Trees can make a city sidewalk prettier, sure. But that’s not even their best trick. A growing pile of research suggests that planting more urban trees, if done right, could save tens of thousands of lives around the world each year — by soaking up pollution and cooling down deadly heat waves.

In fact, as a fascinating new report from the Nature Conservancy details, a well-targeted tree campaign could be of the smartest investments a hot, polluted city can make. Which seems important, given that the world’s cities will add about 2 billion people this century, and they’re only getting hotter.

“A lot of cities still think of trees as just ornamentation,” says Rob McDonald, the lead scientist for the Nature Conservancy’s Global Cities program and a co-author of the report. “But they really do so much more than that. And the evidence suggests that we should start thinking of trees as a crucial part of our public-health infrastructure.”

How trees — yes, trees — actually save lives

As researchers have discovered over the years, across dozens of studies, urban trees do at least two important things for public health:

1) They can soak up fine particle pollution from cars, power plants, and factories — an important job, given that particulates wreak havoc on human lungs and kill some 3.2 million people worldwide each year. The precise effect varies from city to city, but generally trees do improve air quality.

2) Urban trees can also cool down neighborhoods anywhere from 0.5 degrees Celsius to 2 degrees Celsius on the hottest summer days, which is vital during deadly heat waves. (Studies have found that every additional 1 degree Celsius in a heat wave leads to a 3 percent or more increase in mortality.)

The new Nature Conservancy report sifts through all this research and lays out some global scenarios. At the high end, a massive new tree-planting campaign in the world’s 245 largest cities, costing around $3.2 billion in all, could save between 11,000 and 36,000 lives per year worldwide from lower pollution. Those trees would also prevent between 200 and 700 heat-wave deaths per year — with that number presumably going up over time as global warming unfolds.

That’s not all: Because the trees would cool neighborhoods significantly, many households could end up using less energy for air-conditioning — reducing electricity use by between 0.9 and 4.8 percent in some cities and helping slow the pace of global warming. They can also retain storm water, boost real estate values, and may even have important mental-health benefits. Plus, they’d just look nice.

But the trees have to be planted in the right spots

So is that worth the price tag? One intriguing finding in the study is that trees are often a bargain in this regard. On average, a well-targeted tree-planting campaign is roughly as cost-effective as other strategies for cutting pollution, like getting dirty diesel buses off the road. (That said, the precise cost-benefit ratio varies across cities, and most governments will want to employ lots of different strategies for cutting pollution. Trees alone are never sufficient.)

There is one catch, though: The tree-planting campaign has to be well-targeted. And that gets a bit complex.

Trees only improve air quality in their immediate vicinity, about 100 feet or so. That means cities need to figure out which neighborhoods benefit most from new trees (typically the densest areas, but also areas around hospitals and schools). They also have to plant species that are most effective at trapping pollution (typically those with large leaves).

Officials also need to account for things like wind patterns and tree spacing and figure out whether they’ll be able to maintain their trees. Plus, if water is scarce, they’ll want to consider drought-tolerant varieties. And they may want to steer clear of trees that increase pollen and allergies.

The report itself offers broad guidelines on planting and tree selection, and McDonald says the researchers may release more fine-tuned data in the future to help urban planners figure this out. This map, for instance, shows the streets in Washington, DC, where trees have the highest and lowest “return on investment” for pollution reduction. In some cities, the difference can vary by a factor of 100:

At a global level, the return on tree-planting is higher in places like Southeast Asia or Mexico — dense cities with considerable air pollution.

Finally, here are the 10 cities where the return on investment is absolutely highest for both cutting pollution and reducing heat. Note that many of these cities are likely to be some of the most vulnerable to climate change in the future:

So what’s stopping many cities from going on a tree-planting binge? Space is sometimes a major obstacle, as is water availability. The report tries to take this into account, and delves into individual cities, like Nairobi or London or Seoul, to figure out where tree-planting makes most sense.

McDonald points out a couple other big barriers. First, many cities just don’t think of trees as a public-health measure — they often fall under the purview of parks and recreation departments. That can lead to an underinvestment in urban planting.

Finally, there’s maintenance. For many cities, it’s easier to plant trees in the first place — developers may be required to plant them, say. But once they’re planted, they have to be maintained continually, by pruning branches, protecting against disease, and so forth. And that requires proper staffing, higher operating budgets, and so on. Those maintenance costs can be a hurdle for many strapped local governments, especially if trees are mostly seen as an aesthetic luxury.

Hence the argument for thinking of trees as more than just an aesthetic luxury. McDonald notes that our conception of urban trees has already changed dramatically in the past. Before the 1600s, street trees were rare, with the Dutch pioneering the practice to help stabilize their canals. It wasn’t until the 19th century that tree-lined boulevards became common in European cities. Today, they’re ubiquitous.

“So our idea of what trees can bring to cities has changed over time,” McDonald says. “And in the coming urban century, we’re saying that maybe it’s time to rethink their role yet again.”

Further reading