These radiative forcings
arise from changes
in the atmospheric
composition, alteration
of surface reflectance
by land use, and variation
in the output of the
sun. Except for solar
variation, some form
of human activity
is linked to each.
The rectangular bars
represent estimates
of the contributions
of these forcings
- some of which yield
warming, and some
cooling.
Forcing due to episodic
volcanic events, which
lead to a negative
forcing lasting only
for a few years, is
not shown. The indirect
effect of aerosols
shown is their effect
on the size and number
of cloud droplets.
A second indirect
effect of aerosols
on clouds, namely
their effect on cloud
lifetime, which would
also lead to a negative
forcing, is not shown.
Effects of aviation
on greenhouse gases
are included in the
individual bars.
The vertical line
about the rectangular
bars indicates a range
of estimates, guided
by the spread in the
published values of
the forcings and physical
understanding. Some
of the forcings possess
a much greater degree
of certainty than
others. A vertical
line without a rectangular
bar denotes a forcing
for which no best
estimate can be given
owing to large uncertainties.
The overall level
of scientific understanding
for each forcing varies
considerably, as noted.
Some of the radiative
forcing agents are
well mixed over the
globe, such as CO2,
thereby perturbing
the global heat balance.
Others represent perturbations
with stronger regional
signatures because
of their spatial distribution,
such as aerosols.
For this and other
reasons, a simple
sum of the positive
and negative bars
cannot be expected
to yield the net effect
on the climate
system.
The simulations of
this assessment report
(for example, Figure
5) indicate that the
estimated net effect
of these perturbations
is to have warmed
the global climate
since 1750.
[Based
upon Chapter
6, Figure
6.6]"
Source
& © :
IPCC TAR SPM of WG
I
