Europe and the world are facing an increasing number of extreme heat events. Record temperatures in cities are significantly worsening living conditions, increasing energy consumption and threatening health. Three proven strategies – trees, cool roofs and radiative cooling (radiative cooling) – they provide real solutions to reduce urban temperatures while bringing ecological and social benefits.

1. Greenery in cities: tree network (Urban Trees)

Benefits of trees:

• They reduce the warm, sensible temperature by 1.7°C (approximately 1°F) below the tree canopy
• Increasing tree cover by every +1 % brings a temperature reduction of approximately 0.14 °C – with an increase from 10 % to 25 %, the total decrease is up to 2 °C (Phoenix study)
• In addition to providing coolness, they provide shade, purify the air, absorb CO₂, reduce the energy consumption of buildings and increase the quality of life.

Limitations:

• Need for watering in dry cities, high maintenance costs (~$900/year per tree in Boston).
• In dense urban areas, there is often no space for planting. But even small structures – such as green facades – can contribute to local cooling.

2. Cool Roofs

Strategy profile:

• Reflective, light-colored roofs increase albedo and reflect most of the sun's radiation back into the atmosphere. They reduce the internal temperature of buildings by 1–3°C and can reduce air conditioning energy consumption by up to 27 %.

Efficiency in cities:

• Models in London show that city-wide deployment of cool roofs could reduce temperatures by 1.2°C and up to 2°C in some locations, with green roofs and vegetation delivering just 0.3°C of cooling.

Advantages:

• Fast implementation, low operating costs compared to trees, ideal for dense development and private buildings. Initiatives like NYC CoolRoofs or Boston show that the return on investment can be very fast (e.g. in India <3.3 years).

Limitations:

• It only works on suitable roof types (not shingles, pitched roofs). With a limited number of roof areas, the effect is more local and less pronounced than the street or neighborhood level.

3. Passive Daytime Radiative Cooling (PDRC)

What is PDRC?
These are surface technologies (e.g. special panels) that effectively reflect solar radiation while simultaneously radiating heat back into space through the infrared "window" band of the spectrum. They operate without the need for energy and provide cooling even during the day.

Why is this technology the third element?

• They have higher savings potential than a white roof – models show approximately double the effect of traditional cool roofs in dry cities like Phoenix or Las Vegas

Advantages and options:

• It works passively, is effective in hot, dry regions, and can be applied to roofs or facades.
• Can be combined with green roofs or trees to provide synergistic effects. Reduces heat load on surfaces and air.

Challenges:

• New technology with higher initial costs. Requires mass production development and integration into future building codes.

How to choose the right strategy?

Studies recommend combined approaches:

• Research shows that a combination of trees, green roofs and cool roofs can reduce temperatures by 2°C or more and at the same time optimize the effect in different microclimatic zones.

It depends on local conditions:

• In areas with plenty of space and rainfall (e.g. Boston), tree netting has a strong effect, but in dense city blocks or arid regions, cool roofs or PDRC technology have the upper hand.

Citizen engagement and funding:

• Successful initiatives include subsidies, community participation, targeted programs for disadvantaged urban areas – e.g. CoolRoofs subsidies in New York, tree planting in Detroit, or climate centers recommended in Paris.

No single strategy is universal. Trees, cool roofs and passive radiative technologies represent three powerful tools that complement each other. Trees provide shade, cooling potential and ecological benefits. Cool roofs are practical, cheap and quickly deployable in dense urban areas. Passive radiative surfaces bring a technological advantage for very hot and dry climates.

The most effective way is integrated plan, which combines these practices according to the local context. With the right combination, each city can find its own unique mix that will improve the comfort of its residents, reduce energy demands, and transform hot cities into places resilient to extremes.  JRi