Download Global Circulation Models – the basis for climate change science

Global Circulation Models – the basis
for climate change science
Presented by
James Reeler
UWC
Weather prediction
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Vilhelm Bierknes claimed we can
predict weather by calculations.
7 equations that predicted "largescale atmospheric motions.".
Weather processes were too
complex for calculation.
Computers became essential for
this process.
NWP (National Weather prediction)
first carried out by the Royal
Swedish Air Force Weather Service
in Stockholm (December 1954).
Forecasts were three times a week.
NWP was soon available in most
western countries.
NWP vs climate models
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NWPs are designed to predict regional weather conditions in
the short (1-3 days) and medium (4-10 day) term.
Climate models are derived from these models, but are
designed to predict weather conditions years into the future.
Given the moderate accuracy of models in the short term, how
is it feasible to predict weather conditions so far into the
future?
The answer is: STATISTICS.
Climate models are not designed to give an accurate forecast
on a daily basis, but rather to predict means and variability in
climatic indicators – to give a statistically accurate picture of
CLIMATIC, not WEATHER conditions.
NWP vs climate models (cont)
Contrasts
NWP
Goal
to predict weather
Spatial coverage
regional or global
Temporal range
days
Spatial resolution
variable (20-100km)
Relevance of:
Initial conditions
high
Clouds/radiation
low
Surface (land/ocean/ice) low
Ocean dynamics
low
Model stability
low
Time dimension
essential
Climate models
to predict climate
global
years
usually coarse
low
high
high
high
high
ignored
How does the climate work?
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The global climate system is a result of the link of atmosphere,
oceans, the ice sheets (cryosphere), living organisms
(biosphere) and the soils, sediments and rocks (geosphere),
each of which will be considered in greater detail after this.
Each of these systems is integrally connected to the others,
and energy exchanges between and within systems, as well as
other interactions (such as the provision of nuclei for rain
droplet formation) determine climatic condtions.
However, despite the interconnectedness, an explanation must
clearly focus on aspects separately, and the linkages between
these systems.
Climate models allow us to study these aspects of the systems
independently.
GCMs stitch these individual models together through a
process of linkages, the development of which has taken many
years and a great degree of understanding of the climate
The atmosphere I: vertical structure
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The lowest level of the atmosphere
(the troposphere) is where the
majority of weather processes take
place.
It contains 75% of the gases and
almost all water vapour and aerosols.
(Barry & Chorley,1992)
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The tropopause marks the upper limit
of the troposphere.
Temperature change is due to
absorption of UV by the ozone layer.
Consequently very stable.
The atmosphere above this level is
mostly irrelevant in terms of weather.
The action and feedbacks associated
with clouds are still poorly
understood, and only recently have
models begun to incorporate cloud
cover in any comprehensive detail.
Source: Barry & Chorley, 1992
The atmosphere II: energy budget
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1368Wm-2 of incoming radiation hits
the top of the atmosphere.
A black body would reflect all the
radiation, although some would be
absorbed and re-radiated as longer
wavelengths (dotted lines).
However, atmospheric gases absorb
some of the radiation, reducing the
radiated energy (grey area).
Atmospheric gases/aerosols also
scatter incoming radiation.
Although 30% of incoming energy is
reflected, little of the remainder
escapes directly.
The atmosphere consequently heats
up (greenhouse effect).
Source: IPCC Third
Assessment Report
The atmosphere III: Horizontal transfers
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Because of the earth’s curvature,
more radiation falls in equatorial
regions than at the poles.
(Trewartha & Horn, 1980).
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To restore equilibrium, an
interchange of heat from tropics to
poles occurs through movement
of air masses. (Barry & Chorley, 1992)
This latitudinal transfer of energy
occurs in several ways:
- movement of sensible heat
- movement of latent heat
- ocean circulation
Source: NASA
For each packet of air that moves polewards, a similar quantity moves
towards the tropics, setting up circulation cells (also affected by the
coriolis forces of the earth’s rotation).
These energy fluxes are the principal components of the climate –
therefore actions which interfere with the fluxes necessarily affect the
climate.
The oceans
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It is divided into two distinct layers:
- The upper, seasonal layer of warm
mixed water that stretches up to 100m
deep in the tropics, and interacts with
the atmosphere.
- The lower deeps, which contain more
than 80% of the water in the oceans.
The ocean holds more energy than the
atmosphere because:
- Heat capacity is 4.2 times higher
- Density is 1000 times higher.
Heat is transferred to the atmosphere by evaporation of water vapour,
which passes on its energy to the atmosphere when it condenses into
clouds or precipitates.
Vertical energy transfer at the poles – freezing oceans become more
saline, and the water sinks.
The world therefore has extensive global thermohaline circulation,
which warms polar regions and transfers nutrients to the tropics..
Biosphere
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The biosphere is the living component
of the world. It affects many aspects of
the climate:
Plants absorb more light than bare
ground, reducing albedo (coniferous
forest: 0.09-0.15; cf. bare ground: 0.3).
The biosphere also affects the fluxes of
certain greenhouse gases:
- Terrestrial plants fix CO2 in their structure.
- Oceanic plankton remove CO2 from the atmosphere as shells
when they fall to the ocean bottom.
The biosphere also generates large amounts of aerosols such as
spores, viruses, dust, bacteria and pollen that scatter and reflect
incoming radiation.
Primary productivity in the oceans also generates dimethyl sulphides
(DMS), which act as nuclei for cloud formation.
The geosphere
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This is the physical structure of
the earth, from the soils and
rocks of the continental shelf to
the planet’s core
The internal energies of the earth
can cause climate change over
extremely long periods.
Plate tectonics change the shape
of the surface, and transform
ocean basins or mountains,
affecting energy transfers
between coupled systems.
The structure of soil can affect both its
The Mahameru volcano on the
interaction with the air (in terms of gas
island Java of Indonesia. Photo by
Jan-Pieter Nap
fluxes) and its water retention for
biological processes.
Volcanism can emit vast quantities of CO2 from single events, as well as
putting large amounts of aerosols into the atmosphere, which can
reduce incoming radiation for several years. (Sear et al., 1987).
Different types of climate models
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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.
Energy balance models
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These simple models only really concern themselves
with two things:
- Radiation balance (between incoming solar radiation and
heat loss)
- Latitudinal energy transfer
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EBMs may be 0-D, in which case latitudinal
characteristics are ignored.
In 1-D models, the dimension included is latitude.
Temperature for each latitude band is calculated
using the appropriate latitudinal value for the various
climatic parameters
Radiative-convective models
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These models are generally 1-D or 2-D, with
height always present as a dimension. They
model:
- Radiative transformations as energy is
absorbed, emitted and scattered.
- The role of convection and vertical energy
transfer through atmospheric motion.
By considering surface albedo, cloud amount
and atmospheric turbidity, it calculates the
heat absorption in various atmospheric
layers.
If the heating in a layer exceeds a certain
value (the lapse rate), it will convect into the
layer above, transferring heat energy.
The tropical Hadley cells models are an
example of this type of model.
Source:
http://www.newmediastudio.org/DataDiscovery/Hurr_
ED_Center/Easterly_Waves/Trade_Winds/Trade_Wi
nds_fig02.jpg
Statistical-dynamical models
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These are generally 2-D, with one horizontal
and one vertical dimension (although there
are some models with two horizontal
dimensions).
They combine the horizontal energy
transfer of EBMs with the radiativeconvective functions of RCMs.
However, the equator-pole transfer is more
accurately simulated than in EBMs, based
on theoretical and empirical relationships of
the cellular flow between latitudes.
Energy diffusion is simulated using the
laws of motion.
Statistical relationships define the
windspeed and wind direction within the
models.
These models are useful for simulating and
studying horizontal energy flows, and
processes that disrupt them.
SOURCE:
http://www.newmediastudio.org/DataDiscovery
/Hurr_ED_Center/Easterly_Waves/Trade_Win
ds/Trade_Winds_fig01.jpg
Global circulation models
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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, quasigeotrophic 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 CO2 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
Contemporary GCMs: an outline
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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.
However, taken over the whole globe, this resolution results in an
extremely large number of individual cells.
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.
Climatic processes modelled in a GCM
Thermodynamic
equation
ADVECTION
MOISTURE
Heat balance Hydrology
of the earth’s surface
Equation of
motion
ADVECTION
HEATING AND
COOLING
Radiation
transfer
DENSITY
Equation of
water vapour
Flux adjustments
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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.
How many GCMs are there?
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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.
Use of GCMs
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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.
By running the models on
palaeoclimatological data, an
understanding of long-term climate
effects far beyond the age of even
the human race can be established.
GCMs are thus the best tools for all
climate science, and allow
conservationists, planners and
politicians to test different response
scenarios.
Climatic forcing
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The effect of various factors on the climate are expressed as a
forcing value (ie: to what extent they force global warming)
The forcing value is measured in Wm-2 (the increase in effective
energy caused per square metre).
There are many natural forcing factors operating over different
time periods, from desertification increasing albedo of a planet
to the passage of the solar system through the galaxy (Williams,
1975).
Even the movements and placement of continents over time
has some forcing activity.
However, these natural (also called external, or non-radiative)
forcing factors are independent of those which directly affect
the energy balance of the Earth-atmosphere (called radiative
forcing factors) (Shine et al, 1990).
The effects of current radiative forcings
Source:
IPCC online
slide
archive
IPCC future scenarios
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In order to predict future climate responses, the IPCC has
modelled and detailed several different scenarios (IPCC, 1992;
IPCC, 2000).
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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:
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A1FI – fossil fuel intensive.
A1T – fossil fuel use phased out entirely.
A1B – balanced use of all sources ( no one dominates).
Development scenarios (cont).
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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.
Future radiative forcings depend on response
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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
GCM model responses
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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
Click to enlarge
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.
GCM outputs for 2100 (I)
Source: IPCC
online slides
(SYR fig 3-3a &b)
GCM outputs for 2100 (II)
Linear and non linear responses
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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).
Examples of non-linear changes
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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.
Conclusion
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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.
Check your understanding of
Chapter 2
PASS MARK 80%
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The evidence for anthropogenic climate change
Global Climate Models
Climate change scenarios for Africa
Biodiversity response to past climates
Adaptations of biodiversity to climate change
Approaches to niche-based modelling
Ecosystem change under climate change
Chapter 8 Implications for strategic conservation planning
Chapter 9 Economic costs of conservation responses
I hope that found chapter 2 informative, and that you
enjoy chapter 4. Chapter 3 has unfortunately been
omitted for this course