Insights from and about the IPCC’s Sixth Assessment Report

Philip Mote

In August, 2021, the Intergovernmental Panel on Climate Change (IPCC) released the long-awaited first volume of its Sixth Assessment Report (AR6), which focuses on the physical climate system. Since the last assessment report in 2013,

  • Global warming (the change in globally averaged temperature since 1850-1900) crossed the 1.0°C (1.8°F) mark, and is likely to reach 1.5°C between 2030 and 2052.
  • Attribution of that warming to human activity has strengthened from “clear” to “an established fact.”
  • Climate models have continued to improve the representation of physical processes, year-to-year variability, and long-term change, including the notably warm period ~50-60 million years ago, the early Eocene.

Improved understanding of the deep past (thousands to millions of years ago) has led to improved estimates of both climate sensitivity (the increase in global temperature with rising carbon dioxide) and the response of global sea level to climate change, and to the revelation that “key indicators of the climate system are at levels unseen in centuries to millennia”

It is worth saying a few words about the IPCC assessment process, which spans several years. The IPCC, an international body with one member from each of nearly 200 countries, oversees the assessment process which is carried out by three working groups, each tasked with producing one report. The outline for AR6 was approved in 2017. The report of Working Group I, on the physical climate system, was released in August (Fig. 1).

The front cover of the summary for policymakers of the Intergovernmental Panel on Climate Change's Sixth Assessment Report.
Figure 1. The Summary for Policymakers of the Intergovernmental Panel on Climate Change’s Sixth Assessment Report was released in early August, 2021.

Each report consists of 12 or more chapters. The chapter teams for Working Group I—a total of 234 authors from 66 countries—assessed a vast number of peer-reviewed articles on their topic that were published since the last report, or more precisely since the cutoff for inclusion in the last report, in 2012. A subset of articles assessed is actually cited, and even that subset came to over 14,000 articles. IPCC reports use careful and standardized language to describe the strength of evidence for various statements made in each chapter.

Each chapter in the report goes through three rounds of expert peer review. Chapter teams substantially revise the report each time, responding in a comprehensive spreadsheet to every review comment. The level of effort to describe the state of knowledge is unmatched in any field of science. I participated in AR4 and AR5 as an author of the cryosphere (snow, ice, and frozen ground) chapter, and it was impressive to see the care of authors and IPCC leaders in presenting the state of knowledge.

A short summary for policymakers digests the voluminous report’s main messages and is then scrutinized, debated with authors, and ultimately approved by the IPCC in a days-long meeting. The meeting to approve AR6 concluded on 6 August, 2021. The text of the summary for policymakers is available now, but still is undergoing copy edit and layout.

The US Global Change Research Program issues similar reports, the National Climate Assessments. The process of writing the fifth National Climate Assessment has begun, with chapter authors selected. CIRC co-lead Erica Fleishman will participate as an author of the Northwest chapter. The Oregon Climate Change Research Institute also conducts a biennial assessment of the state of knowledge of climate change as it relates to Oregon’s natural and human systems. The fifth Oregon Climate Assessment was released in January 2021.

Why so many reports? Climate change is such a comprehensive subject, and draws on such a large body of research, that it’s easy to be confused by the latest press release. Beginning in 1988, policymakers began requesting guidance on what can be inferred from the science, and these reports are a response to those requests.


Philip Mote is Vice Provost and Dean of the Graduate School at Oregon State University. He remains active in atmospheric science and climate policy and adaptation.


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