New Report Shows Trees Can Help Fight Rising Heat in Arid Cities

As the world grapples with increasingly common and devastating heatwaves, a new science report produced by The Nature Conservancy (TNC) shows that tree planting can be a cost-effective way to help protect communities in arid locations from the worst effects of rising heat.

TNC released the report, Roots of Resilience: Using Trees to Mitigate Rising Heat in Arid, Frontline Communities, while presenting the topic on Sept. 11 in Montreal at the Innovate4Cities Conference, which is co-hosted by the Global Covenant of Mayors for Climate & Energy (GCoM) and UN-Habitat.

The report, which includes the researchers’ assessment of 61 large cities (> 3 million population), found that citizens in arid cities have, on average, 1.5% tree cover in their neighborhoods, while the average for citizens in semi-arid cities is 4.2%. The researchers estimate, however, that targeted tree-planting and maintenance programs could realistically increase tree canopy to 7.1% in neighborhoods in arid cities and 7.3% for those in semi-arid cities.

According to researchers, such an increase from greening programs in arid cities—like the report’s two case studies, Athens and Phoenix—would meaningfully reduce air temperatures near people’s homes by an average of 0.5˚C.

The report also identified 96 million people who live in ‘frontline communities’ in arid and semi-arid regions and who are particularly at risk of extreme heat.

“Frontline communities—or those impacted first and worst by the impacts of extreme heat—are most at risk,” said report co-author Rob McDonald, TNC’s lead scientist for nature-based solutions. “These are communities that are often home to people of lower socioeconomic status who are most vulnerable to climate change and least empowered to adapt.”

For instance, Roots of Resilience cites a 2021 survey of more than 5,700 U.S. municipalities that revealed that 92% of low-income neighborhoods average 15.2% less tree cover than high-income neighborhoods and are, on average, 1.5 ̊C warmer.

“Actions to address extreme heat are urgently needed today,” said report co-author Eleni Myrivili, the United Nation’s first Chief Heat Officer. “As those most at risk, frontline communities must be explicitly made the focus of funding, and it takes years for trees to mature into a robust canopy. Climate change is already here, and every year countries wait to ‘green’ frontline communities is a missed opportunity to save and improve lives.”

Overcoming Water Challenges

Considering its focus on arid and semi-arid cities, the report also highlights potential water solutions for tree plantings and maintenance, as the researchers estimate that increasing tree canopy cover in the 61 cities to the maximal potential would also increase aggregate water demand by 3,200 million cubic meters per year. To address this concern, the researchers estimate that the use of location-appropriate, drought-tolerant species could reduce this water demand to 1,500 million cubic meters per year—with especially large water savings possible in semi-arid climates.

“Any plans to increase tree canopy cover in these cities must also provide viable options for overcoming potential water limitations,” McDonald said. “In arid climates where irrigation for trees will be essential, plans should seek to leverage alternative sources of water, such as the reuse of stormwater as well as wastewater or grey water.”

In addition to its focus on water solutions, Roots of Resilience also includes sections that offer funding and policy recommendations.

Download the full report at nature.org.