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IPCC Report Finds Warming Unequivocal, Human Influence Dominant Cause

IPCC30 January 2014: The Intergovernmental Panel on Climate Change (IPCC) released the full and finalized report of Working Group I's contribution to the Fifth Assessment Report (AR5), titled ‘Climate Change 2013: The Physical Science Basis.' The report concludes that: warming of the climate system is unequivocal; human influence on the climate system has been the dominant cause; limiting climate change will require substantial and sustained emission reductions; and atmospheric greenhouse gas (GHG) concentrations, already at levels not observed in at least 800,000 years, will persist for centuries to come.

The report provides warnings about the melting of the Greenland and Antarctic ice sheets, rising oceans, and extreme weather events, such as cyclones and heat waves. More specifically, it describes it is “extremely likely,” with 95-100% certainty, that human activity, as opposed to naturally occurring phenomena, has been the primary cause of climate change, up 5% from the IPCC's 2007 Fourth Assessment Report (AR4). The report concludes, inter alia, that: heat wave frequency has “likely” increased in parts of Europe, Asia and Australia; annual mean Arctic sea ice decreased over the period 1979 to 2012; Northern Hemisphere snow cover has decreased since the mid-20th century; and global surface temperature change for the end of the 21st century is “likely” to exceed 1.5°C relative to 1850 to 1900 for most scenarios studied by the IPCC, and “likely” or “more likely than not” to exceed 2°C for some of those scenarios. The report also states that: Arctic sea ice cover will “very likely” continue to shrink and thin; Northern Hemisphere spring snow cover will decrease during the 21st century; global glacier volume will further decrease; and global mean sea level will continue to rise during the 21st century.

The report's Summary for Policymakers (SPM) was approved in September 2013 by IPCC member States in Stockholm, Sweden, and is based on the full report, which offers a comprehensive understanding of the physical science basis of climate change. Policymakers, stakeholders and the scientific community are now able to access, use and apply the detailed information contained in the report, which: includes over 1500 pages of text and more than 600 diagrams; cites over 9000 scientific publications; and consists of the SPM, a longer Technical Summary, 14 chapters and six annexes, including an Atlas of Global and Regional Climate Projections. The Atlas, included for the fist time in this assessment, contains time series and maps of temperature and precipitation projections for 35 regions of the world, which enhance accessibility for stakeholders and users. The report also includes for the first time, Thematic Focus Elements, which offer assessments of cross-cutting issues, enhancing the understanding of the climate change's physical science basis, such as water cycle change, irreversibility and abrupt change, climate sensitivity and feedbacks, climate targets and stabilization.

The report contains an explication by Thomas Stocker, Working Group I Co-Chair, who discusses what has changed in the climate system, why it has changed, and how it will change in the future. The report is available electronically, while the print version will be available at the end of March. The IPCC was established by the UN Environment Programme (UNEP) and the World Meteorological Organization (WMO) in 1988 to provide a scientific view on the current state of knowledge in climate change and its potential environmental and socioeconomic impacts [Publication (by chapter): Climate Change 2013: The Physical Science Basis] [Publication (in full): Climate Change 2013: The Physical Science Basis] [UN Press Release] [IPCC Press Release]