Color me cooler

You study urban heat islands (UHIs). What are they, and how do they form?

When cities get systematically warmer than surrounding suburbs, that's an urban heat island. They form because cities typically have lots of dark surfaces that absorb energy in summer, plus a lot of man-made heat from cars and buildings. Suburbs have more forested areas and green space.

How long have you been studying urban heat?

In 1983, I formed the Heat Island Group, part of the Lawrence Berkeley National Laboratory [a U.S. Department of Energy facility managed by the University of California]. We wanted to establish that urban heat islands existed and explore basic mitigation techniques such as pavement, roofs, and vegetation.

At first, we were the only ones studying the issue. But after 10 years, papers we published sparked interest in the field. Today, there are about 20 major institutions worldwide working on various aspects of the UHI problem, including mortality and morbidity related to urban heat.

What are some of your most important research findings?

Our work helped quantify the benefits of cool roofs and pavements, which in turn informed standards for measuring heat-mitigating materials. In California, for example, new roofs must meet cool-roof standards, or buildings must incorporate other energy-efficient measures.

What makes a "cool roof" cool?

White roofs reflect most of the sun's energy, and therefore stay cooler. But they may not be the color of choice for some people, especially for residential sloped-roof buildings. To address this, materials engineers created "cool colors": pigments, paints and coatings that are red, brown, blue — the normal colors you'd see on a roof — but still reflect much of the sun's heat.

How do they do that?

Most solar heat comes from near-infrared radiation, a part of the electromagnetic spectrum that lies just beyond what the human eye can see. Cool-color pigments are engineered to reflect near-infrared wavelengths, which carry most of the sun's heat but are invisible to the human eye. So, they reflect a lot of the sun's energy and most of its heat, without producing a glare.

These cool materials: are they expensive to manufacture?

No, they're readily available. They're made using special pigments designed to reflect the heat-carrying part of sunlight, mixed evenly into paints or coatings so they can be applied like a regular roof or wall finish.

In fact, they are widely available in the US for fiberglass asphalt shingles, metal roofs, and clay and concrete tiles. They use heat-reflective pigments but look like ordinary roofing materials.

How much impact could cool roofs and pavements have on climate change?

Cities make up about three percent of the Earth's land surface. A moderate program to increase the reflectivity of roofs and pavement could increase solar reflectance in cities by about 10%. Under certain assumptions, researchers have said the resulting net global cooling could be equivalent to taking the world's passenger cars off the road for several decades. In hotter regions of the world, the effect would probably be much greater.

Cool-roof programs around the world

Los Angeles, USA
Modeling shows broad adoption of reflective roofs, pavements and modest tree planting could lower average urban temperatures by ~2–3°C.
Sources: Lawrence Berkeley National Laboratory; City of Los Angeles program reports.

Ahmedabad, India
Reflective roof coatings in low-income housing pilots have delivered significant indoor cooling, often 2–5°C lower during peak heat compared to uncoated roofs.
Sources: Ahmedabad Heat Action Plan program evaluations; academic pilot results.

Melbourne, Australia
Municipal trials show cool pavements and reflective roofs reduce surface temperatures by ≈10–20°C at midday, with building cooling-load reductions of ~20–40% in models.
Sources: City of Melbourne / university research.

Mexico City, Mexico
Programs testing reflective waterproof roof coatings have reported 1.5–3.5°C reductions in indoor temperature during heat events in low-income neighborhoods.
Sources: Local pilot studies supported by UN-Habitat / Mexico City government.

Abu Dhabi, UAE
Urban heat island effects account for a substantial share of cooling demand; reflective materials can cut roof-surface temperatures by 10–20°C and reduce energy stress in peak summer.
Sources: Masdar Institute / MIT research.