Cool Factor

Carly Ziter rides a bicycle fitted with an air temperature sensor through an urban park.

Eric Pedersen

Trees can offset the urban heat island effect—that city temperatures are higher than those in surrounding areas due to more impervious surfaces—by providing shade and evapotrans-piration. But how many trees are needed to realize a significant change in temperature?

Researchers led by biologists Carly Ziter and Monica Turner, then both at the University of Wisconsin-Madison, used air temperature sensors attached to bicycles to track how temperatures related to tree and impervious surface cover. They biked more than forty miles of routes through downtown Madison during the summer of 2016, sampling ten different routes at least three times during the hottest part of the day. Four routes were also sampled at night. The scientists corrected for temperature changes due to weather and daily heat fluctuations by comparing recorded temperatures to the average temperature at the city center.

During the day, blocks with 100 percent tree cover were more than 1.5 degrees Celsius cooler than areas with no tree cover. In contrast, blocks with 100 percent impervious surfaces were 1.3°C warmer than blocks with no impervious surfaces. Nighttime temperature differences were less pronounced.

Adding trees had the largest cooling effect in areas that had more than 25 percent impervious surface cover and at least 40 percent tree cover. This effect disappeared when impervious surface cover was too high and there wasn’t enough space for trees to grow. Planting more trees may be most effective in neighborhoods close to the tipping point of 40 percent tree cover and with intermediate levels of impervious surface cover. Repeating this study in other cities of varying sizes and climates may provide broader insights into best practices for combating the effects of rising air temperatures and extreme heat waves. (Proceedings of the National Academy of Sciences)