What is Climate Change?
Modelling the Global Climate
General Circulation Models
A General Circulation Model (GCM) simulates the passage of energy
through the climate system. The workings of the climate system are
represented by sequences of mathematical equations. Essentially,
these describe the earth's radiation budget, its translation into
heat and motion, and the operation of the water cycle. Values are
specified for the certain quantities such as the amount of solar
radiation reaching the top of the atmosphere and the reflectivity
of different types of surfaces. Additional equations or sub-models
may be added to account for other factors that affect the basic
climate processes.
Researchers at Environment Canada have developed one of the most
advanced GCMS being used today. Its improvements over previous models
include a more accurate simulation of cloud properties, solar heat,
ocean temperatures, and ice boundaries.
In spite of its sophistication, a GCM is still only an approximation
of reality. Even the most powerful supercomputers available today
cannot handle all the detail needed to give a complete description
of the climate system. Nor do we fully understand all of the processes
that affect climate. Consequently, the world as seen by a GCM bears
a strong resemblance to a three-dimensional map made up of Lego
pieces. All of the major features are recognizable, but much of
the fine detail is missing. The objective as the models evolve is
to fill in more of the missing detail and make the models more realistic.
What do the models tell us?
The models project an increase in global mean surface temperature
relative to 1990 of about 3-4°C by 2100. Regional temperature
changes could differ substantially from the global mean value. Average
sea level is expected to rise as a result of thermal expansion of
the oceans and melting of glaciers and ice-sheets. Models project
an increase in sea level of about 50 cm from the present to 2100.
Regional sea-level changes may differ from the global mean value
owing to land movement and ocean current changes.
All model simulations, whether they were forced with increased
concentrations of greenhouse gases and aerosols or with increased
concentrations of greenhouse gases alone, show the following features:
greater surface warming of the land than of the sea in winter; a
maximum surface warming in high northern latitudes in winter; little
surface warming over the Arctic in summer; an enhanced global mean
hydrologic cycle, and increased precipitation and soil moisture
in high latitudes in winter. Warmer temperatures will lead to a
more vigorous hydrologic cycle; this translates into prospects for
more sever droughts and/or floods in some places and less severe
droughts and floods in other places.
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