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How Many Trees for a Cooler City? Scientists Develop Urban Canopy Goal Tool

Introduction

Urban centers worldwide are grappling with intensifying heat as concrete and asphalt amplify warming effects. Tree-planting initiatives, a favored natural approach to cooling cities, have often relied on estimation rather than precise data. However, a recent study in Proceedings of the National Academy of Sciences introduces a tool that enables urban planners to establish targeted, science-backed greening objectives.

Research Overview

Lead Researchers

This research was led by Jia Wang, Weiqi Zhou, and Yuguo Qian from the Chinese Academy of Sciences, with Steward Pickett, an urban ecologist at the Cary Institute of Ecosystem Studies, contributing as a co-author.

Importance of Urban Trees

Cooling Benefits of Trees

"Trees bring multiple advantages to urban areas, with cooling being a key  one," Pickett noted. "They are particularly effective at cooling by transferring significant amounts of water from the ground to the air. As this water evaporates on the leaves' surface, it absorbs substantial heat, a process driven by evaporation physics. Additionally, the shade from trees enhances their cooling impact."

Previous Research Limitations

Focus on Localized Areas

To date, studies assessing urban trees' cooling impact primarily focus on localized areas, such as individual streets or neighborhoods. Expanding the tree canopy by 1%, for example, can lower nearby temperatures by approximately 0.04 to 0.57 degrees Celsius.

Addressing Citywide Needs

The Need for Broader Data

"This information is useful, but planners and decision-makers are thinking about citywide impacts," Pickett explained. 'They're wondering, "What tree canopy coverage does the whole city need? What happens when we increase it on a large scale?" Such data has not been accessible until now.'

Research Methodology

Assessing Cooling Efficiency

The researchers were uncertain if findings from fine—scale measurements could be applied on a citywide level. Thus, they aimed to assess how trees' cooling efficiency-defined as the temperature reduction from a 1% increase in urban canopy—varies across broader areas.

Study Cities

The researchers analyzed satellite imagery and temperature data from four climatically diverse cities:

• Beijing (Temperate)
• Shenzhen (Subtropical)
• Baltimore (Temperate)
• Sacramento in the United States (Mediterranean)

Baltimore and Beijing experience temperate climates, Shenzhen has a subtropical climate, and Sacramento falls within a Mediterranean climate zone.

Data Collection Process

1. Pixelation of Cities: The researchers begin by dividing each city into pixelated sections roughly the size of a city block.
2. Temperature and Tree Cover Measurement: For each pixel, measured the land surface temperature and tree cover.
3. Scalability of Analysis: They then expand the analysis to progressively larger areas like the neighborhood, citywide, and beyond. Finally, they calculate how the relationship between green cover and temperature referred to as cooling efficiency varies across these different scales.

Key Findings

Cooling Efficiency Across Scales

The researchers found that trees' cooling efficiency improved as the scale increased, though the rate of improvement slowed with larger unit sizes. For instance, in Beijing, a 1% increase in canopy cover at the block level lowered temperatures by approximately 0.06 degrees, while the same increase at the city scale could yield a temperature reduction of around 0.18 degrees.

Benefits of Larger Tree Collections

The additional advantages associated with larger scales seem to result form the capacity to encompass larger collections of trees, which have a greater ability to cool the environment.

Implications for Urban Planning

Predictive Capabilities

With improved insight into how area, tree canopy cover, and cooling effects interact, this research allows for the prediction of cooling impacts across entire cities.

• Framework for Managers: Equipping managers with an essential framework to establish tree canopy objectives for reducing extreme temperatures.

Power Law of Cooling Efficiency

Co-author Weiqi Zhou explains, "Our research indicates that cooling efficiency adheres to a power law across various scales, ranging from as small as 120 by 120 meters to extensive areas encompassing cities. This relationship is consistent across all four cities studied, which represent diverse climates. It implies that this model could assist in predicting the additional tree cover required to meet specific heat reduction and climate adaptation targets in urban areas globally."

Practical Applications

Case Study: Baltimore

As an example, the authors estimate that:

• Increasing tree canopy cover by 1%, Baltimore could lower land surface temperatures by 0.23°C.
• To reach a cooling target of 1.5°C, a 6.39% increase in tree canopy would be required.

Conclusion and Future Directions

Equity in Urban Tree Distribution

While this study provides critical insights for municipal-level decision-making, Pickett notes that urban planners may need to address smaller geographic scales to ensure an equitable distribution of urban trees and their benefits across the city, accompanied by community support.

Guidance on Tree Placement

"This study does not provide guidance on tree placement," Pickett remarked. "A different type of analysis would be necessary, one that incorporates comprehensive social information and collaboration with communities or individual property owners."

Future Directions

According to Pickett, potential next steps could involve extending the analysis to other cities with varying climates and investigating how effectively this nature-based solution might perform as climate change intensifies heat and aridity in some areas.

Source

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