General circulation model

General circulation model

1. General circulation
Model

2. Contents:
•DEFINITION OF MODEL & GCM…
•FACTORS OF GCM…
•NWP VS CLIMATE MODELS…
•HOW CLIMATE WORKS…
•TYPES OF GCM…
•WORKING OF GCM…
•USES , EFFECTS & CONCLUSION…

3. MODEL;
Smaller representation of a larger object.
GCM;
A computer model that both identifies possible causes of
climate change & predicts climate change into Future.

4. 1;Living organism.
2;Glacier ice.
3;ENERGY FROM SUN.
4;LAND FORMS
5;HOW ALL THESE FACTORS
INTERACT.
FACTORS OF GCMFACTORS OF GCM

6. Different types of climate models
It is often convenient to regard climate models as
belonging to one of four main categories:
energy balance models (EBMs)
one dimensional radiative-convective models (RCMs);
two-dimensional statistical-dynamical models (SDMs)
three-dimensional general circulation models (GCMs).
It is not always necessary to use the most complex model.
Using a simpler model allows more runs to be carried out
as sensitivity tests to assess the accuracy of modelling
assumptions.

7. Global circulation models
GCMs “…are the only credible tools currently available for
simulating the response of the global climate system to
increasing greenhouse gas concentrations” (IPCC-TGCIA, 1999).
The first GCM was a very simple 2 layer, hemispheric, quasi-
geotrophic computer model, developed in the 1950’s by
Norman Philips.
Such early GCMs involved several atmospheric layers and a
very simple oceanic model. The model was run to equilibrium
with a set CO
2
level (such as 300ppm) and then the CO2 level
was increased.
Contemporary models are considerably more complex, and are
capable of being run in a transient mode.
They are 3-D, and may comprise thousands of individual cells

8. Contemporary GCMs: an outline
The most complex current models are known
as coupled atmospheric ocean general
circulation models (AOGCMs).
They have between 10 and 20 layers in the
atmosphere, and as many as 30 layers in the
ocean.
Contemporary AOGCMs have a horizontal
resolution of between 250km and 600km.
For local planning, this is a very coarse scale,
and the underlying topography is poorly
represented.
For a given time step, calculations are carried out for each of these
cells over the whole globe, including energy exchanges between each
of the 26 adjacent cells.
Clearly this is very computationally intensive, and it is no surprise that
atmospheric predictions have been at the forefront of computer
development since the early 1950s.

9. Climatic processes modelled in a GCM
Thermodynamic
equation
Equation of
motion
Radiation
transfer
Equation of
water vapour
Heat balanceHydrology
of the earth’s surface
DENSITY
ADVECTION A
D
V
E
C
T
I
O
N
H
E
A
T
O
F

C
O
N
D
E
N
S
A
T
IO
N
MOISTURE
H
E
A
T
I
N
G

A
N
D

C
O
O
L
I
N
G
H
E
A
T

E
N
E
R
G
Y
E
V
A
P
O
R
A
T
IO
N
S
N
O
W

C
O
V
E
R

F
E
E
D
B
A
C
K
P
R
E
C
IP
IT
A
T
IO
N

10. Flux adjustments
Some GCMs do not correctly provide a stable equilibrium
condition under current climatic conditions.
In order to ensure they accurately do so, a number of “flux
adjustments” are provided.
These are non-physical correction constants that are used as
correction factors to ensure that the models stay on track.
More recently, through intensive exploration of more exacting
physical calculations, some models have been developed that
do not flux adjustments.
Some non-flux adjusted models are now able to maintain
stable climatologies of comparable quality to flux adjusted
models.
Furthermore, there is no systematic difference between the
outputs of flux-adjusted and non-flux-adjusted models in
terms of internal climatic variability.

11. How many GCMs are there?
Considering the incredible computing power necessary to run
a full GCM, one would expect there to be only a few models.
In fact, a number of different groups have developed and
refined models over the years, and the IPCC Third Assessment
Report uses no fewer than 34 AOGCMs, some of which exist in
several refinements.
These models are developed and operated by 18 different
climatology centres, including the UK Meteorological Centre,
National Center for Atmospheric Research,
Goddard Institute for Space Studies and the
Geophysical Fluid Dynamics Laboratory.
These models are run nearly constantly, and the results are
published on the internet in order to allow planners and
response modellers ready access.

12. Use of GCMs
GCMs enable us to better understand the
processes that drive the climate. Models
that work better at describing climatic
conditions generally give us an insight into
how the various physical characteristics of
the earth are interacting.
They allow us to make informed and
scientifically defensible predictions based
on current understanding of the climate.
GCMs are thus the best tools for all climate
science, and allow conservationists,
planners and politicians to test different
response scenarios.

13. The effects of current radiative forcings
Source:
IPCC online
slide
archive

14. IPCC future scenarios
In order to predict future climate responses, the IPCC has
modelled and detailed several different scenarios (IPCC, 1992;
IPCC, 2000).
The SRES scenarios fall into four main “storyline” categories.
A1 – rapid economic growth and introduction of efficient
technologies.
-Global population peaks mid-century, then decreases.
-Global capacity building; difference in per capita income between
regions decreases.
-Three separate sub scenarios depending on energy policy:
•A1FI – fossil fuel intensive.
•A1T – fossil fuel use phased out entirely.
•A1B – balanced use of all sources ( no one dominates).

15. Development scenarios (cont).
A2 – very heterogenous world, focussed on self-reliance.
Constant population growth due to slow fertility rate change
Per capita economic and technological growth slow
Regional responses
B1 – similar population growth and global economy to scenario A1.
Rapid transition to service economies (low-impact)
Focus on provision of clean, resource efficient technology.
Global solutions to economic inequities, but no other climate
initiatives.
B2 – emphasis on local solutions to economic, social, and environmental
sustainability
Constant population growth (slower than A2).
Slower economic/social growth, focussed on a regional scale.
Focussed on environmental solutions and greater equity, but on a
regional rather than global scale.

16. Future radiative forcings depend on response
Current climate change is largely
anthropogenic in origin.
Human activities are likely to
continue to affect the climate in
a similar manner.
Consequently, the human
political and economic response
to global climate change is
essential.
The SRES scenarios demonstrate
how human response is likely to
affect global greenhouse gas
and aerosol emissions.
Source: IPCC online slides

17. GCM model responses
All GCMs are tested to ensure that
they correctly model previous
palaeoclimatological conditions to
the present day.
However, although they often
agree on general trends for a
given scenario, they may predict
moderately different responses
over time.
Consequently, climate scientists
tend to use several different
models and scenarios for any
given set of predictions or plans.
The IPCC TAR (Third Assessment Report) uses an average of as many
as 20 model predictions when stipulating future climate trends,
although as yet not all models have produced runs for all of the SRES
future trend scenarios.
Click to enlarge

18. GCM outputs for 2100 (I)
Source: IPCC
online slides
(SYR fig 3-3a &b)

19. GCM outputs for 2100 (II)

20. Linear and non linear responses
Many climatic responses to changing conditions are linear in nature
(either logarithmically through feedback mechanisms or as a flat line).
However, palaeoclimatological evidence points towards a number of
periods of extremely rapid climate change.
This is typical of non-linear systems with multiple stable equilibria (Lorenz,
1993).
When conditions are pushed towards a “threshold value”, the transition
to a new mode may be exceedingly rapid.
This has also been seen in recent changes in large scale circulation
patterns detected by instrumental readings, and in contemporary
observations of regional weather patterns. (Corti et al, 1999).

21. Examples of non-linear changes
Most GCMs show a slowing of the Atlantic Thermohaline Circulation as the world
heats up. However, some show the circulation stopping entirely as heating reaches
a threshold value. (Manabe and Stouffer, 1988).
Sea ice melting may be accelerated by feedback mechanisms.
Sea level rise may destabilise large polar ice masses, ice sheets, or even entire ice
shelves, accelerating sea level rise.
Observed variability of ENSO indicate a transition to increased occurrence of ENSO
in 1976, although not enough is know to say whether this is an anthropogenic
effect, or even if it is a long-term transition.
Large-scale (possibly irreversible) transformations in the biosphere such as the
growth of the Sahara desert (Claussen et al., 1999), have occurred even with minimal
anthropogenic interaction. These can be seen as non-linear changes triggered by
slow changes in forcing factors, and it seems highly possible that this could occur
given the current level of anthropogenic disturbance. However, not enough is
know about this incredibly complex system to say this with any degree of
certainty.

22. Conclusion
General circulation models are the best
tool we have for determining the range
and extent of climate change, as well as
for working out what is likely to happen
in the future.
All current models agree that current
climatic change is a result of
anthropogenic influences.
Future climate change will depend on the
current human response to that
knowledge.
Although GCM outputs are very large
scale, they can be refined and
downscaled to assist in prediction for
smaller areas.
Thus, the outputs from GCMs can be
exceedingly useful in terms of
conservation planning for responses to
climate change.