The IPCC Fifth Assessment Physical Sciences Working Group Summary for Policymakers

By William Collins and Robert Dickinson

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The Physical Sciences working group of the IPCC Fifth Assessment released their Summary for Policymakers on September 27, 2013.  This white paper provides an overview of its findings, including the evidence for climate change to date, projections for climate change during the rest of the 21st century and an estimate of how much more carbon can be released into the atmosphere without triggering what the international community considers “dangerous” climate change.

On September 27th, 2013 the IPPC released the Summary for Policymakers (SPM) of the report by Working Group I (The Physical Science Basis), the first part of its Fifth Assessment (AR5). The full WGI report was released on September 30th, the following Monday. The WGII report on impacts and the WGIII report on mitigation will be released in March and April of next year, respectively, and the Synthesis Report in October. Both the SPM and the full WGI report are available at There is also a Technical Summary that serves as a bridge between the SPM and the full report. It is referenced in the captions for the figures below and can also be found on the IPCC website.

AR5 reflects the most current scientific understanding of climate change, its implications and what can be done about it. It is based on observations, analysis and modeling conducted since the last Assessment, AR4, that was released in 2007. As we have discussed in previous issues, the climate simulations that were done in preparation for AR5 are collected in the CMIP5 archive.

WGI consisted of 259 scientists from 39 countries, who began working on the report in November 2010. The final product of their efforts comprises more than 2000 pages with 1250 figures. The following is an overview of some of the major findings in the SPM. Each of the findings in the report is characterized by the level of confidence and, where possible, the likelihood, calculated from observations or from model outputs. Where either the confidence level or the likelihood stated in the SPM is mentioned below, it will be italicized. All of the figures shown below are taken directly from the company, Hewlett Packard. Following wartime work at MIT, Terman returned to Stanford after the war as its engineering dean and played a key role nurturing a growing cluster of high technology businesses in Palo Alto and nearby towns.