Motivation: The climate crisis stands as the defining moral and intellectual dilemma of our age. Misguided clean-energy investments expose the fragility of the energy–carbon equilibrium, demanding analysis, ethical foresight, and institutional resolve to preserve our ecological inheritance. Purpose: This study examines the dynamic impact of energy investment on the carbon budget across 28 developed and 37 developing countries, using assembled annual data from 2000 to 2024. Design/methodology/approach: The empirical strategy is framed in time series and dynamic econometric analysis. Findings: The results indicate a concave relationship between clean-energy investment and carbon budgets, where a 1% increase in clean-energy investment lowers carbon budgets by approximately 2.6% in developing economies and 2.1% in developed economies, with diminishing marginal returns. Nonlinear effects and strong persistence in historical carbon trajectories jointly shape the current effectiveness of investments across both groups. Research limitations/implications: The study focuses on developed and developing economies, which limits the detail on individual countries but enhances cross-regional understanding. Originality/value: The study makes a significant contribution to the existing literature by demonstrating that clean-energy investment yields larger marginal carbon reductions in developing economies, implying higher global mitigation efficiency.