On a hot summer afternoon, the difference between one city block and the next can feel like a different world. On one street, the sun beats down from above while the pavement radiates heat from below. Just around the corner, on a sidewalk shaded by trees, the air seems noticeably cooler. The actual temperature change may only be a few degrees. But for people waiting for a bus or walking home from work, that difference matters a lot.As cities get hotter, block-by-block temperature differences are becoming more consequential. In the world’s largest capital cities, the number of extremely hot days each year has risen by 25% since the 1990s. Longer, more intense heat waves are increasing health risks, straining energy systems, and disproportionately affecting lower-income neighborhoods that tend to have fewer trees. City leaders are searching for solutions that are effective, affordable and scalable.Trees are one of the most practical tools cities have to combat heat. Offering both shade and ‘evaporative cooling’ — trees’ version of air conditioning — they provide relief to people outdoors while helping lower surrounding temperatures. A 2023 modeling study found that almost 40% of the 6,700 deaths attributable to urban heat in Europe in 2015 could have been prevented if cities increased tree cover to 30% of their land area. In U.S. cities, urban trees reduce electricity use by nearly 39 million megawatt-hours per year, saving billions of dollars in cooling costs.Our analysis of more than 60 cities worldwide reveals a clear gap: Large areas of plantable land — especially along streets — are underutilized, presenting a clear opportunity for expanding urban tree cover. Within most cities, local neighborhoods already demonstrate what’s possible, with high levels of tree cover delivering measurable cooling benefits. Extending that level of canopy more broadly could meaningfully reduce heat exposure citywide.Of course trees alone won’t solve the problem of urban heat. But they’re an accessible and practical solution that can meaningfully ease its burden for residents. A tree-lined street in Freetown, Sierra Leone. The city’s Freetown the Treetown initiative has planed more than 1 million trees. Photo by em113/Shutterstock How Trees Cool CitiesThe cooling power of trees starts with shade. When surfaces like pavement, rooftops and buildings are exposed to direct sunlight, they absorb and store heat, releasing it back into the air long after the sun goes down. Trees interrupt that process. They shade sidewalks, streets and people, preventing surfaces from heating up in the first place. They also cool the surrounding air through evapotranspiration, the release of water vapor from leaves. A global study of 806 cities found that increases in tree canopy are associated with midday reductions in land surface temperature of around 1.5 degrees C (2.7 degrees F) — demonstrating how shade limits the amount of heat that is absorbed and re-radiated by urban surfaces. A global meta-analysis found that increasing tree cover also cools the air itself, with each 10% increase in canopy lowering ambient air temperatures by about 0.3 degrees C (0.54 degrees F).But ambient and land temperatures aren’t the only factors that determine how hot it feels. The way we experience heat — quantified by ‘thermal comfort indices,’ various measures of how hot an average person feels — is also shaped by factors like humidity, wind and direct solar radiation. Trees make it feel dramatically cooler by blocking direct sunlight. Data from WRI’s Cool Cities Lab shows that trees can improve thermal comfort by 2-8 degrees C (3.6-14.4 degrees F).Together, shade and evapotranspiration allow trees to provide cooling at multiple scales at once — from lowering neighborhood air temperatures to reducing surface heating to easing hyperlocal thermal stress. This all makes outdoor conditions more bearable during the hottest months of the year and reduces heat-induced illness and mortality. Urban trees also deliver wide-ranging benefits beyond cooling. They can improve air quality by removing and dispersing pollutants. They absorb stormwater runoff, helping to prevent flooding. They support biodiversity and sequester carbon, which helps curb climate change. They enhance physical and mental health and strengthen social cohesion by creating comfortable public spaces where people can gather and spend time outdoors. They can even boost property values by making neighborhoods more desirable places to live and spur local economic activity by increasing foot traffic in shaded commercial areas.Overall, more trees make for more resilient, livable cities.Cities Have Space for Many More TreesDespite the clear benefits of urban trees, many cities are not planting at the scale needed to keep pace with rising heat. Tree cover is unevenly distributed in many places — lower-income neighborhoods often have significantly fewer trees than wealthier areas.But how many trees could cities realistically grow?We analyzed existing and potential tree cover in 66 cities across all major world regions, from smaller cities to large urban areas. The results show a clear pattern: Most of these cities have substantial opportunity to increase their tree cover along streets and in places like parks, parking lots and underused land. Our Methodology for Calculating Urban Tree Potential[Read more]To set realistic goals for tree cover, cities first need to know how much space is available for new trees. Using high-resolution land cover data, we can map places that could support more trees — including parks, street right-of-ways, parking lots, and patches of open or underused land.To estimate what level of tree cover is achievable, we look at how much tree cover already exists in the greener parts of each land type within a city — if one neighborhood achieved a tree cover level, other neighborhoods probably can too. That helps set reasonable targets for increasing trees in each land type relative to current local conditions. Targets are also informed by the land type itself; for example, we set higher targets for street right-of-ways and parks because cities often manage these spaces directly, making it easier for planting programs to have a real impact.Mapping the opportunity for increased tree cover gives cities a realistic aspirational benchmark they can use in plans and policies — helping them advance beyond nonspecific or uninformed goals to plant more trees. It also highlights where the biggest gains are possible. Combined with other data, this can help cities focus their efforts where planting is feasible, strategic and most likely to benefit people. Learn more about our methods here. The cities we examined could increase their tree cover anywhere from 2 to 13 percentage points, depending on the type of land available and local growing conditions. For a typical city that means a median increase of about 8 percentage points, for example, going from 10% tree cover to 18%. Because many cities start with relatively low levels of tree cover, even these seemingly modest increases represent significant change. An 8 percentage point increase from 10% amounts to an 80% expansion relative to current conditions. Still, the relative change differs significantly by city — from about a 40% increase from baseline in Charlotte, North Carolina, to a dramatic 244% increase in Bogotá, Colombia.A large share of this opportunity — roughly half of all potential additional tree cover — lies along streets. Starting With Street TreesFor cities looking to expand tree cover, street trees — those planted along street right-of-ways like sidewalks and medians — represent one of the largest and most actionable opportunities. Unlike trees in parks or on private land, street trees are embedded in the everyday spaces where people move through the city.They deliver cooling exactly where it’s needed most. Streets are where much of a person’s daily heat exposure happens — walking to transit, waiting at crosswalks, running errands, etc. The presence of trees can make a tangible difference. A shaded sidewalk can feel up to 8 degrees C (14.4 degrees F) cooler than a sun-exposed one, improving thermal comfort and reducing heat stress for pedestrians.In nearly three-quarters of the cities we analyzed, street right-of-ways account for more than half of the total tree cover opportunity. In Hyderabad, India, the share reaches 72%. Even in cities with lower shares, like Porto Velho, Brazil, streets still represent an impressive 38% of the opportunity for expanding tree cover. Street right-of-ways take up about a fifth of urban land, but only 8% of that area is tree covered today. That could rise to nearly 28% if the cities we analyzed planted trees wherever space allows — a nearly fourfold increase. Here’s a closer look at Monterrey, Mexico—where tree canopy currently covers just 8% of the city. Where trees are present, the difference is immediately evident: shaded areas feel significantly cooler than exposed surfaces. If all suitable areas were planted, Monterrey’s tree cover could nearly double—reaching 15.7% across the city. Street trees alone could increase canopy by nearly 4%, creating cooler corridors for pedestrians and reducing how hot it feels in the shade by up to 6°C (10.8°F). Street trees aren’t just effective; they’re practical. Because they are often located on public land, they are typically managed by a single city department. This means they can be easier to plan, implement and maintain than trees in other locations. They also allow cities to strategically prioritize efforts in areas where heat exposure is highest.Urban Trees’ Benefits Outweigh Their CostsDespite the benefits, challenges remain that keep cities from reaching their full tree-cover potential.In dense urban areas, space for planting can be limited. Streets must accommodate many competing needs — like vehicle travel, sidewalks, drainage infrastructure, bike lanes and parking — which can make it difficult to incorporate trees in some locations. These constraints extend below the surface as well — trees need sufficient soil volume, water access, and space for roots to grow. Cost and long-term care also pose challenges. Young trees often require years of watering and maintenance before they become established. Urban environments can be especially difficult for trees to grow, with compacted soils, heat stress, poor access to groundwater and pollution. Without sufficient planning and resources for maintenance, survival rates can be low.However, these challenges should be considered alongside the growing costs of inaction. Cities face potentially billions of dollars in economic losses if they fail to adequately address rising temperatures, including impacts on health systems, infrastructure, employment and energy demand. Urban trees represent a relatively cost-effective investment. When trees are appropriately selected, planted in suitable locations, and well maintained, the benefits of urban forestry outweigh the costs of planting and care. In fact, a meta-analysis found that cities receive more than $3 in benefits for every $1 invested in urban trees. Viewed this way, trees are a form of urban infrastructure that delivers significant economic, environmental and public health benefits.Cities around the world — like Freetown, Sierra Leone which has planted more than 1.2 million trees as part of its Freetown the Treetown initiative, or New York City which planted a million trees through MillionTreesNYC — have demonstrated that with clear planning, investment and community engagement, urban tree programs can scale successfully. The question for cities is not whether trees are worth planting, but how to implement tree programs in ways that are strategic, long-lasting and equitable. Workers plant a tree in New York City. The city’s MillionTreesNYC initiative has planted more than 1 million trees, bringing cooling shade to hot city streets. Photo by Here Now/Shutterstock Turning Opportunity into ActionWith the right strategies, urban trees can become a central part of how cities manage rising heat. Moving from opportunity to implementation calls for a few practical steps:Set realistic, data-driven canopy targets. High-resolution data on existing canopy and viable planting space can help cities establish goals that are ambitious but achievable. They can guide planting programs toward areas where trees will have the greatest impact.Prioritize streets and public spaces. Because cities manage these areas directly, planting along streets and in parks and other public lands can often scale faster than programs relying on private property.Invest in long-term stewardship. Successful urban forests depend on maintenance as much as planting — the main reason new urban trees don’t survive is a lack of long-term stewardship. Dedicated budgets, trained staff and partnerships with community organizations can help ensure trees survive and thrive.Focus on heat exposure and equity. Directing investment toward neighborhoods with the highest heat exposure and the least existing canopy can deliver the greatest benefits to the people who need relief the most.Urban trees are not a quick fix — growing a healthy urban forest takes time. But they are one of the most accessible and multifunctional tools cities have to make streets cooler, neighborhoods healthier and communities more resilient.Cool Cities Lab delivers local heat data directly to cities. Explore where heat risks are most acute and visualize how these hot spots overlay with vulnerable populations and infrastructure. Estimate the impacts of cooling solutions ranging from street trees to cool roofs. Learn more about Cool Cities Lab. Sign up for updates and share how your city could use the data.

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