Why tomorrow’s power grids may be built for the wrong climate

For a century, a simple metric called cooling degree days has helped utility companies, grid operators, and financial markets estimate how much electricity air conditioning and refrigeration will require.

New research shows that the old metric has been missing something fundamental all along, and the error has real consequences for how power grids get planned and sized.

The study was led by atmospheric scientists at the University of Hawai’i at Mānoa. 

The team developed a new, physics-based version of the metric, called effective cooling degree days, that captures how temperature and humidity work together.

The goal was to determine the actual workload a cooling system faces. 

Applying the new metric across North America, the researchers found that cooling efficiency has been declining by two to four percent per decade since 1971. 

Moreover, the old temperature-only metric misjudges cooling demand differently depending on region, overstating it in some places and understating it in others.

A problematic metric used everywhere

“Estimates of cooling degree days are used everywhere,” said study lead author Jake Casselman, a postdoctoral researcher at UH Mānoa. 

“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 heatwave, Casselman noted.

An old assumption doesn’t hold up

The traditional approach to estimating cooling requirements rests on a simple assumption: every degree of heat takes the same amount of energy to cool, regardless of conditions. 

Real cooling systems don’t actually work that way. They become less efficient as temperatures climb. 

Humidity makes the problem worse, since the system has to expend extra energy wringing moisture out of the air rather than simply lowering the temperature.

A new model of the refrigeration cycle

The scientists combined climate science with refrigeration engineering to build a cooling demand metric that explicitly accounts for how cooling system efficiency shifts with both temperature and humidity. 

The team’s approach incorporates a simplified model of the refrigeration cycle, the same underlying physics that governs every air conditioner⁠ and refrigerator.

The model is used to estimate how efficiently cooling systems remove heat under different combinations of temperature and humidity.

The researchers applied this new metric to 50 years of high-resolution weather data spanning 1971 to 2020 across North America, quantifying how climate-driven shifts in cooling efficiency have already reshaped cooling demand region by region. 

They then repeated the analysis using projections from 19 climate models under a high-emissions scenario to assess how demand might shift going forward. 

Finally, the team mapped the results onto the U.S. electricity grid while accounting for where people actually live.

This allowed them to identify the regions and power systems most likely to experience the largest increases in cooling demand.

Double demands by mid-century

“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 study senior author Christina Karamperidou.

“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.”

That’s a substantial shift for regions that haven’t historically needed to plan around extreme cooling demand the way hotter parts of the country have.

It raises real questions about whether existing grid infrastructure in these areas is prepared for a very different future.

The impact of humidity

The researchers weren’t particularly surprised to find that cooling efficiency drops as temperatures rise. What genuinely surprised them was the role humidity played once they factored it in properly.

In some regions, like the desert Southwest, the team expected to find efficiency declining.

Instead, once they accounted for the fact that hotter air in these regions was also getting drier, that expected decline sometimes vanished entirely. 

In a few spots, efficiency actually held steady or even improved. Drier air turns out to be genuinely easier to cool, which partly offsets the penalty that comes with rising temperatures.

The opposite pattern showed up in humid regions, where heat and moisture compound each other rather than offsetting anything.

This made the actual energy burden considerably worse than a temperature-only analysis 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,” Casselman said.

Strong collaboration with engineers

This research grew directly out of collaboration between climate scientists and real-world engineering challenges. 

The project was both motivated and made possible through the researchers’ interactions with thermal-management engineers at the Environmentally Applied Refrigerant Technology Hub, where UH serves as a key partner institution.

Since cooling systems are genuinely complex pieces of engineering, Casselman and Karamperidou faced a significant balancing act. 

Working with engineers developing next-generation cooling technologies helped the team build a model that stayed true to refrigeration physics while remaining practical enough to apply across an entire continent.

The result gives planners a more realistic way to estimate future cooling demand as the climate continues to warm.

The study is published in the journal Nature Communications.

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