Geoscience Reference
In-Depth Information
temperature and moisture, lake and peat bog
sediments that contain pollen records of regional
vegetation, reconstructed temperature records
from oxygen isotope ratios in cave stalagmite and
annual growth rings in ocean corals.
Major advances have been made in paleo-
climate reconstruction through the use of general
circulation models with past boundary conditions
(paleogeography, paleovegetation) and changed
earth orbital characteristics.
humans into the system has added a new
dimension. Indeed, by the dawn of the twenty-first
century, overwhelming evidence had amassed of
a discernible and growing human impact upon
global climate.
The driving mechanism of global climate
change is referred to as 'radiative forcing'. In an
equilibrium climate state, globally averaged solar
energy absorbed by the earth system is balanced
by the globally averaged longwave radiation
emitted to space. In other words, there is radiative
equilibrium at the top of the atmosphere. An
imbalance, or radiative forcing, is defined as
positive when less energy is emitted than absorbed
and negative for the reverse. In response to
radiative forcing, the system tries to come back
to a new equilibrium, attended by, respectively
warming or cooling at the surface. Radiation
imbalances arise from both natural processes
(e.g., astronomical effects on incoming shortwave
solar radiation, changes in total solar output and
volcanic eruptions, the latter of which load the
atmosphere with aerosols, tiny particles suspended
in the air) and human influences (e.g., changes in
greenhouse gas and aerosol concentrations owing
to fossil fuel burning and a suite of other activities,
such as deforestation and agriculture). Direct solar
radiation measurements have been made via
satellites since about 1980, but the correlation
between small changes in solar radiation and in the
thermal economy of the global climate system is
still somewhat unclear. However, observed
human-induced increases in the greenhouse gas
content of the atmosphere (0.1 percent of which
is composed of the trace gases carbon dioxide,
methane, nitrous oxide and ozone) appear to have
been very significant in increasing the proportion
of terrestrial longwave radiation trapped by the
atmosphere (a positive radiative forcing), thereby
raising surface air temperature over the past
100 years.
Adjustments to a radiative forcing take place in
a matter of months in the surface and tropospheric
subsystems but are slower (centuries or longer) in
the ocean. In turn, the amount of surface warming
I THE GLOBAL CLIMATE
SYSTEM
Undoubtedly the most important outcome of
work in the second half of the twentieth century
was the recognition of the existence of the global
climate system (see
Box 1.1
)
. The climate system
involves not just the atmosphere elements, but
the five major subsystems: the atmosphere (the
most unstable and rapidly changing); the ocean
(very sluggish in terms of its thermal inertia and
therefore important in regulating atmospheric
variations); the snow and ice cover (the cryo-
sphere); and the land surface with its vegetation
cover (the lithosphere and biosphere). Physical,
chemical and biological processes take place in
and among these complex subsystems. The most
important interaction takes place between the
highly dynamic atmosphere, through which solar
energy is input into the system, and the oceans
which store and transport large amounts of energy
(especially thermal), thereby acting as a regulator
to more rapid atmospheric changes. A further
complication is provided by the living matter of
the biosphere, which influences the incoming
radiation and outgoing re-radiation and affects
the atmospheric composition via greenhouse
gases. In the oceans, marine biota play a major
role in the dissolution and storage of CO
2
. All
subsystems are linked by fluxes of mass, heat
and momentum into a very complex whole. The
coupled climate system always has and always will
be characterized by variability on numerous time
and space scales. However, the introduction of
