Geoscience Reference
In-Depth Information
subsystems: the atmosphere (the most unstable and
rapidly changing); the ocean (very sluggish in terms
of its thermal inertia and therefore important in regu-
lating atmospheric variations); the snow and ice cover
(the cryosphere); and the land surface with its vegeta-
tion 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. The terrestrial biosphere influences
the incoming radiation and outgoing re-radiation
and, through human transformation of the land cover,
especially deforestation and agriculture, affects the
atmospheric composition via greenhouse gases. In the
oceans, marine biota play a major role in the dissolu-
tion and storage of CO
2
. All subsystems are linked by
fluxes of mass, heat and momentum into a very complex
whole.
The driving mechanisms of climate change referred
to as 'climate forcing' can be divided conveniently into
external (astronomical effects on incoming short-wave
solar radiation) and internal (e.g. alterations in the
composition of the atmosphere which affect outgoing
long-wave radiation). Direct solar radiation measure-
ments 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 unclear. However, observed
increases in the greenhouse gas content of the atmos-
phere (0.1 per cent of which is composed of the trace
gases carbon dioxide, methane, nitrous oxide and
ozone), due to the recent intensification of a wide range
of human activities, appear to have been very significant
in increasing the proportion of terrestrial long-wave
radiation trapped by the atmosphere, thereby raising its
temperature. These changes, although small, appear
to have had a significant thermal effect on the global
climate system in the twentieth century. The imbalance
between incoming solar radiation and outgoing terres-
trial radiation is termed 'forcing'. Positive forcing
implies a heating up of the system, and adjustments
to such imbalance take place in a matter of months
in the surface and tropospheric subsystems but are
slower (centuries or longer) in the oceans. The major
greenhouse gas is water vapour and the effect of changes
in this, together with that of cloudiness, are as yet poorly
understood.
The natural variability of the global climate system
depends not only on the variations in external solar
forcing but also on two features of the system itself -
feedback and non-linear behaviour. Major feedbacks
involve the role of snow and ice reflecting incoming
solar radiation and atmospheric water vapour absorbing
terrestrial re-radiation, and are positive in character. For
example: the earth warms; atmospheric water vapour
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 role by reflecting
solar (short-wave 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. A further source of variability within
the climate system stems from changes in atmospheric
composition resulting from human action. These have
to do with increases in the greenhouse gases, which
lead to an increase in global temperatures, and increases
in particulate matter (carbon and mineral dust, aerosols).
Particulates, including volcanic aerosols, which enter
the stratosphere, have a more complex influence on
global climate. Some are responsible for heating the
atmosphere and others for cooling it.
Recent attempts to understand the global climate
system have been aided greatly by the development of
numerical models of the atmosphere and of climate
systems since the 1960s. These are essential to deal with
non-linear processes (i.e. those which do not exhibit
simple proportional relationships between cause and
effect) and operate on many different timescales.
The first edition of this topic appeared some thirty-
five years ago, before many of the advances described
in the latest editions were even conceived. However,
our continuous aim in writing it is to provide a non-
technical account of how the atmosphere works, thereby
helping the understanding of both weather phenomena
and global climates. As always, greater explanation
inevitably results in an increase in the range of phe-
nomena requiring explanation. That is our only excuse
for the increased size of this eighth edition.
