Figure
2-2: The influence of external factors on climate can be broadly compared using
the concept of radiative forcing. 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. Radiative forcing continues to be a useful tool to estimate,
to a first order, the relative climate impacts such as the relative global mean
surface temperature response due to radiatively induced perturbations, but these
global mean forcing estimates are not necessarily indicators of the detailed aspects
of the potential climate responses (e.g., regional climate change).
