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Is urban tree planting a significant climate solution?
There’s not enough space in cities for urban tree planting to affect the global climate. But closer to home, trees can relieve urban heat and reduce energy use for cooling.
February 6, 2025
As they grow, trees absorb carbon dioxide, the main climate-warming gas driving today’s climate change. In fact, about 50 percent of a tree’s dry weight is made up of carbon. That means that the world’s forests are an incredibly important tool for countering some of humanity’s climate emissions, absorbing more than a quarter of the carbon we’ve put into the atmosphere in recent decades. But as climate change worsens, trees are becoming less effective at that job, and some groups have proposed planting more trees as a climate solution.
What about planting more trees in cities—would that have an effect on climate change?
Unfortunately, to the extent this would work at all, it would require vast new urban forests to make even a tiny dent in global climate change. Any emissions decline from traditional tree planting in cities—like along streets and in parks—would be incredibly small, says John Fernández, director of MIT’s Urban Metabolism Group, which focuses on the resource intensity of cities.
Cities account for only about three percent of global land area, meaning that the number of trees that could be planted in them is quite small. “The amount of trees that exist in cities and the amount of new trees that you could plant in cities is trivial in comparison to the number of trees on the planet,” says Fernández. “Even if you did the absolute maximum planting of trees in every city on the globe, it would have a trivial effect on pulling carbon dioxide out of the atmosphere, if any effect at all.”
However, city trees do create an important cooling effect locally. At the level of a single city, there are many good reasons to consider tree planting as a response to climate change. Cities suffer from the “urban heat island effect,” the buildup of heat that results from the concentration of transportation, buildings and industrial activities in a small area. Concrete, black asphalt, and other building materials absorb heat during the day and re-radiate heat at night. Air conditioning dumps heat to the outdoors, and cars and trucks blow hot exhaust and radiate heat from their engines.
Trees help counter this effect in two ways. Their shade minimizes the sunlight hitting the ground, streets, buildings and other infrastructure. Trees also absorb heat as they take in water from the air and soil, and then release that water, a process called evapotranspiration.
That is especially important at night, says Fernández. “Climate change is leading to very hot nights, and there are very few ways in which to deal with that,” he says. “One of the ways is to increase the evapotranspiration that comes from plants and trees, especially in the urban space itself.”
So while urban tree planting is not effective at dealing with our climate-warming carbon emissions, Fernández says, it is important for adapting to some of the effects of climate change, like intense episodes of extreme heat.
By taking some of the edge off heatwaves, trees can make cities safer for people, particularly those vulnerable to high temperatures, like children, the elderly, and those who can’t afford air conditioning. Trees also have important biodiversity benefits, providing space for all kinds of wildlife and diverse landscapes that are more resilient to climate change and better at storing carbon.
As a bonus, by making cities cooler, trees can also lower the need for air conditioning, with all its energy demands and climate emissions.
To maximize the benefits of tree planting, Fernández says, cities should plant a wide variety of species and ensure that trees growing in urban areas remain healthy and provide varied habitats for birds and other animal species. Due to space constraints, urban trees can become water- or space-stressed as they grow, and need to be carefully managed. Cities can also educate private landowners about the benefits of trees, encouraging fewer removals.
Ultimately, a climate strategy focused solely on planting trees is a misguided one, says Fernández, but that does not mean that planting and maintaining trees in cities is a bad idea. Heat reduction in cities is very important. And urban areas can also think about their effects on trees beyond their borders. Reducing urban sprawl can limit the amount of forests that are cut down for new development, protecting our existing forests and their capacity to absorb carbon dioxide. This itself, says Fernández, is a vital global climate initiative. “Protecting the trees that exist—versus planting new trees—is much, much more important.”
“The strategy of adding trees is every year becoming less and less valuable, because the need to reduce emissions faster is becoming ever more urgent,” he adds. “We really need to be looking at solutions for reducing emissions within the next five to 10 years, and afforestation—adding to the number of trees today—to capture carbon dioxide is not going to be an effective strategy. Aggressive emissions reduction is by far the most important action we need to take.”
Thank you to Mike from Singapore for the question.
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