Earth's Climate System Today
Physical Climate Models
Simulate behavior of climate system
Ultimate objective
Understand the key physical, chemical and biological processes that govern climate
Obtain a clearer picture of past climates by comparison with empirical observation
Predict future climate change
Models simulate climate on a variety of spatial and temporal scales
Regional climates
Global-scale climate models – simulate the climate of the entire planet
Climate Processes
Three processes that must be considered when constructing a climate model
1) radiative - the transfer of radiation through the climate system (e.g. absorption, reflection);
2) dynamic - the horizontal and vertical transfer of energy (e.g. advection, convection, diffusion);
3) surface process - inclusion of processes involving land/ocean/ice, and the effects of albedo, emissivity and surface-atmosphere energy exchanges
Constructing Climate Models
Basic laws and relationships necessary to model the climate system are expressed as a series of equations
Equations may be
Empirical derivations based on relationships observed in the real world
Primitive equations that represent theoretical relationships between variables
Combination of the two
Equations solved by finite difference methods
Must consider the model resolution in time and space i.e. the time step of the model and the horizontal/vertical scales
Simplifying the Climate System
All models must simplify complex climate system
Limited understanding of the climate system
Computational restraints
Simplification may be achieved by limiting
Space and time resolution
Parameterization of the processes that are simulated
Model Simplification
Simplest models are zero order in spatial dimension
The state of the climate system is defined by a single global average
Other models include an ever-increasing dimensional complexity
1-D, 2-D and finally to 3-D models
Whatever the spatial dimension, further simplification requires limiting spatial resolution
Limited number of latitude bands in a 1-D model
Limited number of grid points in a 2-D model
Time resolution of climate models varies substantially, from minutes to years depending on the models and the problem under investigation
To preserve computational stability, spatial and temporal resolution must be linked
Can pose problems when systems with different equilibrium time scales have to interact as a very different resolution in space and time may be needed
Parameterization
Involves inclusion of a process as a simplified function rather than an explicit calculation from first principles
Sub-grid scale phenomena, like thunderstorms, must be parameterized
Not possible to deal with these explicitly
Other processes are parameterized to reduce computation required
Certain processes omitted from model if their contribution negligible on time scale of interest
Role of deep ocean circulation while modeling changes
over time scales of years to decades
Models may handle radiative transfers in detail but neglect or parameterize horizontal energy transport
Models may provide 3-D representation but contain much less detailed radiative transfer information
Modeling Climate Response
Ultimate purpose of a model
Identify response of the climate system
Change in the parameters and processes that control the state of the system
Climate response occurs to restore equilibrium within the climate system
If radiative forcing associated with an increase in atmospheric CO climate system
2 perturbs the
Model will assess how the climate system responds to this perturbation to restore equilibrium
Model Equilibrium
Model may require many years of simulated change to reach equilibrium
Final years of simulation averaged
Nature of the Model
One of two modes
Equilibrium mode
No account taken of energy storage processes that control evolution of climate response with time
Assume climate responds instantaneously following system perturbation
Transient mode
Inclusion of energy storage processes
Simulate development of a climate response with time
Models typically run twice
In a control run with no forcing
In a test run including forcing and perturbation of the climate system
Climate Sensitivity
Critical parameters
In the most complex models
Climate sensitivity calculated explicitly through simulations of processes involved
In simpler models
Climate sensitivity is parameterized by reference to the range of values suggested by the more complex models
This approach, where more sophisticated models are nested in less complex models, is common in the field of climate modeling
Data-Model Comparisons
Models constructed to simulate Modern circulation
Changes based on Earth History inserted in model
Climate output compared with observations
One-Dimensional Models
Simplified representation of of entire planet
Model driven by global mean incoming solar radiation and albedo
Single vertical column of air divided into layers
Each layer contains
important constituents
(dust, greenhouse gases, etc)
Layers exchange only vertically
Types of Models
Energy balance models (EBMs)
Simulate two fundamental climate processes
Global radiation balance
Latitudinal (equator-to-pole) energy transfer
Radiative-convective models (RCMs)
Simulate detailed energy transfer through the depth of the atmosphere
Radiative transformations that occur as energy is absorbed, emitted and scattered
Role of convection
EBMs
0-D EBMs
Earth is a single point in space
Global radiation balance modeled
In 1-D models latitude is included
Temperature for each latitude band is calculated
Using latitudinal value for albedo, energy flux, etc.
Latitudinal energy transfer estimated from linear empirical relationships
Difference between latitudinal temperature and global average temperature
RBMs
Surface albedo, cloud amount and atmospheric turbidity
Used to determine heating rates atmospheric layers
Imbalance between net radiation at top and bottom of each layer determined
If calculated vertical temperature profile (lapse rate) exceeds some stability criterion (critical lapse rate)
Convection is assumed to take place (i.e. the vertical mixing of air) until the stability criterion is no longer breached
Two-Dimensional Models
Multi-layered atmosphere coupled with Earth’s physical properties averaged by latitude
Allows simulations of climatic processes that vary with latitude
Angle of incoming solar radiation
Albedo of Earth’s surface
Heat capacity changes
Statistical-Dynamical Models
Combine horizontal energy transfer modeled by
EBMs with the radiative-convective approach of
RCMs
Equator-to-pole energy transfer is more
sophisticated
Parameters like wind speed and wind
direction modeled by statistical relations
Laws of motion are used to obtain a measure of energy diffusion
Particular useful to investigate role of horizontal energy transfer and processes that directly disturb that transfer
2-D Models
Advantage
Simulate long intervals of time quickly and inexpensively
Disadvantage
Not sensitive to climate processes that depend on geographic position of continents and oceans
Three-Dimensional Models - GCM
3-D representation of Earth’s surface and atmosphere
Most sophisticated attempt to simulate the climate system
3-D model based on fundamental laws of physics:
Conservation of energy
Conservation of momentum
Conservation of mass
Ideal Gas Law
GCMs
Represent key features affecting climate
Spatial distribution of land, water, ice
Regional variation in heat capacity and albedo of surface
Elevation of mountains and glaciers
Concentrations of greenhouse gases
Seasonal variations in solar radiation
Calculations at interactions of grid boxes
GCMs
Atmospheric variables at each grid point requires the storage, retrieval, recalculation and re-storage of 10 5 figures at every time-step
Models contain thousands of grid points
GCMs are computationally expensive
Can provide accurate representations of planetary climate
Simulate global and continental scale processes in detail
GCMs cannot simulate synoptic regional meteorological phenomena (e.g.,tropical storms)
Play an important part in the latitudinal transfer of energy and momentum
Spatial resolution of GCMs limited in vertical dimension
Many boundary layer processes must be parameterized
Sensitivity Test
Control case established
Modern climate simulated
One boundary condition altered at a time
Model output compared with present day
climate simulation
Information reveals impact of that boundary condition
Boundary condition examples
Continental configuration
Ice sheet expansion
Solar radiation influx
Greenhouse gas concentrations
Model Resolution
Can it image New Zealand? – this is probably now out of date! (2° lat x 3° long)
Atmospheric and Ocean GCMs
Atmospheric GCM more sophisticated
Much detail known about atmospheric circulation, elevations, landmasses, etc.
Ocean GCM primitive
Rudimentary knowledge of oceanic circulation
Deep water formation
Difficult to model important small features
Fast-moving narrow currents
Oceanic GCMs
Similar in construction to atmospheric GCM
Lower boundary seafloor
Water column divided grid boxes
Low resolution, fewer layers/boxes, ±biology
Output temperature, salinity, sea ice, gases
Atmospheric and Ocean GCMs
Oceanic GCMs simulates circulation over several years to decades
Atmospheric GCMs simulates circulation over several hours to weeks
Basic incompatibility between models
A-GCMs may be used to drive O-GCMs
Asynchronous coupling
Atmospheric conditions drive ocean
Oceanic conditions drive atmosphere
Alternation keeps systems from getting wacky
Geochemical Models
Mass balance models
Follow movement of Earth materials from one reservoir to another
Physical or chemical form
Models focus on sources, rates of transfer and depositional fate of materials
Commonly trace fate of materials using a geochemical tracer
Example 18 O content of seawater
One-way Mass Transfer Models
Movement from source to sink
Movement from one reservoir to another
If materials transferred has unique chemical or physical signature
Flux rate (mass transfer time -1 ) can be determined
Example calving of icebergs
Influx of ice-rafted debris
Determined by physical sedimentology
Quantified by point-counts
Mass Balance Equations
Simple mass balance
F total
= F
1
+ F
2
+ F
3
Tracer mass balance
T
R
T
R
= (F
1
T
1
+ F
2
T
2
+ F
3
T
3
)/(F
1
+ F
2
+ F
3
) is the mean value of tagged inputs
Mass balance of two components in system
Tracer entering = tracer leaving
T
R
= ƒ in
T in
+ (1 – ƒ out
)T out
Tracer Mass Balance Example
Global carbon redox balance
Average d 13 C of carbon on Earth = -4.6‰
CO
2 in hydrothermal vents
Average d 13 C of carbonates = +0.6‰
Average d 13 C of organic carbon = -25.4‰
Know
13 C entering = 13 C leaving d
R
–4.6 = ƒ
= ƒ o d o o
+ (1 – ƒ o
) d carb
(-25.4)+ (1 – ƒ organic carbon o
)0.6
20% of carbon buried in marine sediments is
Chemical Reservoirs
Earth reservoirs
Atmosphere, ocean, ice, vegetation and sediments
Ocean most important reservoir
Interacts with other reservoirs
Receives weathering products
New minerals deposited in sediments
Tracer is carried to ocean, mixed and trapped in sedimentary mineral archive
Steady State Tub
If flux of tracer into and out of reservoir are equal, the system is at steady state
Residence Time
Time it takes for tracer to pass through tub
Residence time = reservoir size/flux
Residence time of tracer typically > mixing time of the ocean (1500 y)
Tracer distribution homogenous
Tracer concentration or isotopic composition is everywhere equal
Records whole-ocean chemistry during deposition
Reservoir Exchange Models
Models can be designed to track reversible exchange between different sized reservoirs
Reservoir Exchange
Monitor cycling of tracers between reservoirs through time
Tracer with distinctive value moves freely between reservoirs
Typically between small and large reservoirs
Ocean and atmosphere, vegetation, land
Monitors change in size of smaller reservoir
Tracer exchange detected in sedimentary minerals
Exchange produces change in volume and tracer value in ocean
Reservoir Exchange Example
Change in the d 18 O of seawater d 18 O of glacial ice and seawater different
Change in glacial ice volume
Produces small changes in the oxygen
isotopic composition of seawater
Change in seawater d 18 O recorded
Calcareous shells or sediment porewater
Glacial ice small reservoir and ocean large reservoir
Time-Dependent Models
Most geochemical models assume steady-state conditions
Time-dependent models assume steady-state only during equilibrium conditions
Steady-state conditions imply no change in reservoir size
Time-dependent models allow changes in reservoir size
From one equilibrium state to another
Under equilibrium
Steady-state conditions prevail
CO
2
and Long-Term Climate
What has moderated Earth surface temperature over the last 4.55 by so that
All surface vegetation did not spontaneously catch on fire and all lakes and oceans vaporize?
All lakes and ocean did not freeze solid?
Greenhouse Worlds
Why is Venus so much hotter than Earth?
Although solar radiation 2x Earth, most is reflected but 96% of back radiation absorbed
What originally controlled C?
In solar nebula most carbon was CH
4
Lost from Earth and Venus
Earth captured 1 in 3000 carbon atoms
Tiny carbon fraction in the atmosphere as CO
• 60 out of every million C atoms
2
Bulk of carbon in sediments on Earth
• CaCO (limestone and dolostone) and organic
residues (kerogen)
Venus probably had similar early planetary history
Most carbon is in atmosphere as CO
2
Venus has conditions that would prevail on Earth
All CO locked up in sediments were released to
Earth and Venus
Water balance different on Earth and Venus
If Venus and Earth started with same components
Venus should have either
Sizable oceans
Atmosphere dominated by steam
H present initially as H
H
2
2
O escaped to space
O transported "top" of the Venusian atmosphere
Disassociated forming H and O atoms
H escaped the atmosphere
Oxygen stirred back to surface
• Reacted with iron forming iron oxide
Planetary Evolution Similar
Although Earth and Venus started with same components
Earth evolved such that carbon safely buried in early sediments
Avoiding runaway greenhouse effect
Venus built up CO
2 in the atmosphere
Build-up led to high temperature
High enough to kill all life
• If life ever did get a foothold
Once hot, could not cool
Why Runaway Greenhouse?
Don't know why Venus climate went haywire
Extra sunlight Venus receives?
Life perhaps never got started?
No sink for carbon in organic matter
Was the initial component of water smaller than that on
Earth?
Did God make Venus as a warning sign?
Early Earth: Faint Young Sun
Solar Luminosity 4.55 bya
25% lower than today
Faint young Sun paradox
If early Earth had no
atmosphere or today’s atmosphere
Radiant energy at surface well below 0°C for first 3 billion years of Earth history
No evidence in scant
Archean rock record that planet was frozen
Early Earth: A Greenhouse World
Earth was more Venuslike during Archean
Models indicate that greenhouse required
Several greenhouse gases
H
2 2
, CH
4
, NH
3
,
N
2
O, CO
O
H
2
O and CO likely
2 most
10 2 -10 3 x PAL CO
2
Archean Atmosphere
Faint young Sun paradox presents dilemma
1) What is the source for high levels of greenhouse gases in Earth’s earliest atmosphere?
2) How were those gases removed with time?
Models indicate Sun’s strength increased slowly with time
Geologic record strongly suggests Earth maintained a moderate climate throughout
Earth history (i.e., no runaway greenhouse like on Venus)
Source of Greenhouse Gases
Input of CO
2 and other greenhouse gases from volcanic emissions
Most likely cause of high levels in Archean
