Cities around the world are facing increasing heat waves that are becoming increasingly deadly and causing health problems. This phenomenon is being exacerbated by human-induced climate change. and urban heat islands, where human activities generate heat and impermeable structures trap it. Natural solutions, particularly the expansion of urban forests, have emerged as a key solution to mitigate these impacts. A recent study, published in collaboration with RMIT University, examined the potential for increased tree cover in 10 Italian cities to achieve the recommended minimum 30% tree cover, and quantified the reduction in heatwave severity and associated ecosystem benefits.

Innovative model and metrics

Study used the i-Tree Cool Air model and the “heatwave degree day” (HWDD) metric to accurately estimate the heatwave mitigation benefits of additional tree cover. HWDD is a new metric that quantifies the cumulative intensity and duration of heatwaves, which are key predictors of heat-related mortality. The modeling focused on creating “functional urban forests,” meaning that all additional tree canopy was located on permeable surfaces, allowing for the coupled processes of rainwater infiltration and evapotranspiration. These processes are essential for healthy tree growth and the generation of cooling services, either directly through evapotranspiration or indirectly through shading that reduces radiant heat from impervious surfaces. The study focused on the summer of 2003, an exceptionally severe European heatwave that claimed an estimated 72,210 lives.

Significant reduction in heat wave burden and mortality

The results were impressive: despite dry summer conditions, a scenario with 30 % tree cover reduced HWDD by a median of 34 % (range: 16–84 %). A linear relationship was observed across all urban districts between increased canopy cover and reduced HWDD: for every 1 % increase in canopy cover, there was an average decrease of 0.81 HWDD. The total number of heat waves in the 10 cities decreased from 310 to 198, representing 36 % reduction.

These changes have directly led to a significant decrease in mortality attributed to heat waves. According to estimates, a scenario with 30 % tree cover prevented 574 deaths among adults aged 65 and over, which represents decrease by 29.3 % of the total number of deaths recorded, 1962. Median mortality rates decreased from 108 to 75 per 100,000 population. These mortality estimates compared favorably with previous studies and confirmed the vulnerability of older adults during extreme heat.

Multidimensional ecosystem services

In addition to cooling and mortality reduction, the functional urban forest scenario also brought substantial benefits in the area of ecosystem services, the value of which is estimated at $132.8 million per year in 10 cities, a significant increase from the $47.7 million in the baseline scenario. The largest benefits came from:

• Prevented rainwater runoff: $71.4 million per year, with annual stormwater runoff reduced by 17 %.
• Improved air quality: $41.6 million annually due to reduced concentrations of pollutants such as carbon monoxide, nitrogen dioxide, ozone, particulate matter and sulfur dioxide.
• Increased carbon sequestration: another $19.7 million, directly supporting climate change mitigation goals.

These values do not include the economic value of temperature reduction itself, nor the many other specific ecosystem functions provided by urban trees, such as provisional, regulatory, supporting, and cultural services.

Climate challenges and innovative solutions

The study also took into account challenges such as dry summer conditions that can limit evapotranspiration. Mediterranean cities, such as those in Italy, may have lower tree cooling potential compared to humid northern cities. However, a growing body of evidence suggests that evapotranspiration and cooling persist even during drought.

Irrigation presents nuanced trade-offs; while it consistently reduces dry bulb temperature, its impact on heat stress in humans can vary. Importantly, apparent temperature (heat index), which includes both temperature and humidity and is related to thermal comfort, does not typically decrease with irrigation and often does. However, wet bulb temperature, which represents the physiological limit for evaporative cooling, can increase with irrigation, posing a health risk when certain thresholds are exceeded. Ventilation strategies, such as appropriate street geometry and vegetation height, can be integrated with irrigation planning to mitigate moisture accumulation. The goal is to design efficient irrigation, such as drip systems applied at night, to support tree resilience without increasing nighttime humidity.

Expanding functional urban forests represents a clear path to achieving Sustainable Development Goal 11 (sustainable cities and communities) by providing critical ecosystem services and increasing the climate resilience of cities. Comprehensive reduction of heatwave burden, mortality and large-scale economic benefits confirm that investments in urban forests are essential for building healthier and more resilient urban environments. Spring

The study was published in in the journal Nature