Spatial microclimate effects on residential carbon emissions: Osaka districts versus JMA climatological normals

The transition toward low-carbon practices in the building sector is central to Japan’s goals of peaking carbon emissions by 2030 and achieving neutrality by 2050. In Osaka Prefecture, the Urban Heat Island (UHI) effect and diverse topography generate distinct microclimates that strongly influence heating and cooling energy use — and thus carbon emissions — from intermittent residential HVAC systems. Standard evaluations using Japan Meteorological Agency (JMA) 1991–2020 climatological normals from a single central station ignore this intra-regional spatial variability, leading to systematic distortions in carbon emission estimates. This study quantifies reference-weather deviations containing both spatial and interannual components in heating and cooling loads and CO2 emissions across nine representative districts by comparing 2025 hourly observed temperatures with JMA 1991-2020 climatological normals for the central Osaka station. Accordingly, the results are not interpreted as fully isolated microclimate effects. A steady-state thermal model (heat loss coefficient 3.0 W/m2K) is used with COP values of 4.0 for heating and 3.5 for cooling, together with Kansai Electric's emission factor of 0.336 kg-CO2/kWh. Degree-day metrics, load calculations, and correlation analysis are used to identify the dominant meteorological drivers. Results indicate that the central-station normals overestimate heating requirements in densely urbanized cores by an order of 5%, while underestimating them in higher-elevation suburban zones by an order of 10%. Cooling deviations show the opposite seasonal tendency in urban districts. Daily emission estimation errors can be much larger during short heating or cooling episodes, although these event-scale values should be interpreted as model outputs rather than precise measured carbon differences. The analysis shows that localized meteorological inputs materially affect district-level building carbon assessments and provides a transferable framework for metropolitan regions where standard weather files smooth over urban, coastal, and topographic gradients.