The Fifth Assessment Report (AR5) of the United Nations Intergovernmental Panel on Climate Change (IPCC) is the fifth in a series of such reports. The IPCC was established in 1988 by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) to assess scientific, technical and socio-economic information concerning climate change, its potential effects and options for adaptation and mitigation.
The Fifth Assessment Report was finalized in 2014. As had been the case in the past, the outline of the AR5 was developed through a scoping process which involved climate change experts from all relevant disciplines and users of IPCC reports, in particular representatives from governments. Governments and organizations involved in the Fourth Report were asked to submit comments and observations in writing with the submissions analysed by the panel. The report was delivered in stages, starting with Working Group I's report on the physical science basis, based on 9,200 peer-reviewed studies. The summaries for policy makers were released on 27 September 2013 for the first report, on 31 March 2014 for the second report entitled "Impacts, Adaptation, and Vulnerability", and on 14 April 2014 for the third report entitled "Mitigation of Climate Change". The Synthesis Report was released on 2 November 2014, in time to pave the way for negotiations on reducing carbon emissions at the UN Climate Change Conference in Paris during late 2015.
The Fifth Assessment Report (AR5) consists of three Working Group (WG) Reports and a Synthesis Report. The first Working Group Report was published in 2013 and the rest were completed in 2014.
More than 800 authors, selected from around 3,000 nominations, were involved in writing the report. Lead authors' meetings and a number of workshops and expert meetings, in support of the assessment process, were held. A schedule of AR5 related meetings, review periods, and other important dates was published.
On 14 December 2012, drafts of the Working Group 1 (WG1) report were leaked and posted on the Internet. The release of the summary for policymakers occurred on 27 September 2013. Halldór Thorgeirsson, a UN official, warned that, because big companies are known to fund the undermining of climate science, scientists should be prepared for an increase in negative publicity at the time. "Vested interests are paying for the discrediting of scientists all the time. We need to be ready for that," he said.
Marking the finalization of the Physical Science Basis UN Secretary General Ban Ki-moon addressed the IPCC at Stockholm on 27 September 2013. He stated that "the heat is on. We must act". Jennifer Morgan, from the World Resources Institute, said "Hopefully the IPCC will inspire leadership, from the Mom to the business leader, to the mayor to the head of state." US Secretary of State John Kerry responded to the report saying "This is yet another wakeup call: those who deny the science or choose excuses over action are playing with fire."
In March 2010, the IPCC received approximately 3,000 author nominations from experts around the world. At the bureau session held in Geneva, 19-20 May 2010, the three working groups presented their selected authors and review editors for the AR5. Each of the selected scientists, specialists and experts was nominated in accordance with IPCC procedures, by respective national IPCC focal-points, by approved observer organizations, or by the bureau. The IPCC received 50% more nominations of experts to participate in AR5 than it did for AR4. A total of 559 authors and review editors had been selected for AR4 from 2,000 proposed nominees. On 23 June 2010 the IPCC announced the release of the final list of selected coordinating lead authors, comprising 831 experts who were drawn from fields including meteorology, physics, oceanography, statistics, engineering, ecology, social sciences and economics. In comparison to the Fourth Assessment Report (AR4), participation from developing countries was increased, reflecting the ongoing efforts to improve regional coverage in the AR5. About 30% of authors came from developing countries or economies in transition. More than 60% of the experts chosen were new to the IPCC process, bringing fresh knowledge and perspectives.
On 23 June 2010, the IPCC announced the release of the final list of selected coordinating lead authors, comprising 831 experts. The working group reports would be published during 2013 and 2014. These experts would also provide contributions to the Synthesis Report published in late 2014.
The Fifth Assessment Report (Climate Change 2013) would be released in four distinct sections:
The full text of Climate Change 2013: The Physical Science Basis was released in an unedited form on Monday, 30 September 2013. It was over 2,000 pages long and cited 9,200 scientific publications. The full, edited report was released online in January 2014 and published in physical form by Cambridge University Press later in the year.
A concise overview of Working Group I's findings was published as the Summary for Policymakers on 27 September 2013. The level of confidence in each finding was rated on a confidence scale, qualitatively from very low to very high and, where possible, quantitatively from exceptionally unlikely to virtually certain (determined based on statistical analysis and expert judgement).
|Term||Likelihood of the outcome|
|Virtually certain||99-100 % probability|
|Extremely likely||95-100 % probability|
|Very likely||90-100 % probability|
|Likely||66-100 % probability|
|More likely than not||50-100 % probability|
|About as likely as not||33 to 66% probability|
|Unlikely||0-33 % probability|
|Very unlikely||0-10 % probability|
|Extremely unlikely||0-5 % probability|
|Exceptionally unlikely||0-1 % probability|
The principal findings were:
AR5 relies on the Coupled Model Intercomparison Project Phase 5 (CMIP5), an international effort among the climate modeling community to coordinate climate change experiments. Most of the CMIP5 and Earth System Model (ESM) simulations for AR5 WRI were performed with prescribed concentrations reaching 421 ppm (RCP2.6), 538 ppm (RCP4.5), 670 ppm (RCP6.0), and 936 ppm (RCP 8.5) by the year 2100. (IPCC AR5 WGI, page 22).
The summary also detailed the range of forecasts for warming, and climate impacts with different emission scenarios. Compared to the previous report, the lower bounds for the sensitivity of the climate system to emissions were slightly lowered, though the projections for global mean temperature rise (compared to pre-industrial levels) by 2100 exceeded 1.5 °C in all scenarios.
In August 2020 scientists reported that observed ice-sheet losses in Greenland and Antarctica track worst case scenarios of the IPCC Fifth Assessment Report's sea-level rise projections.
Climate model simulations in support of AR5 use a different approach to account for increasing greenhouse gas concentrations than in the previous report. Instead of the scenarios from the Special Report on Emissions Scenarios the models are performing simulations for various Representative Concentration Pathways.
Public debate after the publication of AR4 in 2009 put the IPCC under scrutiny, with controversies over alleged bias and inaccuracy in its reports. In 2010, this prompted U.N. Secretary-General Ban Ki-moon and IPCC chair Rajendra K. Pachauri to request that the InterAcademy Council (IAC) review the IPCC and recommend ways to strengthen its processes and procedures for the preparation of AR5. The IAC report made recommendations to fortify IPCC's management structure, to further develop its conflict-of-interest policy, to strengthen the review process, to clarify the guidelines on the use of so-called gray literature, to ensure consistency in the use of probabilities for the likelihood of outcomes, and to improve its communications strategy especially regarding transparency and rapidity of response.
The Climate and Development Knowledge Network has produced region-specific toolkits for policy makers, practitioners, journalists and teachers based on the findings in the Fifth Assessment Report. They include summary reports which distill the key findings of the IPCC report; as well as media materials such as infographics, slideshow presentations and images which can be used for training, educational and reporting purposes. The four toolkits that have been developed are:
The IPCC published their "Special Report on Global Warming of 1.5 °C" on October 8, 2018.