CO2 = P × (GDP/P) × (E/GDP) × (CO2/E)
Developed by Japanese economist Yoichi Kaya in 1990, the Kaya Identity decomposes total CO2 emissions into four driving factors: population, economic output per person, energy intensity of the economy, and carbon intensity of energy. It reveals that reducing emissions requires acting on at least one factor faster than the others grow. Global population grows ~1%/year, GDP/capita ~2%/year, so energy intensity and carbon intensity must decline by >3%/year just to stabilize emissions. Historically, energy intensity has fallen ~1.5%/year through efficiency gains, and carbon intensity ~0.3%/year. This is why emissions keep rising despite improvements. To reach net zero by 2050, carbon intensity must decline at 10×the historical rate, requiring a massive shift from fossil fuels to renewables and nuclear energy. The Kaya Identity is the foundational framework used by the IPCC in all emissions scenarios.