Is there a discernible human influence on climate (as claimed by the IPCC)?
Until properly defined, this is not a meaningful question. Yet Keller puts great emphasis on the main conclusion of the IPCC’s Summary for Policymakers [IPCC 1996, page 5]: “The balance of evidence suggests there is a discernible human influence on global climate.” I will argue below that the scientific evidence on warming patterns does not support the IPCC conclusion. This ambiguous phrase does not identify (using Keller’s words) “a small but growing [anthropogenic] signal in a noisy, chaotic climate.” Nor can it be used “to project future warming even with admittedly imperfect climate models.” In fact, there is an explicit denial of such a link in the IPCC report itself [IPCC 1996, page 434]: “To date, pattern-based studies have not been able to quantify the magnitude of a greenhouse gas or aerosol effect on climate.” Yet when politicians at the Geneva climate convention in 1996 asserted such a link and thereby misused the IPCC phrase to claim a substantial future warming and paint a
