The climate agenda in France and several other countries is a complex combination of unilateral commitments with regional and international objectives. When analyzing national policies, the findings of worldwide analyses are of limited accuracy and the large aggregates on which they are built level out most local specificities and inertia. Specific assessments are hence needed. This paper quantifies the dynamic evolution of carbon values for French climate and energy policy. Its time dependency over successive periods and the effects of setting intermediate targets are evaluated using a long-term optimization model. Addressing critical issues for France, we produce consistent energy, emissions and carbon value estimates with a 5-year time step. Our results are situated above the upper range of carbon value estimates of world models with an overlapping zone. We show that the official policy guideline value is only consistent with an optimistic combination of assumptions. The central estimates are 4 times greater than the guideline carbon value for 2050 and up to 14 times greater in 2020 because of short-term inertia that are costly to move. We also find that with intermediate objectives, the carbon value's dynamic is more than a simple upward curve and that its variability is itself time dependent.