This study investigates the causal interactions during heatwaves across 520 urban sites in the U.S., revealing that megacities like New York City and Chicago act as causal pacemakers, mediating the structure of urban networks during heatwaves. The research combines complex network and causal analysis to show that megacities are causally connected with most other cities and significantly influence the propagation of heatwaves. A positive correlation was found between the strength of causality and the population size of megacities, highlighting the role of human activities, such as greenhouse gas emissions and waste heat, in urban heatwaves. The study also identifies long-range teleconnections and supernodes, which are important for predicting and adapting to heatwaves under global climate change. Urban areas are intrinsically connected and co-evolve, with teleconnections playing a key role in the spread of heatwaves. The study uses a network approach to analyze the dynamics of heatwave propagation, revealing hub-periphery structures in urban networks. Megacities are identified as critical nodes in these networks, influencing the spread of heatwaves. The findings suggest that human activities significantly impact the causal interactions of cities during heatwaves, and that urban areas with high population density are more likely to be affected. The study also highlights the importance of megacities in regulating heatwaves, as they serve as heat sinks and sources, influencing the spread of extreme thermal conditions. The results indicate that the propagation of extreme thermal conditions among urban areas is due to the interplay of synoptic climate conditions and anthropogenic stressors. The study provides insights into the role of megacities in urban heatwave propagation and the importance of human activities in exacerbating heatwaves. The findings have implications for urban planning and policy, emphasizing the need for strategies to mitigate extreme heat stress in vulnerable cities. The study also suggests that future research should consider multidimensional measurements and the role of atmospheric moisture in determining heatwave impacts. The framework proposed in this study can be used to predict future urban heatwaves and their causal mechanisms, providing valuable information for climate adaptation and mitigation strategies.This study investigates the causal interactions during heatwaves across 520 urban sites in the U.S., revealing that megacities like New York City and Chicago act as causal pacemakers, mediating the structure of urban networks during heatwaves. The research combines complex network and causal analysis to show that megacities are causally connected with most other cities and significantly influence the propagation of heatwaves. A positive correlation was found between the strength of causality and the population size of megacities, highlighting the role of human activities, such as greenhouse gas emissions and waste heat, in urban heatwaves. The study also identifies long-range teleconnections and supernodes, which are important for predicting and adapting to heatwaves under global climate change. Urban areas are intrinsically connected and co-evolve, with teleconnections playing a key role in the spread of heatwaves. The study uses a network approach to analyze the dynamics of heatwave propagation, revealing hub-periphery structures in urban networks. Megacities are identified as critical nodes in these networks, influencing the spread of heatwaves. The findings suggest that human activities significantly impact the causal interactions of cities during heatwaves, and that urban areas with high population density are more likely to be affected. The study also highlights the importance of megacities in regulating heatwaves, as they serve as heat sinks and sources, influencing the spread of extreme thermal conditions. The results indicate that the propagation of extreme thermal conditions among urban areas is due to the interplay of synoptic climate conditions and anthropogenic stressors. The study provides insights into the role of megacities in urban heatwave propagation and the importance of human activities in exacerbating heatwaves. The findings have implications for urban planning and policy, emphasizing the need for strategies to mitigate extreme heat stress in vulnerable cities. The study also suggests that future research should consider multidimensional measurements and the role of atmospheric moisture in determining heatwave impacts. The framework proposed in this study can be used to predict future urban heatwaves and their causal mechanisms, providing valuable information for climate adaptation and mitigation strategies.