Study Reveals Green and Blue Infrastructure Reduce Urban Heat

The northern hemisphere’s summer 2024 temperatures have broken last year’s record. This summer’s average temperature of 62.2°F, or 16.8°C, is a tad higher than last year’s average by a narrow 0.05°F 0.03°C. 

The year 2023 recorded the highest temperature since 1850, surpassing the pre-industrial average by 1.35°C. Both years constitute the highest two summer averages in annals since pre-industrial times. However, a study of ancient tree rings suggests that the 2023 temperatures and, by extension, 2024 temperatures were the hottest in the past two millennia.

In Japan, 70,000 people were treated for heat stroke in July and August alone. An unrelenting heat wave in Iran forced government agencies, banks, and schools to cut operating hours. The heat sent 200 Iranians to hospitals for heatstroke treatment. In the U.S., cities such as Phoenix, Arizona, and Las Vegas endured temperatures above 100°F or 38°C for weeks. 

Extreme heat waves have also hit Asian cities since April 2024. From Pakistan to the Philippines, Asian and South Asian countries have experienced prolonged extreme heat, affecting everything from people’s health to children’s education and the economy. Temperatures in Bangladesh reached a deadly 43.8°C on April 30. Exceeding the human body’s core temperature of 37°C significantly increases the chance of heatstroke and heat exhaustion.

A study published in The Innovation in March 2024 reviews thousands of papers on green-blue-grey infrastructure (GBGI), such as parks, wetlands, and engineered greening, focusing on their effectiveness in mitigating urban heating and their co-benefits.

Researchers screened 27,486 papers and reviewed 202 based on 51 types of green-blue-grey infrastructure (GBGI) classified under ten broad categories for their cooling benefits. They found that botanical gardens, wetlands, green walls, street trees, and vegetated balconies offered the most efficient air cooling.

Below is an image from the study showing the cooling efficiency of the various green-blue-grey infrastructure (GBGI) types.

A systematic review of hundreds of studies on green infrastructure shows that it regulates urban heating and reduces temperatures through shading, evapotranspiration, and thermal insulation. The cooling associated with green and blue infrastructure is also linked to lower energy consumption in buildings.

Examining where these studies are conducted and the type of GBGI being studied shows that most of the studies of green infrastructure originate from Asia (51.1%), primarily from China (29.95%), followed by Europe (30.4%), Australia (7.5%), and North America (7.0%). Far fewer studies have been carried out in South America (1.8%), Africa (1.8%), and New Zealand (0.4%)

The type of GBGI studied also varied across continents. Wetlands (14.7%) and parks (12.9%) were the most frequently studied GBGI types for combating urban overheating in Asia. In contrast, the most common GBGI measures reported in Europe were street trees (18.8%), green walls (15.9%), and green roofs (13.0%).

In Australia, green wall studies were prominent (17.6%), alongside irrigating backyards, parks, street trees, and green roofs, making up (58.8%) of the total GBGI studies. In North America, parks (31.3%), street trees (18.8%), and wetlands (12.5%) made up approximately (62.5%) of GBGI interventions described.

According to the study, the differences in the types of GBGI studied across continents likely stem from diverse regional contexts, climate conditions, and urban planning priorities reflecting the varying environmental needs and urban challenges specific to each continent and the best GBGI interventions to tackle urban overheating in their respective regions. 

Besides cooling, GBGI’s co-benefits include managing stormwater, reducing flooding and sequesting CO2. However, potential drawbacks can consist of increased maintenance costs and land-use conflicts. Street trees offer the highest co-benefits in cooling and climate adaptation, followed by green roofs and botanical gardens.

In contrast, Playgrounds, green walls, green roofs, and street trees had the highest reported drawbacks, such as pollutants trapped in street canyons and allergic rhinitis. The study highlights the importance of considering a balanced heat mitigation and adaptation approach during their implementation.

The study provides a heat mitigation inventory that could help policymakers and urban planners select the best GBGI approach to reduce heatwaves and lower temperatures in cities facing the rising risks of extreme and deadly heatwaves that are projected to increase in intensity and frequency due to climate change.

Read the study “Urban Heat Mitigation by Green and Blue Infrastructure: Drivers, Effectiveness, and Future Needs” by clicking the link in the “Sources” section below.