The Crucial Role of Trees in Urban Cooling
It's early July but the UK is set for it's third heatwave of the year. As towns and cities brace themselves for further disruption we look at how the right tree in the right place can make a marked difference
With the UK’s 3rd heatwave inbound the Mayor of London's new "cool spaces" map is a welcome public service. It is also an admission that cities are already living with the human costs of a changing climate, and that the longer work of building climate resilience into our urban fabric has never been more urgent.
The cool spaces map directs residents to air-conditioned venues, shaded areas and free drinking water across the capital. It is a practical, well-executed piece of public service, and the team behind it deserve genuine credit for the speed with which they got it out. For many Londoners, including the elderly, young children, and vulnerable adults, it will make a material difference on the days that matter most.
Heatwaves are now no longer rare or exceptional events in the UK; they are becoming a routine feature of the summer, and the public health consequences are well-documented. As temperatures continue to rise, responses like the Mayor's map will become standard municipal infrastructure rather than novel interventions. The more pressing question, and the harder one, is what cities build alongside them so that fewer people need a refuge in the first place.
That is where nature offers something no air conditioning system can replicate.
London is the UK's largest urban heat island, a term that describes what happens when dense concentrations of concrete, asphalt and built infrastructure absorb solar radiation through the day and release it slowly through the night. This prevents the city from cooling down, producing tropical nights (those that remain above 20C) that are particularly hazardous for public health. Tree canopy works directly against this effect through two complementary mechanisms: shade, which intercepts solar radiation before it reaches the ground, and transpiration, through which trees release water vapour and actively cool the air around them. Both processes operate at street level, where people actually live and move, and both are available free of charge once the right trees are established.
The evidence base for this is substantial. Research using London weather-station data found that during the 2022 heatwaves, the most heavily treed areas of the city were approximately 1.2 degrees Celsius cooler on average than the least treed, with the gap widening to around 2 degrees at night, precisely when cooling matters most. The same study attributed roughly 16 per cent of avoided heat-related deaths between 2015 and 2022 to London's existing urban forest. These are not marginal gains. They represent real lives protected by an asset that, in most cases, was planted decades ago by people who could not have anticipated the conditions we now face.
It is also important to be precise about what kind of green space delivers these outcomes. Not all urban greening is equivalent. A lawn is not a tree, and across European cities the research is consistent: treed areas cool surface temperatures two to four times more effectively than treeless green space. Grass and low planting have a role to play in urban biodiversity and amenity, but when the objective is thermal regulation, the canopy is what counts.
This brings us to a complication that is easy to overlook in the urgency of planting programmes. A tree planted for today's climate may not survive the one it will be required to grow into. Drought and heat stress have become leading causes of urban tree mortality across Europe, meaning that a poorly selected species planted now could be dead or severely compromised well before it reaches the canopy size needed to deliver meaningful cooling. Species selection, provenance and soil design are not mere horticultural footnotes; they are the decisions that determine whether a green infrastructure investment is still functioning in 2050 or has been quietly replaced two or three times before then. For councils, developers and landowners, this represents both a financial risk and a reputational one: planting programmes that fail to survive are planting programmes that fail to deliver on their stated outcomes.
Drawing on the scientific expertise of the team at the Royal Botanic Gardens Kew, and the work of Kevin Martin in particular, Kew Reach have designed a tool to solve this precise problem. The tool uses high-resolution climate datasets to model how local climatic conditions, down to a resolution of one square kilometre, are projected to change by 2050 and 2090 under different projected pathways. Against those projections, it evaluates a database of commercially available tree species using two-dimensional climate modelling, assessing how well each species' natural environmental range aligns with the conditions it will face as the climate shifts. It then layers in physiological trait data, including drought tolerance, wood density and leaf composition, to produce a ranked suitability assessment that is specific to each site.
Critically, the tool does not treat climate resilience in isolation. It accounts for the practical realities of urban planting environments: tolerance of paved and compacted surfaces, suitability for sustainable drainage systems, performance in transport corridors and exposed locations. Species can also be filtered by the ecosystem services they deliver, including urban cooling potential, biodiversity value and air quality improvement. The output is not a generic species list but a prioritised, evidence-based recommendation tailored to the conditions of a specific site and a specific future climate. This kind of transparency is increasingly important to investors and public authorities alike, who need to demonstrate that green infrastructure decisions are grounded in robust, auditable evidence rather than convention or aesthetic preference.
The practical consequence is a significant reduction in planting failure and the costly replacement cycles that follow from it. Trees selected and established through this process are not merely planted for today; they are engineered to survive, grow and deliver measurable ecosystem services over the coming decades.
The Mayor's map helps Londoners find cool spaces today, and that matters. The longer ambition is designing a city that generates them continuously on its own, through an urban forest that is not only extensive but genuinely resilient to the future it will have to endure.
The Kew Reach Tree Tool is coming soon - join the waiting list for early access and exclusive discounts!