Singapore: Singapore is planting one million trees to cool its streets, but scientists say the biggest impact could be on air-conditioning bills | World News

Representative Image of lush urban greenery shading a busy street in Singapore (AI-generated image)

As cities across the tropics search for ways to cope with rising temperatures, Singapore has increasingly turned to a familiar, low-tech solution: trees. A specific figure keeps surfacing in planning documents and public discussion: the claim that shading a street with trees can lower street-level temperatures by as much as 2.6 degrees Celsius. That number has real scientific grounding, but tracing exactly where it comes from, and what local Singapore research has since found on its own streets, reveals a more layered and, in some ways, more useful picture than the headline figure alone suggests.

Why Singapore’s tree cooling estimate comes from research in Germany

The number appears in Strategies for Cooling Singapore, a catalogue of 86 heat mitigation measures compiled for policymakers by the Cooling Singapore project, a collaboration between the Singapore-ETH Centre, SMART, TUMCREATE and the National University of Singapore. Its entry on vegetation around buildings estimates cooling from tree-shaded streets at between 0.9 and 2.6 degrees Celsius, set against the context that Singapore’s urban heat island effect already runs at roughly 4 degrees on average and can exceed 7 degrees during certain hours of the day.Crucially, that 2.6 degree figure was never actually measured in Singapore. It traces back to research by Sten Gillner and colleagues, who monitored six tree species along residential streets in Dresden, Germany, over the summer of 2013, finding that trees with dense foliage and high transpiration rates produced the strongest cooling effect on surrounding air. The Cooling Singapore catalogue is transparent about this origin, describing itself as a review of existing international science translated into local planning guidance, meaning a single European study, covering six tree species over one temperate summer, is now doing considerable work informing tropical urban policy.

What researchers actually measured on Singapore’s own streets

Given that gap, locally gathered data carries particular weight. In December 2024, a team led by Lei Xu and Ronita Bardhan published findings in the journal Science of the Total Environment based on 20 paired sites across Singapore, comparing one sunlit and one shaded location per pair, all measured during a single very hot afternoon. Every form of shade tested- trees, bus shelters, awnings, the shadow cast by a tall building- reduced measurable heat stress.The effect on raw air temperature was comparatively modest, close to a single degree. The effect on how hot a person actually feels, however, was considerably larger. Measured using the Universal Thermal Climate Index, a metric that accounts for radiant heat, humidity and air movement rather than air temperature alone, shade reduced the index by an average of 3.1 degrees, and in locations with around 80 per cent coverage from either canopy or built structures, the team estimated a cooling potential of 3 to 5 degrees. Bardhan, who supervised the research through the Cambridge Centre for Advanced Research and Education in Singapore, has argued that street shading deserves to be treated as essential infrastructure rather than decorative planting, particularly for people who have little choice but to spend extended time outdoors in the heat.

Why the real payoff may be indoors, not on the street

Field measurements alone cannot capture the full picture of how tree shade affects a city’s energy use, which is where modelling studies come in. The most detailed attempt so far comes from Naika Meili and co-authors, published in the Journal of Advances in Modelling Earth Systems in March 2025, which combined a building energy model with an urban ecohydrological model to simulate tree cover scenarios across seven hot cities worldwide, including Singapore.Their central finding was that shade falling directly on a building itself, its walls and windows, rather than any resulting drop in outdoor air temperature, is what actually drives electricity savings. For hot, humid cities like Singapore, the modelled savings in summer cooling energy averaged 6 to 9 per cent, notably lower than the roughly 17 per cent modelled for drier cities like Riyadh and Phoenix, largely because trees release moisture into the air, and removing that extra humidity from indoor air during dehumidification itself consumes additional power. In humid climates, the researchers found this benefit peaks at around 40 per cent tree cover before the added humidity begins eating into the savings, though trees did trim electricity demand most sharply during peak afternoon hours, a detail that matters considerably more to grid operators than any average daily figure.

Why tree shade also has a downside worth weighing

Shade is not without trade-offs. A meta-analysis of 182 separate studies, published in the journal Communications Earth and Environment, found that tree canopy performing well during the heat of midday can trap outgoing radiation after dark, holding heat closer to pedestrian level overnight in dense, high-rise urban areas. The Cooling Singapore catalogue raises a second trade-off as well, noting that dense planting placed within a natural wind corridor can act as an unintended windbreak, a meaningful concern in a city where average wind speeds already sit below 3 metres per second, making the loss of a cooling breeze a real cost in exchange for shade.

Why growing that shade takes patience Singapore doesn’t fully have yet

Singapore’s National Parks Board has pursued its OneMillionTrees movement under the wider Singapore Green Plan 2030, originally targeting a million additional trees planted between 2020 and 2030. According to NParks, the agency now expects to hit that target by the end of 2027, three years ahead of schedule. Even so, a tree planted today will not cast meaningful shade for roughly a decade, which is why researchers behind the local Singapore fieldwork recommend a mixed approach, relying on canopy where planners can afford to wait for it to mature, and built structures like awnings and shelters where they cannot.That distinction also reframes how the original 2.6 degree figure should be understood. Read purely as a promise about street-level air temperature, most streets are likely to fall short of it. Read instead as the upper range of what a single, well placed, mature tree can do to a thermometer, with the larger and more immediately useful benefits showing up in reduced radiant heat, improved felt comfort and cooler building surfaces, the figure becomes a more honest starting point for what tree planting can realistically deliver in a tropical city still working out exactly how to grow shade fast enough to keep pace with its own heat.