University of Hawaiʻi at Mānoa atmospheric scientists have developed a physics-based metric for estimating air-conditioning and refrigeration energy demand. Published in Nature Communications, the effective cooling degree days metric accounts for the combined effects of temperature and humidity on the work required from cooling systems.
Researchers Jake Casselman and Christina Karamperidou combined climate science with refrigeration engineering to create the metric. Their approach incorporates a simplified refrigeration-cycle model to estimate how cooling-system efficiency changes under different atmospheric conditions.
The researchers applied the metric to high-resolution weather data from 1971 to 2020 across North America. They found that cooling efficiency declined by 2–4% per decade and that the conventional temperature-only cooling degree days metric overstated demand in some regions while understating it in others.
The team also analyzed projections from 19 climate models under a high-emissions scenario and mapped the findings onto the U.S. electricity grid while accounting for population distribution. The Northwest, Great Lakes and Mid-Atlantic showed the steepest projected increases, with some grid regions expected to see cooling-related electricity demand more than double by mid-century under the scenario.
Humidity produced different regional effects. In parts of the desert Southwest, increasingly dry air offset the efficiency penalty from higher temperatures, keeping efficiency steady or improving it in some locations. In humid regions, heat and moisture increased the energy burden beyond what temperature alone indicated.
“Getting cooling demand right isn’t an academic exercise; it directly affects how we plan, size, and operate future energy infrastructure as the climate changes,” said Christina Karamperidou, atmospheric sciences professor in the UH Mānoa School of Ocean and Earth Science and Technology.
