Climate and Climate-System Modelling - Environmental Modelling: Finding Simplicity in Complexity - page 146

9

Climate and Climate-System
Modelling

L.D. Danny Harvey
Department of Geography and Planning, University of Toronto, Canada

in the upper ocean affect the reflectivity of the ocean sur-
face to sunlight. Both the marine and terrestrial biosphere
play an important role in determining the atmospheric
concentration of a number of climatically important trace
gases, the most important being carbon dioxide (CO 2 )
and methane (CH 4 ). Finally, the Earth's crust influences
the climate through the influence of land topography
on winds and the distribution of rain, through the role
of the ocean-continent configuration and the shape of
the ocean basin on ocean currents, and through the role
of chemical weathering at the Earth's surface and the
deposition of marine carbonates inmodulating the atmo-
spheric concentration of CO 2 at geological time scales.
Even this thumbnail sketch of the climate system leaves
out many important considerations. The occurrence of
clouds, which are part of the atmosphere, is strongly
influenced by conditions at the land surface. Their optical
properties are influenced in part by micro-organisms in
the upper few metres of the ocean, which emit sulphur
compounds that ultimately become cloud-condensation
nuclei. The occurrence of clouds dramatically affects the
flow of both solar and infrared radiation between the
atmosphere, land surface and space. Many chemical reac-
tions occur in the atmosphere and they determine the
concentrations of climatically important trace gases and
aerosols; many of the important reactants in atmospheric
chemistry are released from the terrestrial or marine bio-
sphere. Cloud droplets serve as important sites for many
important chemical reactions, and the optical properties
of clouds are themselves influenced to some extent by the
chemical reactions that they modulate. A more thorough

9.1 The complexity

Climate and climatic change are particularly difficult to
model because of the large number of individual compo-
nents involved in the climate system, the large number of
processes occurringwithin each component, and themul-
tiplicity of interactions between components. The climate
system consists of the atmosphere, oceans, cryosphere
(glaciers, ice caps, sea ice and seasonal snow cover), bio-
sphere, and lithosphere (the Earth's crust). All of these
components affect, and are affected by, the other com-
ponents, so they form a single system. For example, the
atmosphere and oceans influence each other through the
exchange of momentum (by winds), heat and moisture.
The growth and decay of glaciers, and the formation
of sea ice and land-snow cover, depend on atmospheric
temperature and the supply of moisture, but ice and snow
surfaces in turn influence the absorption of solar energy
and hence influence temperature, and modulate the flow
of water vapour and heat between the land or ocean
and the atmosphere. Continental-scale ice sheets, such as
have periodically occupied most of Canada and north-
ern Europe, reflect sunlight, deflect winds and depress
the crust, leading to lower ice-sheet elevations that con-
tribute to their periodic collapse. Meltwater during the
rapid demise of ice sheets is likely to have altered ocean cir-
culation. The biosphere is affected by atmospheric and/or
oceanic conditions but also influences climate through the
effect of land vegetation on surface roughness and hence
on winds, and through its effect on evaporation and the
reflection or absorption of solar energy. Micro-organisms