More trees to combat heat islands: the 3-30-300 rule for more liveable cities

Trees in towns and cities help to combat urban heat islands. Keystone / Andreas Becker

Heat islands are causing Swiss and European cities to overheat. Yet only 13.5% of the urban population lives in neighbourhoods that comply with the '3-30-300' rule, a model that relies on trees and green spaces to combat the heat.

Summer in the city is increasingly synonymous with stifling heatwaves and tropical nights. This discomfort is not merely a subjective perception but the direct result of urban heat islands.

This microclimatic phenomenon, typical of heavily built-up areas, transforms squares and streets into veritable heat reservoirs. Asphalt and concrete absorb solar radiation during the day and then slowly release it as heat during the night. As a result temperatures do not drop and residents find no relief at night.

In Ticino, some of the most telling examples include iconic locations such as Piazza del Sole in Bellinzona, the Rotonda in Piazza Castello in Locarno, and Piazza Bernardino Luini, opposite the LAC museum in Lugano. During the hottest months, these surfaces become extremely hot and almost impassable.

Temporary solutions such as water misters or small patches of mobile greenery are no longer sufficient to address the problem. Faced with climate change that promises increasingly frequent and intense heatwaves, a radical paradigm shift in spatial planning seems ever more necessary.

The ‘3-30-300’ rule

Various solutions can be implemented to tackle the urban heat island effect. Some are architectural and can be integrated directly into buildings, such as green facades or next-generation materials. Others involve rethinking the use of urban green spaces, rediscovering just how important it is to have trees and areas rich in vegetation even within cities.

One of the most practical and elegant responses to this need comes from urban afforestation and is summarised in a numerical formula: the ‘3-30-300’ rule. Devised in 2021 by the Dutch ecologist Cecil Konijnendijk, this guideline sets out three minimum, measurable requirements to ensure that every citizen can benefit from the therapeutic and refreshing effects of nature:

• 3: every person must be able to see at least three trees of a significant size directly from the windows of their home, school or workplace.

• 30: every neighbourhood must have at least 30% tree cover.

• 300: no one should live more than 300 metres away (about a five-minute walk) from a public park or a high-quality, well-equipped green space.

Although it does not enjoy unanimous support within the scientific community due to certain methodological, climatic and ethical limitations, the 3-30-300 rule provides a valuable starting point for rethinking urban green spaces in a modern context.

Only 13.5% of Europe’s urban population (pictured: the Swiss city of Lucerne) lives in neighbourhoods with a sufficient number of trees and green spaces according to the 3-30-300 rule. Keystone / Urs Flueeler

Only 13.5% of Europeans live in green neighbourhoods

But how far do European cities fall short of this standard? A studyExternal link published in the journal Nature Communications, conducted by the European Commission’s Joint Research Centre (JRC) in collaboration with the University of Copenhagen, provides a clear and scientifically sound answer.

The researchers analysed high-resolution satellite data from 862 European cities (including several Swiss cities). The results indicate that just 13.5% of the urban population lives in areas that meet all three criteria simultaneously. Conversely, as many as 21% of citizens live in areas that do not meet even one of these criteria, and only 28% live in neighbourhoods that reach the 30% tree cover threshold.

The study also raises an issue of social injustice. Wealthier neighbourhoods consistently enjoy greater tree cover and are closer to parks, while low-income groups are relegated to heavily built-up areas, leaving them more exposed to the health risks of heatwaves.

A marked geographical disparity also emerges: while cities in Northern Europe (such as Helsinki) come close to excellent levels of compliance with the guideline, in Southern Europe (such as Athens or Barcelona) the share plummets to below 3% due to the arid climate and urban planning that is less favourable to trees.

Why don’t trees grow in cities?

A further studyExternal link, also published in Nature Communications, highlights an often-overlooked problem: it is not enough simply to plant trees; we must ensure they survive.

Many cities fail the 30% because saplings are planted under conditions of severe stress. The tarmac that suffocates the roots prevents rainwater from penetrating the soil, while constant digging for underground utilities and drastic pruning compromise their growth, leaving them perpetually as fragile shrubs incapable of providing any real shade.

From theory to practice: the challenge for Ticino

Switzerland and Ticino fall into an intermediate category, but climate pressure demands swift action to ensure that the 3-30-300 rule does not remain merely an academic utopia. At a local level, the first signs of change are beginning to emerge: the project to transform the Rotonda in Locarno into a “garden for the city” is a step in exactly this direction.

However, to tackle the challenge of heat islands, every new building or road project must incorporate tall trees, green roofs and vertical gardens as key infrastructure elements. Having nature on our doorstep is not an aesthetic luxury, but a fundamental requirement for ensuring liveable and welcoming cities in the near future.

Translated from Italian, sub-edited by Anand Chandrasekhar