Projection of Heating and Cooling Degree Days in Ankara Under Climate Change Using ERA5-Land and Bias-Corrected CMIP6 Models: A Multi-Threshold Approach

Climate change is shifting building thermal demand from heating toward cooling, yet quantitative, design-relevant projections for semi-arid continental cities remain scarce. This study projects heating and cooling degree days (HDDs and CDDs) for Ankara, Türkiye, using ERA5-Land as the reference, and a quantile delta mapping (QDM) bias-corrected CMIP6 ensemble under SSP2-4.5 and SSP5-8.5, across eight base temperatures (HDD 15–18 °C; CDD 23–26 °C) to 2100. By the far future, HDD18 decreases by 23–37% whereas CDD24 increases by +205 to +498 °C·day/year (with wide inter-model spread), and the heating degree days factor declines from 0.984 to 0.765, marking a shift toward cooling within the degree-day budget; the cooling season extends from 25 to as many as 106 days. In a deterministic −1 to +2 °C local-temperature-offset test, the direction of change persists, and the cooling design dry-bulb temperature rises by up to +7.3 °C, raising peak cooling loads. Independent 2020–2022 station observations support the HDD projections (CDD projections were not independently validated). Because a single grid cell and station cannot resolve urban canyon or local climate zone contrasts, the conclusions apply to the campus vicinity. Overall, the study translates the climate signal into HVAC-sizing and retrofit-relevant metrics for a warming semi-arid capital.