Pope Francis, in his highly anticipated call to action against climate change, took an unexpected swipe at the cap-and-trade systems used in both California and Europe to control greenhouse gases. They may sound good, the pontiff argued. But they won’t work.

“The strategy of buying and selling ‘carbon credits’ can lead to a new form of speculation which would not help reduce the emission of polluting gases worldwide,” Francis wrote in his wide-ranging encyclical on the environment and global warming.

“This system seems to provide a quick and easy solution under the guise of a certain commitment to the environment, but in no way does it allow for the radical change which present circumstances require,” he wrote. “Rather, it may simply become a ploy which permits maintaining the excessive consumption of some countries and sectors.”

His comments stunned many California environmentalists. Some had expressed their own reservations about cap and trade before the system launched in 2012, and in several cases, their concerns mirrored the pope’s. But they have largely rallied around the system to defend it from political attack.