Often offering better job opportunities and a refuge from social and political strife, urban areas are currently home to half of the global population. Experts believe that by 2050, that percentage will increase to two thirds. Unfortunately, this growth is typically opportunistic, with minimal consideration of environmental concerns. Sustainable development and green building practices are widely accepted concepts, but existing infrastructure and cost concerns often make it difficult for local officials to adopt such practices.
The heart of a city lies in its built environment: buildings, roadways, and other structures. These structures have substantial direct impacts on the environment, including effects on air and water quality, the distribution of natural habitats, and the richness of biological diversity. Recently, direct impacts of the urban built environment on local temperatures have been identified as a particular threat to human health. The urban heat island (UHI) effect is caused by a high density of surfaces like pavement and buildings that absorb and retain heat, increasing local temperatures to well above those of surrounding landscapes. With climate change already affecting large cities, it is imperative to plan and develop greener built environments to reduce the threat posed by UHIs. In this unit, you’ll explore how UHIs may be managed in a warming climate.
The Problem
While there are many environmental issues associated with how cities are designed and built, this unit will focus on the urban heat island (UHI) effect, with a look at how and why it forms,
Different surfaces absorb and reflect solar radiation in different ways.
This leads to significant additional heating in pockets or “island” areas which are largely covered by built surfaces.
Not only do trees provide cooling shade, but they also help reduce ground–level temperatures via evapotranspiration.
The global nature of the problem was underscored by a report by Santamouris (2020) which indicated that the magnitude of urban temperature increases in global cities may exceed 4 to 5 oC and, at peak, may reach 10 oC
In addition to increasing urban temperatures, UHIs can lead to other impacts.
Low–income populations are also disproportionately affected due to poor housing conditions and limited access to medical resources.
High urban surface temperatures can also impact water quality, primarily by increasing the temperature of stormwater runoff that enters nearby waterways.
A recent report indicated that climate change is likely to be twice as costly in cities compared to rural areas.
Scientists are now working to develop new color-changing building materials that could reduce the UHI effect in the summer, while keeping colder cities warmer in the winter.
The Role of Climate Change
Climate change and urban heat islands are directly connected, with positive feedbacks working in both directions.
Higher temperatures associated with climate change are already leading to longer, more severe heat waves.
But UHIs are also impacting climate change.
Solutions
While tackling UHIs will be challenging for many cities, given the limitations of existing infrastructure and considerable economic investment required to tackle the problem, there are many ways that cities can work to mitigate impacts.
For example, a 2010 heat wave killed 1,300 in Ahmedabad, a city of 8 million in western India. In 2013, the city implemented a heat action plan, with steps such as opening public buildings and mosques as cooling centers and providing ice packs to outdoor workers.
Revegetating built environments can help reduce temperatures in UHIs. In some cities, replanting shade trees has been underway for decades.
According to Shickman and Rogers (2020), to date, Sacramento Shade has planted over half a million trees throughout Sacramento County CA.
For example, officials in Los Angeles have done a comprehensive study of the UHI in that city (Borunda, 2021).
Baltimore, MD is hiring residents to plant trees in under-represented neighborhoods, while Phoenix, AZ is targeting primarily poorer Hispanic areas for tree plantings and shade structures.
.Efforts are also underway to minimize energy required to cool buildings through passive cooling design.
Green roofs or rooftop gardens can remove heat from the air as well as provide additional shade for the buildings they cover.
Build Your Foundational Knowledge
Shade Trees:
First of its kind study quantifies how tree shade can cancel urban heat island effect,
Using trees and vegetation to reduce heat islands
Green Roofs:
How green roofs and cool roofs can reduce energy use, address climate change and protect water resources in southern California
“Cool Roof” reflective surfaces:
Cool roofs fight climate change
Small Group Guided Worksheets
Additional sources:
Shade trees: Shade Trees Reduce Impacts From Urban Heat Islands | by Sabriga Turgon | Medium
Green roofs: Using Green Roofs to Reduce Heat Islands | US EPA
“Cool roof” reflective surfaces: Increasing Heat Resilience in the Built Environment with Cool Roofs, Cool Walls, and Cool Pavements