Planting trees to solve China’s air pollution would be a very ineffective way.
Let me break down the effect of trees into (1) sucking up particulate and (2) giving particles a place to land.
1. How Much Particulate Air Pollution Do Trees Suck Up?
Let’s talk about the most commonly talked about pollutant: particulate pollution (PM 2.5 and PM 10). According to the World Health Organization, particulate affects more people than any other type of pollution. So that’s a good place to start.
Can trees take particles out of the air? They probably do a bit, but trees really aren’t made to take particles out of the air. If they do, they probably do so when they suck in air and water through their roots. While they’re sucking in, some particles may get filtered out, but it’s probably not much.
When Are Trees Active?
There’s another problem: during the winter, trees are less active, so they’d be even less effective. This comes at the worst time possible, because China’s air pollution is worse during the winter.
2. Giving Particles A Place to Land
Research reports that I’ve read focus on how trees give particles a place to settle or impact on, rather than sucking up particles. For example, researchers at the USDA Forest Service explain: “most particles that are intercepted are retained on the plant surface.”
How big is that effect? I don’t think anyone knows of a way to measure this directly, but researchers have estimated that all of the tree cover in London reduces larger PM 10 particles by 0.7–1.4%. Other researchers estimated that more than doubling the tree cover in Glasgow would reduce PM 10 levels by about 2%. Across 10 US cities, the highest estimate was that tree cover reduces the smaller PM 2.5 particles by 0.24%.
At that rate, how much more tree cover would we need to reduce PM 2.5 by 50%? Let’s start with the highest PM 2.5 effect estimate of 0.24%. At that rate, we’d need to multiply tree cover by 208 times! Maybe we could plant trees on top of trees?
Don’t get me wrong. I love trees, and I would love it if cities had more trees. But they won’t be solving our air pollution problems any time soon.
3. A third use: Blocking sandstorms
In fairness, governments have planted trees for reasons other than sucking up particles. Another goal is to block sandstorms originating in Mongolia from making it all the way to Beijing. In this case, trees are probably somewhat effective. However, sandstorms really aren’t what most people are concerned about in China.
What Would Work Better than Planting Trees to Solve China’s Air Pollution Problem
From a big-picture perspective, it makes the most sense to cut down on pollution at the source. It’s a heck of a lot cheaper to (1) put emissions control technology on smokestacks and (2) remove old polluting cars that lack catalytic converters than it is to try to suck up all those particles after they’re already in the air. So if I were in charge of solving the world’s particulate problem, I’d start at the source rather than go planting trees.
Thomas is an Associate Professor of Behavior Science at the University of Chicago Booth School of Business and the founder of Smart Air, a social enterprise to help people across the world breathe clean air without shelling out thousands of dollars for expensive purifiers.