The net forcing by WMGHGs other than CO
shows a small
increase since the AR4 estimate for the year 2005. A small growth
in the CH
4
2
concentration has increased its RF by 2% to an AR5 value
of 0.48 (0.43 to 0.53) W m
–2
. RF of nitrous oxide (N
O) has increased
by 6% since AR4 and is now 0.17 (0.14 to 0.20) W m
2
–2
. N
O concentrations
continue to rise while those of dichlorodifluoromethane (CFC12),
the
third
largest
WMGHG
contributor
to
RF
for
several
decades,
is
falling
due to its phase-out under the Montreal Protocol and amendments.
Since
2011
N
O has become the third largest WMGHG contributor
to RF.
The
RF from all halocarbons (0.36 W m
2
) is very similar to
the value in AR4, with a reduced RF from chlorofluorocarbons (CFCs)
but increases from many of their substitutes. Four of the halocarbons
(trichlorofluoromethane (CFC-11), CFC-12, trichlorotrifluoroethane
(CFC-113) and chlorodifluoromethane (HCFC-22)) account for around
85% of the total halocarbon RF. The first three of these compounds
have declining RF over the last 5 years but their combined decrease
is compensated for by the increased RF from HCFC-22. Since AR4, the
RF from all HFCs has nearly doubled but still only amounts to 0.02
W m
–2
. There is high confidence
4
–2
that the overall growth rate in RF
from all WMGHG is smaller over the last decade than in the 1970s and
1980s owing to a reduced rate of increase in the combined non-CO
RF. {8.3.2; Figure 8.6}
2
2