Background/Objectives
Globally, heat waves are increasing in both frequency and duration. Extreme heat events are the leading driver of weather-related fatalities in the US, and in 2003 alone more than 70,000 people died as a result of the heatwave that impacted Europe. The construction of urban centers further exacerbates these heat events, with concrete and impervious surfaces retaining heat resulting in what is known as the urban heat island (UHI). By midcentury, cities are expected to expand by 1.3 million km2 by 2050, to house more than 68% of the population, globally.
Increasing vegetation in cities, and specifically tree cover, is a proposed nature-based solution to mitigate the effects of the UHI. The 3-30-300 rule is commonly used as a target for developing the urban forest canopy. The rule states that a resident should be able to see at least three trees from their home, have 30 percent canopy cover in their neighborhood, and not live more than 300 m from a park or green space.
This is a worthwhile target as green spaces and trees have been shown to have positive physical and mental health benefits, reduce pollution, and mitigate acute and chronic impacts of natural disasters. However, it remains unclear the degree to which urban tree canopies are limited by bioclimatic constraints versus factors that could be mitigated through improved urban design and vegetation stewardship.
Approach/Activities
We used 20 years of satellite vegetation and land surface data to quantify the cooling effects of tree and non-tree vegetation on land surface temperature in over 2,000 global cities across all 14 major land biomes. By evaluating how much tree cover would be needed to fully offset UHIs, we assessed the viability of the 30% canopy cover target from the 3-30-300 rule. We also benchmarked the feasibility of using tree planting and conservation efforts as the only adaptation strategy for warming summer temperatures. Finally, we quantify the amount of water needed to support current and target urban vegetation under current and future climate regimes.
Results/Lessons Learned
Trees have a strong cooling effect in all major biomes and are five times more effective at cooling than non-tree vegetation. Although the 30% tree cover target is close to the global average of what would be needed to offset UHI, this would require more than doubling the current urban tree cover. The cooling capacity of trees and vegetation that cities can support varies greatly among terrestrial biomes, which describe plant-climate relationships. Although trees are most effective at cooling cities in arid- and semi-arid regions, these regions are by definition the most water-limited and can sustain the least amount of tree cover. In contrast, many cities in North America and Europe are climatically capable of supporting dramatically higher tree cover than is currently observed in cities. In these regions, space, political support, and long-term care of urban tree canopies are the biggest barriers to vegetation as effective nature-based solutions for urban heat.