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We build, calibrate and simulate a stylised energy-economy model designed to evaluate the level of carbon tax that would allow the French economy to reduce its CO2 emissions by a factor of four over a forty-year horizon. We estimate the substitution possibilities between fossil energy and other factors for households and firms. We show that if energy-saving technical progress continues at its recent historical pace, the initial level of the carbon tax recommended by the Quinet report is totally inadequate.