Hitting the energy-efficiency targets would reduce demand for those permits, in which case their price might fall too low to encourage further investment into low-carbon technologies.
The biggest polluters (who happen to be big and politically well-connected industries, such as chemicals) have had generous exemptions from the levy, mostly in return for signing up for long-term targets to improve their energy efficiency.
"The U.S. commitment to specific, mid-term emission cut targets and China's commitment to specific action on energy efficiency can unlock two of the last doors to a comprehensive agreement, " Yvo de Boer said.