Summary: A groundbreaking study has developed a new tool to help cities plan their tree-planting initiatives for maximum cooling impact. The research, analyzing data from four major cities across different climate zones, reveals that trees’ cooling efficiency increases at larger scales, providing crucial guidance for urban planners tackling rising city temperatures.
Journal: Proceedings of the National Academy of Sciences, November 4, 2024, DOI: 10.1073/pnas.2401210121 | Reading time: 4 minutes
As cities worldwide face increasing heat challenges, urban planners have long turned to tree-planting as a natural cooling solution. However, until now, they’ve had to rely largely on guesswork when planning city-wide initiatives.
The Natural Air Conditioning Effect
“Trees offer many benefits to cities, and cooling is one of them,” explains Steward Pickett, an urban ecologist at Cary Institute of Ecosystem Studies and study co-author. “Trees are good at cooling because they pump a lot of water from the ground into the air, and when that water evaporates at the leaf surface, it absorbs a vast amount of heat. That’s just the physics of evaporation. The shade provided by trees also helps with cooling.”
Previous research has shown that when urban tree canopy expands by 1% at the neighborhood level, nearby temperatures can drop by 0.04 to 0.57 degrees Celsius. But city planners needed more comprehensive data.
Scaling Up the Research
The research team analyzed satellite imagery and temperature data from four diverse cities: Beijing and Shenzhen in China, and Baltimore and Sacramento in the US. These cities represent different climate zones – temperate, subtropical, and mediterranean.
Starting with block-sized sections, the researchers measured land surface temperature and tree coverage, then expanded their analysis to larger areas. The results were surprising: trees’ cooling efficiency actually increased at larger scales, though at a decreasing rate as areas grew larger.
Real-World Applications
The study provides practical guidance for urban planning. For example, in Beijing, while a 1% increase in canopy at the block level decreases temperature by about 0.06 degrees, the same percentage increase at the city level could reduce temperature by about 0.18 degrees.
Co-author Weiqi Zhou explains, “We found that cooling efficiency follows a power law across scales – from as small as 120 by 120 meters to as large as regions covering the entire city. The relationship holds across all four of the studied cities, which are in very different climates. This suggests that it could be used to predict the amount of additional tree cover needed to achieve specific heat mitigation and climate adaptation goals in cities worldwide.”
Looking Ahead
While this research provides valuable planning tools, implementation requires careful consideration. “This paper doesn’t tell you where to put the trees,” notes Pickett. “That’s another sort of analysis, which would have to involve a lot more social information and engagement with communities or with individual property owners.”
Glossary:
– Urban tree canopy: The layer of tree leaves, branches, and stems that cover the ground when viewed from above
– Land surface temperature: The temperature of the ground and built surfaces in an area
– Power law: A mathematical relationship showing how one quantity varies as a power of another
– Mediterranean climate: A climate type characterized by warm, dry summers and mild, wet winters
Quiz:
1. How much could Baltimore reduce its surface temperature by increasing tree canopy by 1%?
Answer: 0.23°C
2. What are the two ways trees help cool cities according to the study?
Answer: They pump water into the air which absorbs heat when evaporating, and they provide shade
3. Which four cities were studied in the research?
Answer: Beijing, Shenzhen, Baltimore, and Sacramento
4. Why does the cooling efficiency of trees increase at larger scales?
Answer: Due to being able to include large groups of trees, which have a larger cooling capacity
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