Urban areas face mounting threats from extreme heat, which claimed approximately 356,000 lives worldwide in 2019. This challenge intensifies through the urban heat island effect, where cities experience elevated air temperatures compared to surrounding rural areas. The phenomenon occurs because urban surfaces like concrete and asphalt absorb solar energy and release it as thermal radiation. With climate change already raising global temperatures by 1.1 degrees Celsius and projections indicating life-threatening heat will affect half to three-quarters of the global population by 2100, understanding how to mitigate urban heat has become essential for public health planning.
This study addresses critical gaps in knowledge by conducting a comprehensive assessment of tree canopy cooling benefits across 8,919 large urban areas globally. The researchers used a hierarchical statistical model combining remotely sensed data on tree coverage and land surface temperature with gridded estimates of air temperature. This methodology represents an advance over previous work that often relied solely on land surface temperature, which inadequately captures heat risk to human health. The analysis examined all major urban areas using consistent methods, enabling meaningful comparisons across diverse climatic, geographic, and socioeconomic contexts.
The findings reveal that urban tree cover currently mitigates approximately half of the maximum potential urban heat island effect. Without trees, the average temperature difference between urban and rural areas would reach 0.31 degrees Celsius globally. Current tree coverage, which averages 18.3 percent in populated urban areas, reduces this effect to approximately 0.16 degrees Celsius. This mitigation represents substantial protection, as trees cool their environment through two mechanisms: evapotranspiration, which dissipates heat through moisture release, and shading, which prevents solar radiation from heating paved surfaces. These effects can lower air temperature by 1 to 2 degrees Celsius directly under street trees.
Looking toward future scenarios, the researchers modeled a maximum plausible tree planting scenario that would increase global average urban tree coverage to 32.8 percent. This expansion would lower population-weighted urban air temperatures by 0.295 degrees Celsius. However, when compared against projected climate impacts, these cooling benefits appear modest. Under a middle-range emissions scenario aligned with Paris Accord pledges, urban areas face a median temperature increase of 1.5 degrees Celsius by 2050. Current tree coverage offsets only about 10 percent of this projected warming, while maximum feasible tree planting would offset approximately 20 percent.
The study also highlights substantial inequities in both heat exposure and tree coverage. Globally, lower-income countries face higher population exposure to heat stress, partly due to location in hotter climates but also because they possess lower tree canopy coverage than higher-income regions. Within cities, similar disparities emerge. Research in US cities shows that low-income neighborhoods typically have 15 percent less tree cover and experience surface temperatures 1.5 degrees Celsius higher than wealthier neighborhoods. These patterns appear in multiple countries, though exceptions exist depending on local context.
The implications suggest that while urban tree planting provides meaningful cooling benefits and should remain part of heat action planning, trees alone cannot fully address the warming cities will experience from climate change. The research indicates that even with maximum feasible tree planting, urban areas would mitigate only one-fifth of projected temperature increases by mid-century. This limitation stems from physical constraints of climate and urban form that restrict how much additional tree coverage cities can accommodate. Therefore, comprehensive urban heat strategies must incorporate multiple approaches beyond tree planting, including modifications to surface materials, building design, and infrastructure that enhances evaporative cooling. The unequal distribution of current cooling benefits also underscores the need for targeted interventions in vulnerable communities.