Urban trees almost halve the urban heat island in cities

Cities and towns are typically 1–3°C warmer than nearby rural areas, because surfaces such as asphalt, concrete and brick soak up the sun’s heat and then release it slowly.

In some places, the gap can reach 7°C. This phenomenon is called the urban heat island.

That extra heat can be hazardous, particularly in hotter countries. When conditions are extreme, dehydration and heat exhaustion become genuine dangers.

If temperatures climb high enough, the consequences can be fatal.

There is a straightforward countermeasure: urban trees. City authorities across the world have increased tree planting to help push back against rising heat.

But how much difference do trees actually make? How much warmer would our cities be if they had no trees at all?

To answer this, we examined data from nearly 9,000 cities worldwide, representing about 3.6 billion residents.

Our new research indicates that trees reduce the amount of heat retained by the urban heat island effect by almost half.

This cooling is clearly beneficial, but it is not shared evenly. On average, tree cover is higher in wealthier, suburban and more humid cities.

Why focus on trees?

Trees function as nature’s air conditioning. By shading the ground, they prevent asphalt and buildings from heating up as much in the first place.

They also cool the surrounding air by releasing water vapour through their leaves via transpiration, which lowers temperatures nearby. The difference can be easy to notice, particularly on scorching summer days.

Because of this, trees provide a relatively simple way to reduce urban heat - and that matters. The United Nations reports that more than half the global population (55%) now lives in urban areas.

By 2050, this share is projected to rise to 68%. Meanwhile, climate change is bringing hotter conditions, with heatwaves becoming more frequent and more intense. On top of that, the urban heat island effect further increases city temperatures.

What did we do?

We set out to answer a basic question: how much hotter would cities be without trees?

To do so, we combined global datasets on air temperature with fine-scale measurements of tree cover across almost 9,000 cities. We then modelled a "what if" scenario in which all tree cover was removed, and compared the results with today’s conditions.

This approach let us quantify the real-world cooling that trees provide for air temperature - the main way people experience heat.

Many earlier global analyses have relied on surface temperatures, often derived from satellites. Yet surfaces such as roads and roofs can become far hotter than the air above them, especially in direct sunlight. That can lead to an overestimate of the cooling trees deliver.

By contrast, air temperature is a better match for what people actually feel, making it a more dependable indicator of heat.

So what effect do trees really have?

The impact was far bigger than we expected.

Worldwide, trees reduce the urban heat island effect by almost 50%. Given that the average urban heat island effect commonly adds about 1–3°C, this equates to roughly 0.5–1.5°C of cooling in many cities.

For over 200 million people, trees lower local air temperatures by at least 0.5°C - enough to matter during episodes of extreme heat.

However, the amount of cooling differs greatly between locations.

In hot, arid cities such as Phoenix in the United States, variations in tree cover can produce clear differences in air temperature. In milder places such as Lisbon in Portugal or Gothenburg in Sweden, the overall cooling remains meaningful, but it is usually smaller and more uniform across the city.

Trees are not evenly distributed

Urban trees are not spread equally. They are often clustered in wealthier districts and suburban areas, and cities in cooler or more humid climates generally have more.

Tree cover is thinner in lower-income cities or in fast-expanding regions. The same inequality shows up within many cities too: leafy suburbs are often several degrees cooler than nearby areas with little vegetation.

Wealth is strongly associated with canopy cover. In the United States, lower-income areas have, on average, 15% fewer trees than richer areas - and they are 1.5°C hotter. In other words, those who most need the free cooling trees provide are frequently the least likely to benefit from it.

Planting more trees isn't enough

Tree planting is often presented as an easy fix for overheated cities. Trees are highly visible, relatively inexpensive, and they bring additional advantages such as cleaner air and improved mental health.

It is therefore unsurprising that authorities turn to urban trees to help counter the heat intensified by climate change. Anyone who has stood beneath a tree on a sweltering day knows the cooling can feel immediate and substantial.

Yet our findings suggest their contribution is more limited when set against ongoing climate change. We estimate that existing urban trees worldwide would offset only 10% of the additional heat expected by mid-century under moderate climate change scenarios. With ambitious planting, that could increase to around 20%.

That is important - but still insufficient. Most of the extra heat would remain unmitigated.

What else can be done?

If cities are to manage rising temperatures, trees need to be treated as one element of a wider plan, not a standalone solution.

Smart urban design can reduce heat by using reflective materials, expanding green space and improving airflow between buildings. Green roofs and shaded streets can help as well.

New plantings should be directed towards hotter neighbourhoods with limited existing canopy, where the gains will be greatest.

Naturally, none of these steps remove the need to address climate change itself by reducing greenhouse gas emissions.

Using trees wisely

Billions of trees are already growing in the world’s cities. Their value is immense: they cool urban areas, support biodiversity and help make cities more liveable.

For residents and decision-makers, the task is to use trees strategically - planting them where they will have the greatest impact and pairing them with other heat-reduction measures.

Trees are extraordinary. But they cannot do everything.

Manuel Esperon-Rodriguez, Researcher in Urban Transformation, Western Sydney University; Rob McDonald, Research Scientist, City University of New York, and Tirthankar Chakraborty, Earth Scientist, Pacific Northwest National Laboratory

This article is republished from The Conversation under a Creative Commons licence. Read the original article.