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). The principal findings were:[5]
- General
- Warming of the atmosphere and ocean system is unequivocal. Many of the associated impacts such as sea level change (among other metrics) have occurred since 1950 at rates unprecedented in the historical record.
- There is a clear human influence on the climate
- It is extremely likely that human influence has been the dominant cause of observed warming since 1950, with the level of confidence having increased since the fourth report.
- Historical climate metrics
- It is likely (with medium confidence) that 1983—2013 was the warmest 30-year period for 1400 years.
- It is virtually certain the upper ocean warmed from 1971 to 2010. This ocean warming accounts, with high confidence, for 90% of the energy accumulation between 1971 and 2010.
- It can be said with high confidence that the Greenland and Antarctic ice sheets have been losing mass in the last two decades and that Arctic sea ice and Northern Hemisphere spring snow cover have continued to decrease in extent.
- There is high confidence that the sea level rise since the middle of the 19th century has been larger than the mean sea level rise of the prior two millennia.
- Concentration of greenhouse gases in the atmosphere have increased to levels unprecedented on earth in 800,000 years.
- Total radiative forcing of the earth system, relative to 1750, is positive and the most significant driver is the increase in CO2's atmospheric concentration.
- Models
- Climate models have improved since the prior report.
- Model results, along with observations, provide confidence in the magnitude of global warming in response to past and future forcing.
- Predictions
- 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 in all scenarios exceeded 1.5 °C.[11][12]
From UN climate change report http://en.wikipedia.org/wiki/IPCC_Fifth_Assessment_Report