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]
