Minneapolis already has a lot of trees. But compared to other major U.S. cities faced with rising populations, air pollution and summer heat waves, it would get a much greater bang for its public health buck by planting even more.

In fact, in a ranking of how much 32 different cities would benefit from more urban forest, Minneapolis was tied for second place for the greatest impacts on reducing illness caused by air pollution, and third for the reduction in deaths and health risks from heat.

“If you want healthier air, trees are part of that solution,” said Rob McDonald, a scientist at the Nature Conservancy who studies urban environments. On Monday he presented his findings from a global analysis of the financial returns to cities from trees at the American Public Health Association annual meeting in Denver.

He said that along with other researchers, he conducted the study in response to the questions they were getting from health officials in cities around the world. Most recognized that as temperatures and air pollution rise, trees are increasingly viewed as a key part of the public health infrastructure. Along with water treatment, sewers, and streetlights, they play a critical role in keeping citizens healthy.

“Beautification be damned,” said Don Willeke, a member of the Minneapolis Tree Advisory Commission and longtime advocate for urban forests. “They are a public utility.”

They are nature’s all-purpose tool. They slow runoff, hold carbon, filter air pollution — especially the microscopic particles from gasoline engines that are a major contributor to lung and heart disease — offer cooling shade to people and buildings, and take humidity out of the air.

“The questions to us are how real is this,” McDonald said. “How big a slice of the problem can they solve? Which cities is it most useful for? Which neighborhoods?”

They looked at population and climactic trends for 245 cities around the world. They factored in density and availability of water and a host of other factors.

The huge, crowded cities like Dhaka, Bangladesh, and Mumbai would see the greatest benefits. But surprisingly, in the U.S. and Canada, Minneapolis along with other northern cities like Boston, Detroit, Chicago and Toronto would see big returns on tree investments, McDonald said.

“Cities in the tropics are already hot and will get hotter,” he said. “But people and societies there are already adapted to higher temperatures.”

Those living in northern cities, however, are not, he said. Yet as Minnesotans are already discovering, climate change will bring increasingly frequent summer heat waves and high humidity. Many in northern cities don’t have air conditioning or other ways of dealing with those potentially dangerous temperatures.

The World Health Organization estimates that already some 12,000 people die every year from heat-related causes, a number that could rise to 260,000 by 2050.

Particulate matter, the tiny particles that come from fires, leaf blowers, lawn mowers, cars and diesel trucks, causes an estimated 4 percent of diseases, primarily heart disease and lung conditions like asthma.

Trees are already doing their job, McDonald said, by catching the air pollution on their leaves before people inhale it and cooling the air around them by up to 3.6 degrees on average.

But many cities, Minneapolis included, are struggling to keep the forest canopies they have, much less add more.

“We are actively maintaining our existing canopy and trying to grow it slightly,” said Justin Long, an assistant superintendent of the Minneapolis Park and Recreation Board. “We don’t have a lot of resources to really grow the urban forest.”

This year the park board removed 5,000 trees and replanted twice that many, he said. And the city gives away 1,000 trees a year to homeowners through the Tree Trust, a nonprofit group.

But Minneapolis’ trees face a gauntlet of problems. The elms that once towered over neighborhood streets with cathedral-like arches are falling one by one to Dutch elm disease and old age. Then there’s emerald ash borer, which will march through some 250,000 trees in Minneapolis, likely killing every one of them.

The park board is replacing the 40,000 growing on the boulevards during the next eight years. But that leaves 200,000 on private property that eventually will succumb to the bug, said Long.

Healthy trees also fall to the chain saw as new developments come along. For example, a new design for the sculpture garden at the Walker Art Museum resulted in the loss of about 400 trees, and the Wirth Adventure and Welcome Center at Wirth Park will take out some 300, including a number of old oaks.

“Combining trees with city infrastructure gets increasingly difficult,” Willeke said.

But McDonald said his report can help cities provide their specific financial and health arguments on behalf trees. Maximum future tree planting around the world would reduce deaths of up to 37,000 per year, and at a cost of only $4 per person.

In short, Willeke said, “trees save you money.”