Heatwaves affect billions of people around the world, and with increasing global temperatures, more people are set to face heatwaves in the coming years. For example, in comparison to average years, 2015 saw 175 million more people exposed to heatwaves.
Heat is a serious threat to public health — 70 thousand people lost their lives in the 2003 European heatwave, and on average, over six hundred people die every year in the United States due to excessive natural heat. As summers get hotter, longer and more extreme due to our planet heating at an unprecedented rate, heat-related death will continue to rise in the future.
Urban residents are especially vulnerable to heat as cities are significantly hotter than rural areas because their trees are usually felled and replaced with heat-absorbing concrete and asphalt that also take longer to cool down. That’s why cities, also known as “urban heat islands”, need to identify new ways to keep their citizens cool. Approaches to keep cities cooler include painting cities in white to act as a reflective coating, having sky gardens in tall urban buildings, or water fountains in city squares. Some countries have taken extreme measures; for example, Qatar is even air conditioning the outdoors to keep its residents cool.
Tree canopy is effective in reducing urban heat as well as saving costs on air conditioning. Additionally, trees can also purify urban air to reduce air pollution from vehicles and the industry in cities. But it seems that even nature is a luxury saved for the rich. Research shows that high-income neighbourhoods are more likely to have high tree canopy cover than low-income communities. So, expanding tree canopy cover in urban areas is vital in developing resilience to climate change and reducing social inequality.
Despite the apparent benefits of tree canopies, many cities around the world don’t have any blueprints for tree planting. One such city is Boston — in Massachusetts, United States — which has already failed in it’s “Grow Boston Greener” project to plant 100 thousand trees.
Recognising the need for urban canopy planning, the City of Boston partnered with Boston University to develop new tools to maximise the benefits of tree planting in the city. The project reviewed the logistics and feasibility of tree planting to ensuring long-term tree survival. The two aims of the study were: (1) protect the most vulnerable people to heatwaves and reduce heat-related deaths, which requires identifying priority areas to take action; and (2) planting trees in the hottest areas to reduce the urban heat island effect.
Heat-related illness and death aren’t spread uniformly across cities due to regional demographics and environmental factors. For example, elderly or young children are vulnerable to extreme heat. The “Heat Vulnerability Index” or HVI uses the following vulnerability components to identify heat-vulnerable populations and regions in a city:
- Social/language vulnerability — representing minority populations with language barriers, who might miss official warnings on weather extremes.
- Socioeconomic vulnerability — includes the population with low education, unemployment and poverty.
- Environmental/urban vulnerability — includes urban areas and old homes made with heat-absorbing concrete and asphalt.
- Elderly/social isolation — includes the elderly and elderly living alone (one-person household).
An interdisciplinary team of scientists at Boston University working in environmental sciences and public health used the HVI or high surface land temperature to support tree planting decision-making strategies. Effectively identifying the most heat-sensitive areas can give authorities vital information to develop effective, targeted interventions. But scientists found that there was little overlap between areas of high heat and the heat-vulnerable regions in Boston.
The findings are available to read in bioRxiv preprint.
To reduce the urban heat island effect, scientists recommend ways to cool individual buildings, for example retrofitting buildings with cool roofing materials or integrating more green roofs and green walls. The research identifies that the current list of 34 approved trees in Boston requires a review to improve biodiversity and disease resistance. Scientists advise authorities to prioritise protecting older trees and prevent the death of young trees by increase professional maintenance guarantee contracts.
Researchers used publically available data to create an interactive decision support tool, Right Place, Right Tree | Boston. The app takes the following decision-making steps:
- Choose areas to plant trees — things to target here are the hottest areas or the most heat-vulnerable areas, and also consider feasibility for planting trees.
- Regional considerations — after selecting an area to intervene, evaluate existing factors such as land ownership, existing canopy cover, community partners, etc.
- Choose the right tree species — it’s vital that the trees grow in the area, which helps reduce tree death and lowers maintenance costs.
- Keep the tree healthy — maintain trees through resources from officials or residents, educate locals, etc.
At a request from the City of Boston, researchers at Boston University Biogeoscience and URBAN graduate program created the web application to life barriers to tree canopy expansion in Boston. Scientists developed the tool both for city officials and residents, who can request City-funded tree planting on public land.
The study says that the decision-making tool and the app framework is designed so that it can be implemented to almost any city in the United States. The code for the application is in GitHub, with commentary for adapting the text and data to other cities.
The research is published in bioRxiv preprint. Click the link to read the full manuscript titled: “A tree-planting decision support tool for urban heat island mitigation” https://doi.org/10.1101/821785
See the code for the application in GitHub: www.doi.org/2F10.5281/2Fzenodo.3515227