Exacerbated by climate change, extreme city heat is an increasingly common concern. Thankfully, trees can help. For decades, city managers have turned to oak, maple, linden, and other species to provide shade and to cool city streets. A large body of research shows that increasing neighborhood tree cover decreases temperatures. However, until recently, scientists knew little about how these benefits scale to entire cities, leaving policymakers without robust tools for planning citywide greening.
To address this gap, a multinational team of researchers from the Chinese Academy of Sciences in Beijing and Cary Institute of Ecosystem Studies in New York modeled how the impact of tree cover changes with scale. The new research, published in Proceedings of the National Academy of Sciences in November 2024, extends scientific understanding of the cooling power of urban trees from local scales to entire cities.
Fueled by human activity and man-made materials such as asphalt and concrete, city temperatures often soar past those in greener surrounding areas, which can impact the health and quality of life of city residents. Tree planting is one of the most common ways cities combat extreme heat in the long term. Trees shade buildings and sidewalks. They also move water from the ground into the air through their leaves, absorbing heat along the way. Given the well-supported benefits of urban trees in city blocks and neighborhoods, the scientists wondered: How does this local-scale cooling relate to cooling across an entire city?
To investigate, the research team considered cooling efficiency, or the reduction in temperature observed with every 1 percent increase in urban tree canopy. The study included four cities with different climate conditions: Sacramento and Baltimore in the United States and Beijing and Shenzhen in China. The team looked at each city at multiple scales, dividing them into squares that ranged in size from roughly a city block to an entire city. They gathered information on tree canopy cover and land surface temperature using remote sensing and thermal satellite data. Then, they calculated cooling efficiency for the block-size squares all the way up to the city-size squares. Finally, they graphed the change in cooling efficiency as the scale increased.
The results showed that at larger scales, the cooling efficiency of urban trees went up, with increasing tree cover leading to greater cooling – but it was not a one-to-one relationship. Instead, the data could be described with a statistical relationship called a power-law form: As the team transitioned from analyzing many small squares to fewer, larger squares, cooling efficiency rose rapidly before leveling out as the squares approached city-scale. In Baltimore, their results predicted that a 1 percent increase in tree cover at the block level would lead to about 0.26 degree Fahrenheit of cooling, whereas the same increase at the city level would lead to about 0.35 degree of cooling. The power-law form fit the data across cities, indicating cooling efficiency scales in a predictable pattern across a wide array of climates. This exciting result suggests that our existing knowledge of how trees cool neighborhoods may be able to be extrapolated to make predictions about the city-wide scales considered in urban planning.
The researchers suggest that cooling efficiency may have increased at greater scales because large areas can include large groups of trees. Larger groups of trees tend to facilitate more cooling though evaporation.
By bridging the gap between data-driven science and on-the-ground planning, this study provides a powerful tool for municipal leaders, as city planners may be better able to predict how much tree cover they need to achieve city-cooling goals.