In their Policy Forum, “The costs of ‘costless’ climate mitigation” (1 December 2023, p. 1001), M. J. Kotchen et al. assert that the greenhouse gas mitigation potential identified by the Intergovernmental Panel on Climate Change (IPCC) in its latest assessment report is larger and cheaper than what is found in widely used cost-benefit–based (as opposed to processbased) integrated assessment models such as DICE, FUND, and PAGE. They suggest that the IPCC report underestimates the costs of mitigation by excluding hidden or nonmonetary costs in various sectors. Their criticism of the IPCC assessment is unfounded.@en