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. A metric shows air conditioners work less efficiently as temperatures rise, while humidity can worsen or offset losses. Across North America, future cooling energy demand may grow far faster than standard temperature-based estimates suggest.