Scientists challenge cooling degree days metric
5th August 2026
USA: Atmospheric scientists claim that the established use of cooling degree days has led to irregularities in estimating air conditioning and refrigeration energy demand.
The study by scientists at the University of Hawaiʻi at Mānoa, published in Nature Communications, reports a new, physics-based version of the metric – effective cooling degree days – that captures how temperature and humidity together impact the actual work a cooling system is required to do.
Applying the metric across North America, they found that cooling efficiency has been declining by 2-4% per decade since 1971, and that the old temperature-only metric misjudges how much cooling is required in different regions. This has led to overstating demand in some places and understating it in others.
“Estimates of cooling degree days are used everywhere,” said Jake Casselman, atmospheric sciences postdoctoral researcher in the UH Mānoa School of Ocean and Earth Science and Technology (SOEST).
“Utility companies, grid operators, energy planners, and engineers use them to anticipate electricity demand, and financial markets trade futures on them to hedge against unusually hot summers. If that yardstick is biased in ways that depend on a region’s climate, then the planning decisions built on it are biased too. That can mean building the wrong amount of power generation in the wrong place, or misjudging where the grid is most at risk during a heat wave.”
The previous approach to estimating cooling requirements assumes every degree of heat takes the same amount of energy to cool, no matter the conditions. The study insists that this ignores the fact that cooling systems get less efficient as it gets hotter, and humidity makes it worse, because the system has to spend energy wringing moisture out of the air, not just lowering the temperature.
The new cooling demand metric explicitly accounts for how cooling system efficiency changes with temperature and humidity. It incorporates a simplified model of the refrigeration cycle to estimate how efficiently a cooling system can remove heat under different atmospheric conditions.
They applied the new metric to 50 years of North American weather data, used used projections from 19 climate models to assess future changes, and mapped the results onto the US electricity grid, accounting for where people live.
“Our results show that the regions facing the steepest future increases in cooling demand are the northwest, Great Lakes, and mid-Atlantic, where some grid regions are projected to see cooling-related electricity demand more than double by mid-century in this high-emissions worst-case scenario,” said Christina Karamperidou, a SOEST atmospheric sciences professor.
The impact of humidity was significant. They reported that in some regions, like the desert southwest, they expected efficiency to decline However, they found that as the air grows drier, the ease of cooling offsets the penalty of higher temperatures, keeping efficiency steady or even improving it in some spots. In humid regions, heat and moisture compound each other, so the energy burden is worse than temperature alone would suggest.
“Seeing these two effects pull in opposite directions across the continent, what we ended up calling a ‘tug-of-war,’ is something a temperature-only view would never reveal,” said Casselman.
To ensure their model was both applicable and accurate, the researchers incorporated a refrigeration framework that captures the specific physics of how temperature and humidity affect efficiency.
