Air conditioning cools the room by heating the street, and a UNSW physicist wants paint first

An air conditioner does not destroy heat. It moves it, and where it moves it is outdoors, which means every unit on a block makes the street and the neighbouring buildings slightly hotter ^[1]. That is the awkward physics behind a review published in Nature Reviews Clean Technology by Mat Santamouris, a physicist at the University of New South Wales in Sydney, and colleagues, who surveyed cooling technologies that work without drawing power ^[2].

Santamouris told Discover that if buildings adopted special coatings, improved ventilation, ceiling fans and similar measures, cities could cut their average temperature by several degrees and so reduce the overall demand for air conditioning ^[3]. Treat that number as what it is: an expert estimate offered in an interview, without a stated baseline city, measurement period or deployment share. It is a reason to look at the underlying measures, not a delivered result.

The cost case is easier to check. Figures published several years ago by the U.S. Energy Information Administration put air conditioning at about 12 percent of total household energy consumption ^[4]. Some research estimates air conditioning accounts for roughly 3 percent of global greenhouse gas emissions, with demand set to rise as poorer countries develop, according to Our World in Data ^[5]. Emissions raise temperatures, higher temperatures raise cooling demand, and Santamouris argues zero-carbon targets become extremely difficult if that loop is left to run ^[6][7]. In single-family homes, the units alone run to several thousand dollars before the monthly bills ^[8].

The technologies in the review are unglamorous, which is the point. Radiative paints reflect most incoming sunlight instead of absorbing it, and on a roof they push heat up through the atmosphere and out to space ^[9]. Evaporative coatings are newer: a hydrogel-based polymer paint that takes up moisture during the cooler night and releases it as the day heats up ^[10]. External shading and better internal ventilation block sunlight and redistribute heat, and Santamouris says advanced ceiling fans extend comfort to higher temperatures ^[11]. Green roofs and reflective pavement move the same lever at street level ^[12].

Santamouris is blunt that the familiar municipal answer is insufficient. Trees and parks absorb some of the excess heat, but he said cities now need much more radical solutions, and that heat waves carry a tremendous impact on both economies and health ^[13][14]. Livia Fritz, a geographer at the University of Geneva who was not involved in the review, said the study is timely and agrees passive cooling should take centre stage, with mechanical cooling reserved as a backup or short-term response rather than the default ^[15].

For anyone managing a portfolio of buildings, the operative detail is that many of these measures retrofit onto existing stock, with older heritage buildings the harder case ^[16]. That makes this a procurement and code question rather than a research one.