Figure 3: Many external
factors force climate change. 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]
