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Cities would literally be much cooler with more trees

IN THE MIDDLE of winter, on a cold, grey day, it’s easy to find yourself yearning for some summer warmth. It’s also easy to forget the deadly heat.

2022 was Europe’s hottest summer on record, leading to more than 20,000 excess deaths across western Europe, and this summer could be even worse with the return of El Niño.

The effects may be felt most in our cities, which are on average 1.5 degrees Celsius warmer than surrounding areas due to so-called urban heat islands (UHIs). That’s when building materials (concrete, asphalt, metal) and machinery (cars, trucks, air conditioning units) absorb and produce heat, turning city blocks into baking ovens.

The heat is just one way in which cities can be unhealthy places to live; 4.3% of premature deaths in cities during the summer months are attributable to UHIs. Outdoor air pollution kills more than 4 million people a year. Noise causes the loss of more than 1.8 million healthy life years in Europe alone. Depending on where you live, you might be surrounded by shades of gray or trapped in a food desert. For many, when it comes to raising children or settling down, health, space, and financial pressures make moving out of the city a very desirable option.

That’s a shame because cities aren’t just brilliant hubs of human activity; they also offer an incredibly efficient way of living. Studies have shown that people who live in cities have smaller carbon footprints than those in the suburbs and countryside. Urban dwellers walk more, cycle more, and use public transport more than their suburban and rural counterparts. They also live in smaller homes with less stuff. Encouraging denser living could be a key part of the race to net zero carbon emissions. But if we’re going to convince people to live in them, we have to make cities more livable first.

Plenty of solutions exist, but there’s one powerful tool at our disposal that could help cool cities, reduce pollution, and improve our mental and physical health: trees.

A new study published in The Lancet shows just how effective trees would be at lessening the effects of climate change in urban areas. Increasing tree canopy cover to 30% of the city could reduce premature summer deaths in cities by about 40%, it finds, by reducing the temperatures through a combination of shading, evaporating water, and removing sources of heat such as concrete and asphalt.

Satellite data has shown that 30% of tree coverage is a feasible target in the 93 cities covered in the study, but the average in Europe is still only 14.9%.

Despite the relative simplicity, there are real barriers holding back urban forestry. Part of it comes down to competing urban interests, says Mark Nieuwenhuijsen, co-author of the study and director of urban planning, environment, and health at the Barcelona Institute for Global Health. Cities have historically given priority to space for vehicles. Wide roads and car parks could be reduced to make way for foliage but, as battles over cycle lanes and low traffic neighborhoods in the UK have shown, that fight is politically charged.

There’s also the expense. After buying and planting, new trees require maintenance. Like people, they can also find the urban environment difficult to survive in. In Lisbon, landscape architect Ana Luísa Soares estimates that each new tree costs the city about €2,000 ($2,180).

There are other ways of cooling the city — via reflective roofs or whiter-than-white paint. But while these interventions should play a role in building climate-resilient cities, they don’t quite deliver the full benefits package that trees do.

In a 2011 study, Soares estimated the cost of maintaining trees at $1.9 million a year. The benefits came in at $8.4 million. Nieuwenhuijsen ran through a list of proven benefits: Not only are streets more aesthetically pleasing when they’re lined with trees, but there are proven benefits for mental health. Urban green spaces are also associated with a lower prevalence of diabetes, heart problems, better birth outcomes, and improved cognitive functioning. Plus, trees are handy carbon stores that can also help prevent flash flooding — another climate risk.

The vital thing will be to ensure there’s an equitable distribution of trees throughout the city. One huge forest park won’t have the same beneficial effects of many tree-lined streets. Richer areas already tend to be greener than poorer ones, so there’s a social justice element too in ensuring everyone has equal access to nature.

Cecil Konijnendijk, professor of urban forestry at the University of British Columbia, has suggested a 3-30-300 rule: Everyone should be able to see three trees from their window, live in a neighborhood with 30% tree cover, and be 300 meters from a green space. But there’s a lot of work to be done. In Barcelona for example, only 5% of people live somewhere that meets that standard.

Making our cities more livable and sustainable will involve a lot of difficult decisions, about more than plant life. But considering trees can take decades to grow, we should get planting now.


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