Climate Change 2013: The Physical Science Basis (Working Group I Contribution to the IPCC 5th Assessment Report)

One of the main activities of the United Nation's Intergovernmental Panel on Climate Change (IPCC) is the preparation of comprehensive Assessment Reports about the state of scientific, technical and socio-economic knowledge on climate change, its causes, potential impacts and response strategies. Since its inception in 1988 the IPCC has prepared four multi-volume assessment reports and is in the process of finalizing the Fifth Assessment Report (AR5). AR5 is being released in four parts between September 2013 and November 2014. It will be the most comprehensive assessment of scientific knowledge on climate change since 2007 when Fourth Assessment Report (AR4) was released.

AR5 is made up of the full reports prepared by the Working Groups (I, II and III) and their Summaries for Policymakers as well as the Synthesis Report. The Working Group I (WGI) contribution on the physical science basis of climate change has been finalized, and both the Summary for Policymakers and unedited accepted final draft of the Full Report are available. 

This new assessment by the IPCC WGI concludes that human influence on the climate system is clear, and has been the dominant cause of the observed warming since the mid- 20th century.  According to the report, warming in the climate system is unequivocal and since 1950 many changes have been observed throughout the climate system that are unprecedented over decades to millennia, and each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850.

Projections of climate change are based on a new set of four scenarios of future greenhouse gas concentrations and aerosols, spanning a wide range of possible futures. The Working Group I report assessed global and regional-scale climate change for the early, mid- , and later 21st century. 

Some of the prominent predictions in the report are: 

  • Further warming will continue if emissions of greenhouse gases continue.
  • The global surface temperature increase by the end of the 21st century is likely to exceed 1.5°C relative to the 1850 to 1900 period for most scenarios, and is likely to exceed 2.0 °C for many scenarios
  • The global water cycle will change, with increases in disparity between wet and dry regions, as well as wet and dry seasons, with some regional exceptions.
  • The oceans will continue to warm, with heat extending to the deep ocean, affecting circulation patterns.
  • Decreases are very likely in Arctic sea ice cover, Northern Hemisphere spring snow cover, and global glacier volume.
  • Global mean sea level will continue to rise at a rate very likely to exceed the rate of the past four decades
  • Changes in climate will cause an increase in the rate of CO2 production. Increased uptake by the oceans will increase the acidification of the oceans.
  • Future surface temperatures will be largely determined by cumulative CO2, which means climate change will continue even if CO2 emissions are stopped.

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.

The Twelfth Session of Working Group I (WGI-12) was held from 23 to 26 September 2013 in Stockholm, Sweden. At the Session, the Summary for Policymakers (SPM) of the Working Group I contribution to the IPCC Fifth Assessment Report was approved and the underlying scientific and technical assessment accepted. 

Publication Date: September 27, 2013

Related Organizations:

  • The Intergovernmental Panel on Climate Change (IPCC)

Resource Category:

Resource Types:

  • Assessment

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