Cities in India, Bangladesh and Pakistan stand to benefit most from tree planting, in terms of both heat and particulate matter reduction, according to a study that identifies the potential return on investment (ROI) from tree planting in 245 global cities.
The 'Planting Healthy Air: Global Return on Investment Rankings' study ranks all 245 cities for the potential per capita impact of planting trees, as defined by degrees of temperature reduction or the amount of fine particulate matter (PM2. 5) pollution that could be removed with a given investment.
"Many cities in these countries have high return on investment from tree planting, in terms of both heat and PM reduction, " said Rob McDonald, lead scientist for Global Cities initiative at The Nature Conservancy (TNC) and the study's primary author.
TNC in collaboration with C40 Cities Climate Leadership Group developed the study with the aim of providing urban leaders with the data they need to demonstrate that investments in tree planting can improve public health in their cities.
"We focused our analysis on street trees, since our review of the scientific literature indicated that proximity between trees and people was needed to deliver meaningful reductions in PM or temperature, " McDonald said.
These 245 cities currently house about a quarter of the world's urban population.
Top 10 ROI cities in terms of particulate matter include Delhi, Mumbai, Ahmedabad in India, Karachi and Faisalabad in Pakistan and Dhaka in Bangladesh.
Dhaka, Mumbai, Karachi and Faisalabad figure among the top 10 ROI cities with regard to heat reduction.
"This (high ROI) occurs because the cities have relatively high population density (so planted trees benefit a larger number of nearby residents), planting costs are relatively lower and (in the case of PM) because the higher PM concentrations mean the removal of PM by trees is relatively higher, " explained McDonald.
"An investment in tree planting of just $4 per resident in some of the world's largest cities could improve the health of tens of millions of people by reducing air pollution and cooling city streets, " the report said.
The study states that an annual global investment of $100 million in tree planting could provide 77 million people with cooler cities and 68 million people with measurable reductions in particulate matter pollution.
"India has high potential return on investment from tree planting, relative to the global average.
Tree planting cannot and should not replace other efforts to clean up the air or to prepare for heat waves.
But it deserves to be considered as one tool in the toolbox for urban planners, " added McDonald.