New physics-based work by atmospheric scientists at the University of Hawai‘i at Mānoa challenges a widely used planning tool for cooling: cooling degree days. For decades, the metric has estimated how much energy air conditioners and refrigerators may need by treating temperature as the key driver. But real cooling systems also respond to humidity, and their efficiency changes with operating conditions. Published in Nature Communications, the study introduces an “effective cooling degree days” framework that couples temperature and moisture to the physics of refrigeration cycles.
The research team argues that the traditional temperature-only yardstick embeds a fundamental bias. Cooling demand does not scale linearly with heat alone because compressors and heat exchangers operate differently as it gets hotter, and humidity forces systems to do extra work by removing water from indoor air. Ignoring these effects can mislead utilities and grid operators about both regional peak loads and long-term infrastructure needs.
To build the new metric, the authors combine climate science with refrigeration engineering. They use a simplified, physics-grounded model of the refrigeration cycle to compute how efficiently cooling can remove heat across varying atmospheric states. The result is a demand metric that weights cooling requirements by the expected efficiency under each temperature–humidity combination.
Applying the approach to high-resolution weather records from 1971–2020 across North America, the team finds that climate-driven changes have quietly altered cooling efficiency for decades. Efficiency has been declining by roughly 2–4% per decade since 1971, reshaping where cooling loads rise and how strongly they increase. In parallel, region-to-region comparisons show that the older method can overestimate cooling demand in some climates while underestimating it in others.
The “humidity twist” produces counterintuitive behavior in certain areas. In the desert Southwest, warmer conditions typically reduce efficiency, but a drying atmosphere can partially offset that penalty. In some locations, the net effect is that efficiency decline weakens or even steadies. In humid regions, however, heat and moisture compound, making cooling energy burdens significantly worse than temperature alone suggests.
This competing influence—temperature pushing efficiency down and humidity sometimes pulling it back—creates what the authors describe as a “tug-of-war.” Such structure is invisible to temperature-only approaches, which cannot capture moisture-driven changes in cooling work.
Looking ahead, the study uses projections from 19 climate models under a high-emissions scenario. By mid-century, it estimates that the steepest increases in cooling-related electricity demand are likely in the Northwest, Great Lakes, and Mid-Atlantic, where some grid regions may see cooling demand more than double.
Finally, the team links the metric to electricity grid realities by translating cooling shifts into likely changes in demand patterns where people live. The goal is not just refined climate interpretation, but better sizing, planning, and operation of energy infrastructure as heat waves intensify and weather extremes evolve.
Subject of Research: Cooling degree days; air-conditioning and refrigeration energy demand
Article Title: Efficiency-weighted cooling degree days reveal opposing temperature and humidity effects on energy demand
News Publication Date: 11-Jul-2026
Web References: https://www.nature.com/articles/s41467-026-75388-9
References: 10.1038/s41467-026-75388-9
Image Credits: National Park Service.
Keywords: cooling degree days; humidity; refrigeration efficiency; energy demand; climate models; electricity grids; thermodynamics
