It is unequivocal that anthropogenic increases in the well-mixed
greenhouse gases (WMGHGs) have substantially enhanced
the greenhouse effect, and the resulting forcing continues to
increase. Aerosols partially offset the forcing of the WMGHGs and
dominate the uncertainty associated with the total anthropogenic
driving of climate change.
As in previous IPCC assessments, AR5 uses the radiative forcing
(RF) concept, but it also introduces effective radiative forcing
(ERF). The RF concept has been used for many years and in previous
IPCC assessments for evaluating and comparing the strength of the
various mechanisms affecting the Earth’s radiation balance and thus
causing climate change. Whereas in the RF concept all surface and
tropospheric conditions are kept fixed, the ERF calculations presented
here allow all physical variables to respond to perturbations except
for those concerning the ocean and sea ice. The inclusion of these
adjustments makes ERF a better indicator of the eventual temperature
response. ERF and RF values are significantly different for anthropogenic
aerosols owing
to
their
influence
on clouds
and
on
snow cover.
These
changes to clouds are rapid adjustments and occur on a time
scale
much faster than responses of the ocean (even the upper layer) to
forcing.
RF and ERF are estimated over the Industrial Era from 1750 to
2011
if other periods are not explicitly stated.
{8.1,
Box 8.1,
Figure
8.1}
1
2
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