IPCC Fifth Assessment Report

The Fifth Assessment Report (AR5) of the United Nations Intergovernmental Panel on Climate Change (IPCC) is the fifth in a series of such reports and was completed in 2014.^[1] 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.^[2][3] Projections in AR5 are based on "Representative Concentration Pathways" (RCPs).^[4] The RCPs are consistent with a wide range of possible changes in future anthropogenic greenhouse gas emissions. Projected changes in global mean surface temperature and sea level are given in the main RCP article.

The IPCC Fifth Assessment Report followed the same general format as the Fourth Assessment Report, with three Working Group reports and a Synthesis report.^[5][1] The report was delivered in stages, starting with the report from Working Group I in September 2013.^[5] It reported on the physical science basis, based on 9,200 peer-reviewed studies.^[6][7] The Synthesis Report was released on 2 November 2014,^[8] in time to pave the way for negotiations on reducing carbon emissions at the UN Climate Change Conference in Paris during late 2015.

The report's Summary for Policymakers stated that warming of the climate system is 'unequivocal' with changes unprecedented over decades to millennia, including warming of the atmosphere and oceans, loss of snow and ice, and sea level rise. Greenhouse gas emissions, driven largely by economic and population growth, have led to greenhouse gas concentrations that are unprecedented in at least the last 800,000 years. These, together with other anthropogenic drivers, are "extremely likely" (where that means more than 95% probability) to have been the dominant cause of the observed global warming since the mid-20th century.^[9]

Conclusions of the fifth assessment report are summarized below:

• Working Group I: "Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia".^[10] "Atmospheric concentrations of carbon dioxide, methane, and nitrous oxide have increased to levels unprecedented in at least the last 800,000 years".^[11] Human influence on the climate system is clear.^[12] It is extremely likely (95–100% probability)^[13] that human influence was the dominant cause of global warming between 1951 and 2010.^[12]
• Working Group II: "Increasing magnitudes of [global] warming increase the likelihood of severe, pervasive, and irreversible impacts".^[14] "A first step towards adaptation to future climate change is reducing vulnerability and exposure to present climate variability".^[15] "The overall risks of climate change impacts can be reduced by limiting the rate and magnitude of climate change"^[14]
• Working Group III: Without new policies to mitigate climate change, projections suggest an increase in global mean temperature in 2100 of 3.7 to 4.8 °C, relative to pre-industrial levels (median values; the range is 2.5 to 7.8 °C including climate uncertainty).^[16] "(T)he current trajectory of global annual and cumulative emissions of GHGs is not consistent with widely discussed goals of limiting global warming at 1.5 to 2 degrees Celsius above the pre-industrial level."^[17] Pledges made as part of the Cancún Agreements are broadly consistent with cost-effective scenarios that give a "likely" chance (66–100% probability) of limiting global warming (in 2100) to below 3 °C, relative to pre-industrial levels.^[18]