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)1
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. {2.2.1.1}