Beyond the Degree Day: Bridging Climate Science and Energy Systems Engineering
The cooling degree day (CDD) has served as the standard climatological measure of building cooling demand for decades. It captures sensible temperature excess, while ignoring both the latent heat burden of humidity and the thermodynamic efficiency penalties that warmer outdoor conditions impose on cooling systems. As cooling becomes a dominant link between weather and grid stress, that omission increasingly matters. Here, we introduce efficiency-weighted cooling degree day (eCDD), which incorporates compressor performance and the sensible-latent partitioning of cooling load directly into the index. Applied to hourly historical atmospheric reanalysis data (ERA5) and future climate model projections (CMIP6), North America exposes a tug-of-war: temperature and inlet vapor pressure exert opposing controls on cooling efficiency, sometimes reinforcing and sometimes cancelling, producing systematic under- and over-estimation of cooling trends by traditional metrics depending on which side dominates. Extending this globally, that tension resolves into four distinct thermodynamic regimes: moisture-dominated, drying-dominated, compound amplification, and compensating. Weighting for population shows where people actually sit within these regimes and mapping that population into an efficiency-load phase space reveals that humanity’s collective operating point has drifted toward warmer and more humid conditions over the last 25 years, a shift driven by demographics rather than climate alone. These trajectories continue further under future emissions scenarios by 2100. Overall, we show that cooling efficiency is also governed by climate, not merely engineered into machines.
Bio: Jake Casselman is a postdoctoral researcher in the Department of Atmospheric Sciences at the University of HawaiÃÂûi at MÃÂÃÂnoa, where he works with Christina Karamperidou on the intersection of climate variability and building energy systems. His research develops industry-facing climate metrics that bridge atmospheric science and energy infrastructure planning, and is the lead developer of CEDAR, an open-source Python toolkit for building-climate analysis. Prior to his postdoc, Jake completed his Ph.D. at ETH Zürich in atmospheric physics and holds degrees from McGill University and Columbia University.