Amplified fossil dependence and environmental burden during temperature extremes amid the low-carbon energy transition

Climate change, driven largely by fossil fuel combustion, is making temperature extremes more frequent and more severe 1 , 2 . Cooling and heating are among the most immediate ways societies adapt to those extremes 3 , 4 , 5 , yet the resulting surge in electricity demand can raise CO₂ and air pollutant emissions and the associated health burdens 6 , 7 , 8 , feeding back into the warming that made the extremes more likely 3 , 9 . The challenge is that this adaptation is now taking place inside power systems that are simultaneously decarbonizing. Wind and solar capacity is expanding rapidly 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , but variability in their output makes reliability harder to secure 18 , 19 , and in the absence of cost-effective, carbon-free flexibility, fossil units originally built for baseload operation have increasingly served as balancing resources 20 , 21 . Extreme temperatures may therefore do more than raise demand: because thermal generation remains an important source of flexibility for meeting short-lived peaks, these events may deepen the very fossil dependence that the transition is meant to end (Fig. 1 ). Combining fuel-specific electricity generation and demand data for U.S. balancing authorities (BAs) with temperature, emissions and health-impact data, we find that daily fossil generation is, on average, 43% higher on extreme hot days and 19% higher on extreme cold days than on mild days, and the amplification is largest—not smallest—in systems with the lowest annual fossil shares, where daily CO₂ and air pollutant emissions on hot days can more than double. Fig. 1: Conceptual diagram of the linkages between climate change, extreme weather, fossil reliance, and the low-carbon energy transition. Full size image Climate change increases the occurrence of extreme weather events, and adaptation to extremes unintentionally intensifies fossil generation and reliance during weather extremes, which in turn further exacerbates climate change and air pollution. Increasing non-fossil energy (particularly variable wind and solar power) for climate mitigation increases the variability of fossil generation and inadvertently leads to greater fossil reliance under extreme weather and to high retention of installed thermal capacity, which brings about low capacity utilization rates. Energy storage, non-fossil forms of flexible energy and demand-side measures therefore need to be deployed alongside expanding wind and solar energy to avoid unintended fossil reliance, carbon and air pollution, and health burdens during extreme weather in the low-carbon energy transition.