IPCC extends review of Working Group I Sixth Assessment Report Second Order Draft

GENEVA, April 7 – The Intergovernmental Panel on Climate Change (IPCC) has extended government and expert review of the second order draft of the Working Group I contribution to the Sixth Assessment Report (AR6) by six weeks to allow reviewers more time in the light of disruption caused by the COVID-19 pandemic. 

The review, which opened on 2 March 2020 and was originally scheduled to end on 26 April, will now close on 5 June. Interested experts can register until midnight CET on 29 May at

The Working Group I Co-Chairs requested an extension after surveys of government representatives and the scientific community indicated that retaining the original end date given the COVID-19 pandemic would jeopardize IPCC requirements for a successful review: inclusion of the the best possible scientific and technical advice; a wide circulation process, ensuring representation of independent experts from developing and developed countries and countries with economies in transition; and an objective, open and transparent review process.

“The review process is crucial for the quality of IPCC assessment reports. It is essential for its exhaustivity, objectivity and rigour,” said Valérie Masson-Delmotte and Panmao Zhai, Co-Chairs of Working Group I in a joint statement. 

“We have two priorities: on the one hand, supporting scientists who are facing multiple challenges in this crisis situation to work for IPCC based on their volunteer efforts, and on the other, making sure that we keep the highest scientific quality standards for the Working Group I report. We are extremely thankful to the dedication of scientists and experts who are willing to contribute to the review process and will have more flexibility with this six-week extension.” 

The extension of the Working Group I review is one of the first actions agreed by the IPCC as it undertakes a strategic review of the timing and format of all reports and activities in the Sixth Assessment Report cycle.

IPCC reports go through repeated drafts and reviews to help ensure that the report provides a balanced and comprehensive assessment of the latest scientific findings.

The Working Group I contribution to the AR6, Climate Change 2021: The Physical Science Basis, will assess large-scale climate changes, climate processes and feedback and regional climate information. The Working Group I report includes an interactive online atlas, which is also open for review comments.

The draft report has been prepared by 233 authors from 65 countries.

During the review, experts may comment on the structure and comprehensiveness of the report and suggest improvements on the presentation of materials graphically or through tables. They may also propose revisions, relevant additional papers with their full citation, and editorial changes for readability and concision. This version also includes the first drafts of the Technical Summary and Summary for Policymakers.

IPCC reports undergo multiple stages of reviews, first by experts and then by both governments and experts.  Experts who commented on the first draft will also be invited to comment on this second order draft. All expert reviewers will be acknowledged in the published report. This was originally due to be finalized in April 2021, but along with other IPCC reports and activities may be delayed because of the pandemic.

The first order draft of the Working Group I report received 23,462 comments from 750 expert reviewers. So far, 2,277 experts have registered to review the second order draft.

For more information about the IPCC’s review of its strategic planning schedule, please see this statement.

More information on the role of an expert reviewer is available in Annex 1 of Appendix A to the Principles Governing IPCC Work.

Additional information explaining the Expert Review process is available here:

For more information contact:

IPCC Working Group I Technical Support Unit,

IPCC Press Office, Email:

Jonathan Lynn, +41 22 730 8066

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Notes for editors

 About the IPCC

The Intergovernmental Panel on Climate Change (IPCC) is the UN body for assessing the science related to climate change. It was established by the United Nations Environment Programme (now UN Environment) and the World Meteorological Organization (WMO) in 1988 to provide policymakers with regular scientific assessments concerning climate change, its implications and potential future risks, and to put forward adaptation and mitigation strategies. In the same year the UN General Assembly endorsed the action by the WMO and UNEP in jointly establishing the IPCC. It has 195 member states.

IPCC assessments provide governments, at all levels, with scientific information that they can use to develop climate policies. IPCC assessments are a key input into the international negotiations to tackle climate change. IPCC reports are drafted and reviewed in several stages, thus guaranteeing objectivity and transparency.

The IPCC assesses the thousands of scientific papers published each year to inform policymakers about the state of knowledge on climate change. The IPCC identifies where there is agreement in the scientific community, where there are differences and where further research is needed. It does not conduct its own research.

To produce its reports, the IPCC mobilizes hundreds of scientists. These scientists and officials are drawn from diverse backgrounds. Only a dozen permanent staff work in the IPCC’s Secretariat.

The IPCC has three working groups: Working Group I (the physical science basis of climate change); Working Group II (impacts, adaptation and vulnerability); and Working Group III (mitigation of climate change). It also has a Task Force on National Greenhouse Gas Inventories that develops methodologies for estimating anthropogenic emissions and removals of greenhouse gases. All of these are supported by Technical Support Units guiding the production of IPCC assessment reports and other products.

IPCC Assessment Reports consist of contributions from each of the three working groups and a Synthesis Report. Special Reports undertake a shorter assessment of specific cross-disciplinary issues that usually span more than one working group.

About the Sixth Assessment Cycle

At its 41st Session in February 2015, the IPCC decided to produce a Sixth Assessment Report (AR6). At its 42nd Session in October 2015 it elected a new Bureau that would oversee the work on this report and Special Reports to be produced in the assessment cycle.

The Special Report on Global Warming of 1.5ºC was released in October 2018, the 2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories in May 2019, the Special Report on Climate Change and Land in August 2019, and the Special Report on The Ocean and Cryosphere in a Changing Climate in September 2019.

The three working group contributions to the Sixth Assessment Report will be released in 2021, and the Synthesis Report, integrating all the products in this assessment cycle, will be finalized in the first half of 2022. Previously announced release dates may shift because of the COVID-19 pandemic.