Efficiency-weighted cooling degree days reveal opposing temperature and humidity effects on energy demand

Cooling degree days (CDD) are widely used to estimate air-conditioning energy demand, yet they implicitly assume constant refrigeration efficiency neglecting its strong dependence on temperature and humidity. Here, we introduce an efficiency-aware cooling metric --effective cooling degree days (eCDD)-- that links ambient temperature and humidity conditions to the physical work required for cooling. Applying this framework across North America, we show that cooling efficiency has declined by 2–4% per decade since 1971, and that regionally opposing trends in temperature and humidity cause CDD to misrepresent cooling demand. During hot extremes, efficiency losses are amplified under humid-heat conditions but partially offset under dry-heat conditions. Projections further reveal a continent-scale shift in humidity regimes, with an eastward extension of dry heat that locally enhances cooling efficiency during extremes, even as eCDD increases by 10–80% across the continent. These results demonstrate that temperature-based metrics alone are insufficient and that efficiency-aware metrics such as eCDD are essential for accurately assessing cooling demand, especially considering differing electricity generation mixes.