New research published in The Lancet has found that increasing tree cover in European cities to 30 percent could have reduced premature deaths from urban heat islands by 40 percent.
Currently, the average tree canopy in cities across the continent is approximately 15 percent. Increasing tree coverage to 30 percent would cool cities by 0.7°F (0.4°C) — showing that even seemingly small reductions in summer temperatures could save many lives.
The climate crisis has increased temperatures across Europe and led to more dangerous heatwaves. Last year, the continent experienced its hottest summer and second warmest year overall, which only increased the impacts of urban heat islands.
Heat islands form in urban environments comprised of dark, heat-absorbing materials like asphalt, concrete, and tar-covered roofs. These islands are also found in areas where is a high population density, which means a high number of air conditioners expelling heat, and a lack of trees and plants.
Urban heat islands “intensify the adverse effects of high temperatures on health,” the researchers write, particularly for heat-vulnerable populations like the elderly and those who cannot afford air-conditioning.
And this is confirmed by previous research. In a study of the West Midlands, United Kingdom cited by the authors, “urban heat islands were estimated to have contributed around 50 percent of the total heat-related mortality during a 2003 heat wave.”
“We already know that high temperatures in urban environments are associated with negative health outcomes, such as cardio-respiratory failure, hospital admission, and premature death,” explained lead author Tamar Iungman, with the Institute for Global Health in Barcelona, Spain. And hotter temperatures and deadlier heat islands will lead to an even “bigger burden to our health services over the next decade.”
The researchers used data from 2015, the latest continent-wide population study, to estimate the summer mortality rates of 57 million Europeans in 93 cities, aged 20 years and older. From June to August 2015, the cities were 2.7°F (1.5°C) warmer on average than surrounding rural areas.
The researchers first looked at a range of strategies to reduce heat islands. The majority are solutions landscape architects plan and design:
- Green roofs and walls
- Light-colored building roofs, walls, and landscape pavers
- Replacing impervious surfaces with plants and soils
- Increased tree coverage
But for this study, the researchers decided to hone in on trees. They argue that “planting urban trees offers an important opportunity to mitigate high temperatures and, compared with other strategies, is relatively simple and cost-effective to implement.”
Trees are known to be effective at cooling cities — through the shade they provide and the water vapor they release through transpiration. A study cited by the researchers found that in 600 cities trees cooled urban neighborhoods by an average of 1.8°F (1.1°C) — and up to 5.2°F (2.9°C).
Other studies have backed the 30 percent tree canopy target, which is why many cities have set that as a policy goal as well. “Previous epidemiological studies have suggested health benefits associated with tree coverage of at least 30 percent, including reduced odds of incident psychological distress, and non-communicable diseases, such as diabetes, hypertension, and cardiovascular disease.”
Trees have other co-benefits: they can reduce air and noise pollution and increase biodiversity. Exercising in green spaces has been linked with better health outcomes than just working out in a gym. And just spending time in a spaces filled with trees offer significant benefits for mental health and well-being.
Another study found that if European city leaders met World Health Organization recommendations and ensured all their populations lived within 1,000 feet (300 meters) of a green space, 20 deaths per 100,000 people could be avoided each year. This speaks to the importance of distributing trees equitably across all neighborhoods and focusing on historically marginalized and underserved communities with lower tree canopies.
According to The Lancet study, the cities with the highest number of deaths that can be attributed to urban heat islands were in “southern and eastern Europe, particularly in Spain, Italy, Hungary, Croatia, and Romania,” while the lowest were located in “northern Europe including Sweden, Estonia, UK, and northern France.”
Most of the cities with the worst heat islands were also the most densely populated. Dangerous heat islands were found in Paris, France; Thessaloniki and Athens, Greece; Bilbao, Spain; and Brussels, Belgium. These cities have population densities that range from 7,272 to 21,462 people per square km.
The researchers argue that denser cities with lower than average canopies can therefore see even greater benefits from increasing their tree cover than other cities. The issue is finding ways to retrofit these cities to add more trees — a challenge landscape architects can address by adding in new parks, recreational areas, green roofs, and green streets.
Premature deaths that can be attributed to hotter urban temperatures also vary greatly across cities. For example, there were no summer heat-related deaths in Goteborg, Sweden, but 32 premature deaths per 100,000 people in Cluj-Napoca, Romania, The Lancet notes.
Trees were also found to significantly reduce the effects of heat islands during the day because they provide shade along with transpiration. But at night, “the urban canyon (i.e., the geometry formed by a city street and its flanking buildings) more strongly determines urban heat island effects.”
“The night-time intensity of the urban heat island effect is on average three times the daytime intensity. Therefore, urban green infrastructure strategies need to be accompanied by other interventions—especially those that reduce night-time urban heat island effects—to achieve health benefits, such as changing ground surface materials (e.g., from asphalt to granite) and structural interventions that change the sky view factor (i.e., the fraction of visible sky relative to street geometry and building density),” the researchers write.
The researchers’ analysis is based in a “coarse spatial resolution” (1,600 feet by 1,600 feet squares), so it’s not precise. The researchers also note that data on urban tree transpiration rates is hard to measure at a city scale. Typical urban trees’ transpiration rates may be more limited than trees in large parks or the suburbs, because they are “often exposed to harsh conditions (paved soils, air pollution).” The researchers also didn’t factor in how transpiration rates or the shade generated differ by tree size or species. And there was no discussion on how water bodies and features in cities could help further cool communities.