the traditional measurement tools give a direct measure of conditions near the surface as they change through time as they vary with other surface characteristics. A more detailed picture of temperature could offer key insight into what drives the urban heat island and what tools we might use to reduce it.
Science by Bicycle
To get a more detailed look at the urban heat island, Dr. Ziter and her team went for a bike ride. In the summer of 2016 the authors repeatedly rode several transects crisscrossing the city of Madison, Wisconsin, on a bicycle fitted with a compact air temperature sensor. Their instrument package took frequent temperature measurements as they crossed into different terrains over the course of the ride (Figure 1). Combining these measurements with GPS location and detailed maps of the surface cover across the city revealed how temperature varied with location, time, and surface cover.
What they found was that daytime temperatures varied by as much as 3.5 °C from one part of the city to another, but was consistently lower in the presence of tree canopy cover – especially with canopy cover of more than 40% (Figure 2). At night, in the absence of any effect of shading, temperature rose more than 0.7 °C between areas with 0% and 100% impervious cover. And if you were hoping to keep cool by planting a tree just in front of your own house, try thinking more neighborly: Because the researchers had such fine-scale data, they were also able to show that the best predictor of temperature was the combined tree canopy and pavement cover within about a city block (60–90 m) of the sensor, rather than cover only in the immediate vicinity.
These results of this study suggest that it may be possible to meaningfully reduce the temperature extremes of the urban heat island. By increasing tree canopy cover and reducing pavement cover, cities like Madison might achieve up to 2.5 °C of temperature reduction from daytime extremes. These reductions would likely work most effectively, though, changes in cover were made on a larger scale of cities blocks or whole neighborhoods.
Staying Cool in the Urban 21st Century
Adding trees was the most efficient strategy for reducing daytime high temperature, particularly to parts of city that already have at least 40% tree cover. Alternatively, reducing heat at night – which may blunt the worst health impacts of the urban heat island – may require reducing impervious cover or lowering the amount of heat it can absorb, similar to ongoing experiments in applying brighter street coatings in Los Angeles. The authors acknowledge that these results also raise questions of social equity, since the hottest parts of cities often include lower-income areas with little existing tree canopy, and which are potentially home to more vulnerable populations.
As the climate warms and people increasingly move to town, reducing high temperatures is an increasingly important municipal responsibility. The work of Dr. Ziter and her colleagues sheds valuable light on just how city planners and policymakers can act to protect the health of future urban multitudes from excessive heat.
Harlan, S.L. and D.M. Ruddell. 2011. Climate change and health in cities: impacts of heat and air pollution and potential co-benefits from mitigation and adaptation. Current Opinion in Environmental Sustainability 3: 126–134. DOI: https://doi.org/10.1016/j.cosust.2011.01.001
Ziter, C.D., E.J. Peterson, C.J. Kucharik and M.G. Turner. 2019. Scale-dependent interactions between tree canopy cover and impervious surfaces reduces daytime urban heat during summer. PNAS 116 (15): 7575–7580. DOI: https://www.pnas.org/content/116/15/7575