Heat island

Studied for over two centuries, the urban heat island (UHI) phenomenon has recently seen a resurgence of interest. While between 1990 and 2000 around 30 studies were published each year, this number exceeded 300 in 2015. Its inclusion in 2019 among the performance indicators for resilient cities in the ISO 37123 standard illustrates this growth.

Today, when we talk about adapting to climate change, heat islands are almost always mentioned, often in a negative light. Indeed, the phenomenon is commonly preceded by the phrase “combating,” which presents it as an enemy to be defeated. This crusade seems justifiable in view of the images associated with it: a particularly hot place in the city, a factor in the deterioration of urban comfort, an amplifier of heat waves, a health risk to living beings, or even “red spots” appearing on microclimate simulation maps. However, these representations are far removed from its scientific definition: we end up condemning a concept based on what it is not.

Scientifically, an urban heat island is defined as the difference between the air temperature in the city and that of a surrounding rural or natural area. The phenomenon is permanent, but its intensity varies according to the seasons and the time of day. However, while the rise in air temperature increases the vulnerability of populations during heat waves, it also improves winter comfort and reduces heating needs. Ignoring these benefits leads to an incomplete analysis. Even when limiting the study to the summer months, a misunderstanding persists: reducing the heat island effect does not necessarily improve living comfort or reduce energy consumption in cities. In fact, one of the quickest and least expensive ways to reduce its intensity would be to... set fire to the surrounding countryside! The city would then become an “island of coolness,” while the living conditions of its inhabitants would deteriorate and the need for air conditioning would increase. This example, deliberately absurd, is nevertheless mathematically accurate: the gap between two configurations can be reduced by acting on one or the other of the terms of the subtraction. In other words, lowering the temperature in the city or increasing the temperature in the countryside are, in theory, two equally effective strategies for reducing a heat island. If an absurd answer turns out to be correct, it is the relevance of the question that must be questioned.

Thus, the goal of cities should not be to “reduce the urban heat island,” but to control summer overheating and ensure the comfort of its inhabitants. As long as the concepts remain confused, adaptation policies will focus on the indicator rather than on well-being. The precision of words determines the precision of actions.