Revisiting the IPCC’s “Discernible Human Influence” Finding: History and Lessons Learned

Abstract:

At a November 1995 Plenary Meeting in Madrid, the Intergovernmental Panel on Climate Change (IPCC) reached the historic conclusion that “the balance of evidence suggests a discernible human influence on global climate”. Prof. Bert Bolin, the Chairman of the IPCC, played a critical role in obtaining acceptance of this conclusion by all government delegates present in Madrid. Prof. Bolin was also instrumental in defending the “discernible human influence” finding – and the scientific process by which it had been reached – after the Second IPCC Assessment Report was published in 1996. The first part of my talk covers Prof. Bolin’s involvement with the 1995 IPCC finding.

Support for the 1995 IPCC statement came from the physical understanding of heat-trapping properties of greenhouse gases, observations of warming, and the comparison of modeled and observed climate-change patterns (“climate fingerprinting”). Initial criticism of the 1995 IPCC statement stemmed from the paucity of fingerprint studies and the focus on surface temperature. Other concerns were related to poorly quantified uncertainties in observed climate records and in model estimates of climate signals and internal variability.

The scientific community responded by performing the research required to address the criticism. After Madrid, human-caused temperature fingerprints were identified from the stratosphere to the deep ocean. Fingerprint research rapidly moved beyond “temperature only” studies, exploring changes in dozens of different aspects of the climate system. Large multi-model and single-model ensembles became available, leading to improved quantification of uncertainties in human-caused warming signals and in the noise of natural climate variability. In tandem with progress in Earth System modeling, observational climate change was more systematically assessed using satellite data and reanalysis products developed by different research groups. The second part of my talk summarizes these and other post-Madrid advances in climate fingerprinting. I will show that the cautious “discernible human influence” finding has been confirmed and strengthened since 1995. The third and final part of my talk provides personal reflections on “lessons learned” from my involvement with the 1995 IPCC report.