Geoscience Reference
In-Depth Information
to a given radiative forcing (termed climate
sensitivity) depends critically on feedbacks that
amplify or dampen the climate response to the
forcing. In the case of greenhouse gases, the issue
is further complicated in that the radiative forcing
is itself changing. Major feedbacks involve the role
of snow and ice reflecting incoming solar radiation
and atmospheric water vapor absorbing terrestrial
re-radiation, and are positive in character. For
example: the earth warms; atmospheric water
vapor increases; this, in turn, increases the
greenhouse effect; the result being that the earth
warms further. Similar warming occurs as higher
temperatures reduce snow and ice cover allowing
the land or ocean to absorb more radiation.
Clouds play a more complex and still incompletely
understood role by reflecting solar (shortwave
radiation) but also by trapping terrestrial outgoing
radiation. Negative feedback, when the effect of
change is damped down, is a much less important
feature of the operation of the climate system,
which partly explains the tendency to recent global
warming. The impact of aerosols is one of the
biggest areas of uncertainty. While the cooling
effect of aerosols through scattering solar radiation
back to space is well known, and in part masks the
warming effect of greenhouse gases, some aerosols,
such as soot, absorb solar radiation. Aerosols also
affect the number and density of cloud droplets,
changing the optical properties of clouds.
An important factor in weather and climate
processes is unpredictabiliy. Weather systems
display sensitivity to their initial conditions,
meaning that a very small change in the initial
state of a weather system can have a large,
disproportionate effect on the whole system. This
was first recognized by E. Lorenz (1963), who
pointed out that a butterfly flapping its wings
in Beijing could affect the weather thousands of
miles away some days later. This sensitivity is now
called the 'butterfly effect'. It is addressed in
numerical model experiments by running many
simulations with minute variations in the initial
conditions and then examining the results of an
ensemble of projections.
The Intergovernmental Panel on Climate
Change (IPCC), jointly established in 1988 by the
WMO and the United Nations Environmental
Programme (UNEP), has served as a focal point for
climate change research, and released its Fourth
Assessment Report in 2007. One of the most
important tools of the IPCC is numerical models
of the climate system. Since the initial development
of atmospheric general circulation models (GCMs)
in the 1960s, the current models have become
very sophisticated, and are essential for untangling
the complexities of radiative forcing, feedbacks
and climate response. They now incorporate
coupled ocean, land and biosphere submodels.
The emerging picture that these models paint is of
a much wamer and different world by the end of
this century, posing challenges for society that
include, but are not limited to, higher sea levels and
shifts in agricultural zones. Major uncertainties
nevertheless remain, particularly of climate change
on regional scales.
The first edition of
Atmosphere, Weather and
Climate
appeared in 1968 before many of the
advances described in later editions were even
conceived. However, our continuous aim in
writing it is to provide a largely non-technical
account of how the atmosphere works, thereby
helping the understanding of both weather
phenomena and of global climates. As noted in the
eighth edition, greater explanation inevitably
results in an increase in the range of phenomena
requiring explanation. As a result, this topic
continues to thicken with time.
•
How have technological advances
contributed to the evolution of meteo-
rology and climatology?
•
Consider the relative contributions
of observation, theory, and modeling
to our knowledge of atmospheric
processes.
