How much can air conditioning increase air temperatures for a city like Paris, France?

TLDR

Urban heat islands drive higher air‑conditioning use in cities, and while AC cools indoor spaces it releases waste heat that warms the surrounding atmosphere. A coupled MESO‑NH meteorological model and TEB urban energy balance model simulated street‑level temperature changes for four Paris scenarios, including current AC inventories and future cases with all evaporative/free‑cooling systems replaced by dry units and a doubled AC power. The simulations show that AC waste heat raises street air temperature by 0.5 °C under current conditions, 1 °C if only sensible heat is released, and 2 °C with doubled AC power, with the largest nighttime increases due to a shallower boundary layer, underscoring the need for spatially explicit planning of future AC deployment.

Abstract

Abstract A consequence of urban heat islands in summer is an increase in the use of air conditioning in urbanized areas, which while cooling the insides of buildings, releases waste heat to the atmosphere. A coupled model consisting of a meso‐scale meteorological model (MESO‐NH) and an urban energy balance model (TEB) has been used to simulate and quantify the potential impacts on street temperature of four air conditioning scenarios at the scale of Paris. The first case consists of simulating the current types of systems in the city and was based on inventories of dry and evaporative cooling towers and free cooling systems with the river Seine. The other three scenarios were chosen to test the impacts of likely trends in air conditioning equipment in the city: one for which all evaporative and free cooling systems were replaced by dry systems, and the other two designed on a future doubling of the overall air conditioning power but with different technologies. The comparison between the scenarios with heat releases in the street and the baseline case without air conditioning showed a systematic increase in the street air temperature, and this increase was greater at nighttime than day time. It is counter‐intuitive because the heat releases are higher during the day. This is due to the shallower atmospheric boundary layer during the night. The increase in temperature was 0.5 °C in the situation with current heat releases, 1 °C with current releases converted to only sensible heat, and 2 °C for the future doubling of air conditioning waste heat released to air. These results demonstrated to what extent the use of air conditioning could enhance street air temperatures at the scale of a city like Paris, and the importance of a spatialized approach for a reasoned planning for future deployment of air conditioning in the city.