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The Kaya identity was developed by Japanese energy economist Yoichi Kaya and is a cornerstone of emissions scenario analysis, particularly within the Intergovernmental Panel on 气候变化 (IPCC). It is not a predictive model but an identity—a tautological equation that must be true by definition. Its power lies in decomposing complex CO2 emissions into understandable, quantifiable drivers. The four terms represent: (P) population growth, (G/P) economic growth or affluence, (E/G) energy efficiency of the economy, and (F/E) the 碳足迹 of the energy supply. This decomposition allows policymakers and researchers to analyze the leverage of different strategies. For instance, reducing emissions (F) can be achieved by controlling population (P), reducing per capita consumption (G/P), improving energy efficiency (lowering E/G), or decarbonizing the energy supply (lowering F/E). The identity highlights the immense challenge of decarbonization, as economic and population growth have historically driven emissions upward, requiring massive improvements in energy and carbon intensity to achieve net reductions.