Fifty shades of green: Trees, buildings and urban heat islands

Greenery and open space offer a nature-based solution to the problem of extreme urban temperatures. It is an uncomfortable reality for many cities that access to such relief remains the preserve of the wealthy.

At a global level, July 2021 has the dubious distinction of being the hottest month in recorded history. And that honour may yet prove to be short lived: in early August, large tracts of North America sweltered under a “heat dome”. By mid-month, in the ancient Sicilian city of Siracusa, the mercury was tipping 50 degrees Celsius, a mid-afternoon ordeal now believed to be Europe’s hottest ever. And, in a clear sign of the times, the UK’s national weather agency, the Met Office, has added an extreme heat warning to its list of advisory notices. In many parts of the world, even those countries where the climate has traditionally been classed as temperate, summer has become a dangerous time of year.

In cities and towns, rising temperatures have been elevated further. Solar radiation is absorbed by the mass of hard surfaces – concrete, brick and asphalt – and released slowly back into the atmosphere. This is a phenomenon known as the urban heat island effect, and its consequences are measurable. A recent Climate Central study showed New York City, in the summer, is 4.2° degrees Celsius warmer on average than its less developed surroundings.

Green spaces and heat inequality

In 2019, by combining satellite footage and census data for 25 global cities, Yale School of the Environment was able to prove the existence of urban heat inequality. According to the study, lower-income neighbourhoods suffer disproportionately from elevated temperatures. Greenery, open space, and lower densities, all more commonly found in affluent city neighbourhoods, can help militate against the worst effects of intense urban warming.

A separate analysis of 14,000 US cities undertaken by the Trust for Public Land found that in large parks, surface temperatures can be 10 degrees Celsius cooler than nearby built-up areas. Streets within a 10-minute walk of a park were up to 3.3 degrees Celsius cooler than those further away. In most cases, the size of the parks and green expanses, and the density of woodlands they contain correlated positively with the scale of the cooling effect. Perhaps unsurprisingly, the study also revealed that parks serving majority low-income households were, on average, four times smaller and four times more crowded than those serving high-income neighbourhoods. This wealth disparity is replicated elsewhere. In England, 59% of households in the top 10% income bracket are within a 10-minute walk of a publicly-accessible green space. This compares with just 35% of households in the bottom 10% income bracket. Moreover, the quality of parks, other green spaces and rivers is often lower in areas suffering higher instances of social and economic deprivation.

The effectiveness of trees at temperature reduction

Trees combat heat in two ways: by providing shade; and via the process of evapotranspiration, in which air is cooled by water evaporating through leaves and foliage. Cooling performance is dependent on tree type, coverage, location, and surface type. Figures cited by the U.S. Environmental Protection Agency (EPA) estimate that shaded surfaces are 11–25 degrees Celsius cooler than non-shaded surfaces at times of peak temperature. Trees are most effective at cooling buildings when shading windows and roofs. The EPA also cites findings that evapotranspiration, alone or in combination with shading, can help reduce peak summer temperatures by 1–5 degrees Celsius.

A study using weather stations mounted on bicycles by University of Wisconsin-Madison and Memorial University of Newfoundland found trees could lower air temperature by as much 5 degrees Celsius. Optimal temperature reduction was achieved when canopy cover exceeded 40%. Again, the report demonstrated the reality of heat inequality, by highlighting the disproportionately greener character of wealthier neighbourhoods .

Canopy coverage can also be a life saver. Research published in the Lancet Planetary Health journal estimates that a 30% increase in tree cover across Philadelphia by 2025 would prevent 403 premature deaths annually. The benefits will be most pronounced in areas of lower socioeconomic status, where 244 lives will be saved each year. This modelling outcome, it should be noted, is also dependent on the removal of impervious surfaces across the city.

Alternatives to trees

Man-made solutions are preferable to trees in certain circumstances. Depending on the structure, they can be cheaper to assemble and maintain. What’s more, they are often more practical, especially where it is difficult to plant trees due to subterranean restrictions such as cabling.

An Arizona State University study compared the shade performance of trees against lightweight or engineered shade, for example umbrellas, and other urban forms such as building overhangs, tunnels and breezeways. Shade from urban forms proved to be the most effective, followed by trees . Dr Ariane Middel, Assistant Professor School of Arts, Media and Engineering, lead researcher on the project says: "We hope that our results can help cities pick the right shade for the right place and provide more shade cover in places where it's needed." It is still important that cities continue to plant trees, which provide a range of benefits beyond shading, she stresses.

Additional benefits provided by trees include air quality enhancements, carbon sequestration and increased resilience to stormwater. In Madrid, for example, local authorities are planting a “green wall” of 500,000 trees around the city. The plans will see the absorption of around 175,000 tons of CO2 per year and help alleviate the urban heat island effect.

In the UK, heatwaves are now 30 times more likely than in pre-industrial times. According to the Met Office, there is a 12% chance that extreme temperatures, such as those experienced during the UK heatwave of 2018, could become ‘normal’ by the 2050s . While a 12% chance may seem to amount to a remote possibility, that figure would be less than 1% in a natural climate scenario.