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 dependence on temperature and humidity. We introduce an efficiency-aware cooling metric -effective cooling degree days (eCDD)- that links ambient temperature and humidity to the physical work required for cooling. Here we show that, across North America, 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 but partially offset under dry-heat. Projections reveal a continent-scale shift in humidity regimes, with an eastward extension of dry heat that enhances cooling efficiency during extremes, while eCDD increases by 10-80%. These results demonstrate that temperature-based metrics alone are insufficient and efficiency-aware metrics are essential for accurately assessing cooling demand, especially considering differing electricity generation mixes.