HONOLULU, Hawaii —  Researchers at the University of Hawaiʻi at Mānoa have developed a new method for estimating air-conditioning and refrigeration demand that accounts for both temperature and humidity.

The study, published Aug. 4, in Nature Communications, introduces “effective cooling degree days,” a physics-based metric designed to better represent the actual work required from cooling systems.

The researchers found that cooling efficiency across North America declined by 2–4% per decade from 1971 to 2020. They also found that the traditional cooling degree day metric can overestimate cooling demand in some regions and underestimate it in others.

Traditional cooling degree days assume that each degree of increased temperature requires the same amount of cooling energy. However, actual cooling-system efficiency changes with atmospheric conditions.

Higher temperatures can reduce cooling efficiency, while humidity adds another energy burden because systems must remove moisture from the air in addition to lowering its temperature.

Jake Casselman, an atmospheric sciences postdoctoral researcher at the University of Hawaiʻi at Mānoa’s School of Ocean and Earth Science and Technology, and Christina Karamperidou, an atmospheric sciences professor, developed the new metric by combining climate science with refrigeration engineering.

The researchers incorporated a simplified refrigeration-cycle model to estimate how efficiently cooling systems can remove heat under different temperature and humidity conditions.

They applied the metric to 50 years of high-resolution weather data from 1971 through 2020 across North America. They also analyzed projections from 19 climate models under a high-emissions scenario.

The analysis found that some regions could experience substantial increases in cooling-related electricity demand. The Northwest, Great Lakes, and Mid-Atlantic are projected to face some of the largest increases, with cooling-related electricity demand in some grid regions potentially more than doubling by mid-century under the high-emissions scenario.

The research also found that humidity can produce different effects depending on regional climate. In the desert Southwest, increasing dryness can offset the efficiency penalty associated with higher temperatures. In some locations, efficiency could remain steady or improve.

In humid regions, higher temperatures and moisture can compound the energy required for cooling, creating a greater burden than temperature alone would indicate.

The researchers mapped their findings onto the U.S. electricity grid while accounting for population distribution. The goal was to identify regions and power systems most likely to experience significant changes in cooling demand.

The project involved interactions with thermal-management engineers at the National Science Foundation-funded Environmentally Applied Refrigerant Technology Hub, of which the University of Hawaiʻi at Mānoa is a key partner institution.