Planting more trees in cities could have saved 1.1 million lives in two decades, study suggests

Trees in cities aren’t just a ‘nice-to-have’; they have a huge bearing on public health by keeping us cooler.

A new study has calculated that increasing vegetation in urban areas by 30 per cent could have prevented more than one third of all global heat-related deaths between 2000 to 2019. In total, 1.16 million lives may have been saved with more greenery during this 20-year period.

While the health benefits of green spaces are well-known, the modelling study from researchers at Monash University in Melbourne is the first to estimate both the cooling effect of greenness on daily temperatures, and its modifying effect on the heat-mortality relationship.

"This gives us a more comprehensive assessment of its benefits in mitigating heat-related mortality," says study lead Professor Yuming Guo.

"These findings indicate that preserving and expanding greenness might be potential strategies to lower temperature and mitigate the health impacts of heat exposure."

How do city trees save lives?

Heat exposure is a major public health threat, increasingly so due to human-caused climate change. 

Between 2000 to 2019, high temperatures were linked to half a million deaths per year, accounting for 0.91 per cent of all global deaths.

According to Professor Guo, "under the most extreme global warming scenarios”, estimates of heat-related deaths are projected to range from 2.5 per cent of all deaths in North Europe to 16.7 per cent in South-East Asia in the 2090s. 

Stopping Earth-heating emissions at the source is the most effective way to reduce heat-related deaths. But how can increased greenery help to protect people too?

A fuller spread of grasses, trees and other plants in dense urban areas has a cooling effect. Such vegetation shades surfaces, deflects radiation from the sun, and increases ‘evapotranspiration’ - evaporation from the ground and plants - which promotes air convection. 

This cools the ambient temperature, leading to a drop in how many people are exposed to extreme heat. 

Professor Guo says there is also emerging evidence that greenness can modify heat-related mortality risks, potentially due to improving mental health, upping social engagement and exercise, and reducing air pollution.

How many lives could have been saved in Europe?

The study found that increasing vegetation levels by 10 per cent, 20 per cent and 30 per cent would have decreased average daily temperatures by 0.08°C, 0.14°C, and 0.19°C, respectively. 

In turn, that could have prevented 0.86, 1.02, and 1.16 million deaths, representing 27.2 per cent, 32.2 per cent, and 36.7 per cent of all heat-related deaths from 2000 to 2019.

Under the model, the positive effect of more greenery depended on a range of factors, including climate type and the socioeconomic and demographic makeup of a particular city. 

If the level of vegetation rises by 30 per cent, the average number of lives saved from 2000 to 2019 is 396,955 in Europe. Asia sees the most lives saved, at 527,989.

More specifically, it is urban areas in Southern Asia, Eastern Europe, and Eastern Asia that would have seen the greatest reduction in heat-related deaths.

Heat-mortality associations were assessed by using data from 830 locations in 53 countries and extrapolated to each urban centre, the researchers explain. While greenness was measured via the Enhanced Vegetation Index (EVI), a satellite-based vegetation index derived from NASA satellite images.