Climatic Research Unit : Information sheets
8: Modelling Climate Change
David Viner
GCMs
Global Climate Models also known as General Circulation Models
(GCMs) are the most complex of climate models, since they attempt
to represent the main components of the climate system in three
dimensions. GCMs are the tools used to perform climate change
experiments from which climate change scenarios (possible representations
of how the climate will evolve) can be constructed. The design
and structure of an individual GCM determines the climate change
experiments that can be performed. These characteristics are limited
by our scientific understanding of the climate system and by the
available computing resources.
The historical evolution of GCMs, computing resources, and
the nature of climate change experiments, are inextricably linked.
This evolution of the Hadley Centre models can be viewed in a
historical context through as shown in Table 1.
Table 1: Evolution of the Hadley Centre GCMs (see text for explanation of the terms)
| Model Name and Experiments |
Year |
Ocean |
Resolution
lat. x long. |
UKLO
Equilibrium 10 year integration |
1987 | Slab-ocean | 5.0 x 7.5 |
UKHI
Equilibrium 10 year integration |
1990 | Slab-ocean | 2.5x3.75 |
UKTR
Transient cold-start
Multi-decadal integrations |
1992 | 20 layer full ocean | 2.5 x 3.75 |
HadCM2
Transient warm-start
Historically forced
Multi gas
Multi-century integrations
Multi-member ensembles |
1995 | 20 layer full ocean | 2.5 x 3.75 |
HadCM3
As HadCM2 but including gas life cycle models
and early version of a biosphere model
No flux correction |
1998 | 20 layer full ocean at
1.25x1.25° resolution | 2.5 x 3.75 |
Note: For further details of the Hadley Centre's
GCMs, climate change experiments
and available data sets please see the
Climate Impacts LINK Project
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GCMs can be split into three main types:
- Atmospheric GCMs coupled with a simple slab ocean (a single
fixed layer representation of the ocean) and simple land-surface
parameterisation schemes (e.g. UKLO and UKHI).
- Atmospheric GCMs coupled to a three-dimensional representation
of the the ocean system (one in which ocean currents and heat
transport are represented) and with simple land-surface parameterisation
schemes (e.g. UKTR).
- Atmospheric GCMs coupled to a three-dimensional representation
of the ocean and a three dimensional terrestrial biosphere model
(e.g. HadCM2 and HadCM3).
Figure 1: A conceptual structure of a coupled ocean-atmosphere GCM
In order to develop a fully coupled ocean-atmosphere GCM and
run climate change experiments with it, there is a need for very
large high performance computing resources. As a result there
are only a few centres around the world with the necessary facilities.
For example, the Hadley Centre has access to two CRAY 3TE Supercomputers,
both with over 900 processors each. Coupled to this there is the
need for multi-terabyte data storage and processing facilities.
Climate Change Experiments
Climate change experiments performed with GCMs can be split into
two generic types:
Equilibrium Climate change experiments
The first generation of climate change experiments were performed
with atmosphere only GCMs. These were used to simulate the equilibrium
response of the climate system to an instantaneous increase (usually
a doubling) of the atmospheric carbon dioxide concentration. Because
of the absence of an ocean component, and because only slow computers
were available, these experiments could only be performed to simulate
short periods (e.g. 10 years) and their results could not be fixed
to calendar years.
Transient Climate change
With the advent of more powerful computers, climate change experiments
could be performed with coupled ocean-atmosphere and, more recently,
coupled ocean-atmosphere-biosphere GCMs. The most recent climate
change experiments have consisted of multi-century ensembles of
integrations which have been used to investigate the response
of the climate system to different scenarios of radiative forcing.
One of the major implications of these experiments was that results
could be fixed to calender years.
Results and Application of the results from GCM Climate Change Experiments
The results of climate change experiments have been used widely
to investigate how ecosystems and / or socio-economic systems
will respond to an evolving climate. The spatial resolution of
GCMs is relatively coarse, of the order of 2.5 latitude x 3.75
degrees longitude. This often leads to a mismatch, as the majority
of impacts assessments are carried out at resolutions of 50km
or less. To overcome this difference in scales (as indicated by
Figure 2) there is a need to construct scenarios (see information sheet 9).
Figure 2
© Copyright 2000, Climatic Research Unit.
You may copy and disseminate this information, but it remains the property
of the Climatic Research Unit, and due acknowledgement must be made.
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For further information on this subject please contact:
cru@uea.ac.uk
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Last updated: June 2000, David Viner
