Impact of climate warming on passive night cooling potential

Night-time ventilation is often seen as a promising passive cooling concept. However, as it requires a sufficiently high temperature difference between ambient air and the building structure, this technique is highly sensitive to changes in climatic conditions. In order to quantify the impact of climate warming on the night-time ventilative cooling potential in Europe, eight representative locations across a latitudinal transect were considered. Based on a degree-hours method, site-specific regression models were developed to predict the climatic cooling potential (CCP) from minimum daily air temperature (T-min). CCP was computed for present conditions (1961-90) using measured T-min data from the European Climate Assessment (ECA) database. Possible time-dependent changes in CCP were assessed for 1990-2100, with particular emphasis on the Intergovernmental Panel on Climate Change (IPCC) 'A2' and 'B2' scenarios for future emissions of greenhouse gases and aerosols. Time-dependent, site-specific Tmin scenarios were constructed from 30 Regional Climate Model (RCM) simulated data sets, as obtained from the European PRUDENCE project. Under both emissions scenarios and across all locations and seasons, CCP was found to decrease substantially by the end of the 21st century. For the six Central and Northern European locations (47N) CCP was found to decrease in summer (June-August) by 20-50%. For the two Southern European locations (Madrid and Athens), future CCP was found to become negligible during the summer and to decrease by 20-55% during the spring and the autumn. The study clearly shows that night-time cooling potential will cease to be sufficient to ensure thermal comfort in many Southern and Central European buildings. In Central and Northern Europe, a significant passive cooling potential is likely to remain, at least for the next few decades. Upper and lower bound estimates for future CCP were found to diverge strongly in the course of the 21st century, suggesting the need for flexible building design and for risk assessments that account for a wide range of emissions scenarios and uncertainty in climate model results.