Lord Monckton was kind enough to assist me in deciphering the meaning of the ruling and writes:
In France, if at least 60 Deputies of the House and 60 Senators appeal to the Constitutional Council, it has the power to pronounce on the constitutionality of a proposed law – in the present case, the 2010 national budget of France, which contained enabling provisions (loi deferee) for a carbon levy. The Council found that these enabling provisions were unconstitutional on two grounds: that the exemptions contained within the provisions for a carbon levy vitiated the primary declared purpose of the levy, to combat carbon emissions and hence "global warming"; and that the exemptions would cause the levy to fall disproportionately on gasoline and heating oils and not on other carbon emissions, thereby breaching the principle that taxation should be evenly and fairly borne.
The Press release from the French Constitutional Council is here in English (Google Translated) and in original French
Here's a Deustch-Welle news article on the reversal.