As urban populations surge and property demands grow, American cities increasingly miss out on green space. That loss comes with a cost: Losing nature doesn't just change the aesthetics of a city, it radically affects the health of its occupants.

According to a recent study, green spaces don't just make cities feel more liveable — an increase in green space can actually save lives.

This jibes with previous research, which shows that urban parks, yards, and gardens crucially offer city dwellers opportunities for physical activity, social interaction, and stress relief.

This study, published Monday in the journal The Lancet Planetary Health, found that increasing tree canopy to 30 percent of the land area of Philadelphia, Pennsylvania could prevent 403 premature deaths across the city every year. Adding green space would also yield an estimated economic benefit of almost four billion dollars .

"While we have long suspected that trees helped us live longer healthier lives, we have not had until now invested in the science to quantify that," co-author Michelle Kondo, a social scientist at the United States Forest Service, tells Inverse.

This city-wide analysis is that investment. In turn, the team was able to connect the dots between the "unseen and uncounted benefits" of urban trees, financial impact, and mortality.

"Increasing green space can bring a great return on investment," Kondo says. "The simple message is that more is better."

To determine how green space influenced mortality, the researchers analyzed Philadelphia, Pennsylvania. It is one of the largest cities in America with a relatively high risk of mortality.

Using aerial and satellite imagery, the researchers mapped existing tree coverage across the city, looking at the leaves and branches from above. In 2014, when the study was conducted, trees made up 20 percent of the city's total land area.

Tree cover in 2014 by census tract. High-income neighborhoods are associated with more green space, while trees and vegetation are more scarce in low-income neighborhoods. Lancet Planet Health

The team also leveraged data from a massive meta-analysis, which included over eight million people in seven different countries. This analysis suggested that residential green spaces can protect against premature all-cause mortality.

In the new study, researchers applied this international data to a local context. The team estimated the number of deaths from any cause that could be prevented if green spaces in a whole city were increased, through a greening program called Greenworks Philadelphia. To estimate economic costs, scientists also calculated the monetary value of lives lost prematurely.

Then they forecasted three scenarios of how the cityscape might look in 2025. The first prediction is based on the current goal set by Philadelphia's City Council to increase tree coverage to 30 percent of land area in each of the city's neighborhoods by 2025, in turn increasing the city-wide canopy by 9.7 percent. The second scenario increased the tree canopy in areas with non-tree vegetation by five percent (a total city-wide increase of 3.7 percent). The third scenario upped tree canopy city-wide by 7.4 percent.

The results showed staggering effects linked to planting trees: If Philadelphia achieves its 30 percent goal, 403 premature adult deaths would be prevented each year, the study suggests. This represents 3 percent of the city's annual mortality.

Even just a five percent increase in areas without trees could result in an annual reduction of 302 deaths city-wide, with a value of 2.9 billion dollars. Meanwhile, a ten percent increase in canopy city-wide was associated with an estimated reduction of more than 376 deaths and a value of 3.6 billion dollars.

Areas with lower socioeconomic status would benefit the most from an increase in tree canopy cover, as these neighborhoods are dramatically tree-starved compared to high-income neighborhoods, the researchers argue.

This study can't pin down exactly which causes of death extra tree cover would mitigate. It likely comes back to the effect nature has on physical activity, social cohesion, and mental health. Meanwhile, a 2015 study did find that adding green spaces across Philadelphia from 2000 to 2012 lowered rates of crime and gun violence.

Still, the researchers emphasize that saving lives isn't as simple as planting a few extra trees.

Since the study was completed, Kondo and her colleagues learned that Philadelphia’s tree canopy cover declined over the past decade despite "intense tree planting efforts." This decline is most likely due to climate-related events, invasive pests and diseases, and development and construction, Kondo explains.

To catch up and reach the 30 percent goal by 2025, Greenworks Philadelphia will have to expand rapidly, planting trees not only in parks and public gardens but also in residential yards or privately-owned spaces.

Venturing into private spaces adds another layer of complexity. Greening programs can often signify gentrification and eventual displacement, Kondo says. Getting community members to buy-in and ensuring they get to stay in their homes can be a difficult but necessary part of building a successful green space program.

Ultimately, the research in Philadelphia offers a roadmap to other city policy-makers on the magnitude of health benefits of real-life green space intervention proposals, and how to overcome these challenges.

"All cities face a wide range of challenges not only in increasing but in just simply maintaining their tree canopy cover," Kondo says. "It will take more than grassroots efforts to increase tree canopies."

Summary:

Background Cities across the world are undertaking ambitious projects to expand tree canopy by increasing the number of trees planted throughout public and private spaces. In epidemiological studies, greenspaces in urban environments have been associated with physical and mental health benefits for city dwellers. Greenworks Philadelphia is a plan to increase tree cover across Philadelphia (PA, USA) by the year 2025. We aimed to assess whether an increase in tree canopy or greenspace in Philadelphia could decrease mortality.

Methods We did a greenspace health impact assessment to estimate the annual premature mortality burden for adult residents associated with projected changes in tree canopy cover in Philadelphia between 2014 and 2025. Using up-to-date exposure–response functions, we calculated the number of preventable annual premature deaths city- wide, and for areas of lower versus higher socioeconomic status, for each of three tree canopy scenarios: low, moderate and ambitious. The ambitious scenario reflected the city’s goal of 30% tree canopy cover in each of the city’s neighbourhoods; and low and moderate scenarios were based on the varying levels of plantable space across neighbourhoods.

Findings We estimated that 403 (95% interval 298–618) premature deaths overall, including 244 (180–373) premature deaths in areas of lower socioeconomic status, could be prevented annually in Philadelphia if the city were able to meet its goal of increasing tree canopy cover to 30%.

Interpretation Bringing all of Philadelphia, and particularly its poorer neighbourhoods, up to the 30% goal of tree canopy cover is not without challenge. Nevertheless, policies are warranted that value urban greening efforts as health-promoting and cost-saving measures.