Executive Summary

The evidence of climate change from observations of the atmosphere and surface has grown significantly during recent years. At the same time new improved ways of characterizing and quantifying uncertainty have highlighted the challenges that remain for developing long-term global and regional climate quality data records. Currently, the observations of the atmosphere and surface indicate the following changes:

Atmospheric Composition

It is certain that atmospheric burdens of the well-mixed greenhouse gases (GHGs) targeted by the Kyoto Protocol increased from 2005 to 2011. The atmospheric abundance of carbon dioxide (CO2) was 390.5 ppm (390.3 to 390.7) in 2011; this is 40% greater than in 1750. Atmospheric nitrous oxide (N2O) was 324.2 ppb (324.0 to 324.4) in 2011 and has increased by 20% since 1750. Average annual increases in CO2 and N2O from 2005 to 2011 are comparable to those observed from 1996 to 2005. Atmospheric methane (CH4) was 1803.2 ppb (1801.2 to 1805.2) in 2011; this is 150% greater than before 1750. CH4 began increasing in 2007 after remaining nearly constant from 1999 to 2006. Hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulphur hexafluoride (SF6) all continue to increase relatively rapidly, but their contributions to radiative forcing are less than 1% of the total by well-mixed GHGs.