Physical Climate Models Simulate behavior of climate systemSimulate behavior of climate system

Ultimate objectiveUltimate objectiveUnderstand the key physical, chemical Understand the key physical, chemical

and biological processes that govern and biological processes that govern climateclimate

Obtain a clearer picture of past climates Obtain a clearer picture of past climates by comparison with empirical observationby comparison with empirical observation

Predict future climate changePredict future climate change Models simulate climate on a variety of spatial Models simulate climate on a variety of spatial

and temporal scalesand temporal scales Regional climatesRegional climates Global-scale climate models – simulate the Global-scale climate models – simulate the

climate of the entire planetclimate of the entire planet

Climate Processes Three processes that must be considered Three processes that must be considered

when constructing a climate modelwhen constructing a climate model 1) radiative - the transfer of radiation 1) radiative - the transfer of radiation

through the climate system (through the climate system (e.ge.g. . absorption, reflection);absorption, reflection);

2) dynamic - the horizontal and vertical 2) dynamic - the horizontal and vertical transfer of energy (transfer of energy (e.ge.g. advection, . advection, convection, diffusion);convection, diffusion);

3) surface process - inclusion of 3) surface process - inclusion of processes involving land/ocean/ice, and processes involving land/ocean/ice, and the effects of albedo, emissivity and the effects of albedo, emissivity and surface-atmosphere energy exchangessurface-atmosphere energy exchanges

Constructing Climate Models Basic laws and relationships necessary to Basic laws and relationships necessary to

model the climate system are expressed as a model the climate system are expressed as a series of equationsseries of equations Equations may beEquations may be

Empirical derivations based on Empirical derivations based on relationships observed in the real worldrelationships observed in the real world

Primitive equations that represent Primitive equations that represent theoretical relationships between variablestheoretical relationships between variables

Combination of the twoCombination of the two Equations solved by finite difference methodsEquations solved by finite difference methods

Must consider the model resolution in time Must consider the model resolution in time and space and space i.ei.e. the time step of the model and . the time step of the model and the horizontal/vertical scalesthe horizontal/vertical scales

Simplifying the Climate System All models must simplify complex All models must simplify complex

climate systemclimate system Limited understanding of the climate Limited understanding of the climate

systemsystem Computational restraintsComputational restraints

Simplification may be achieved by Simplification may be achieved by limitinglimiting Space and time resolutionSpace and time resolution Parameterization of the processes Parameterization of the processes

that are simulatedthat are simulated

Model Simplification Simplest models are zero order in spatial dimensionSimplest models are zero order in spatial dimension

The state of the climate system is defined by a single The state of the climate system is defined by a single global averageglobal average

Other models include an ever-increasing dimensional Other models include an ever-increasing dimensional complexitycomplexity 1-D, 2-D and finally to 3-D models1-D, 2-D and finally to 3-D models

Whatever the spatial dimension, further simplification Whatever the spatial dimension, further simplification requires limiting spatial resolutionrequires limiting spatial resolution Limited number of latitude bands in a 1-D modelLimited number of latitude bands in a 1-D model Limited number of grid points in a 2-D modelLimited number of grid points in a 2-D model

Time resolution of climate models varies substantially, Time resolution of climate models varies substantially, from minutes to years depending on the models and the from minutes to years depending on the models and the problem under investigationproblem under investigation

To preserve computational stability, spatial and temporal To preserve computational stability, spatial and temporal resolution must be linkedresolution must be linked

Can pose problems when systems with different Can pose problems when systems with different equilibrium time scales have to interact as a very different equilibrium time scales have to interact as a very different resolution in space and time may be neededresolution in space and time may be needed

Parameterization Involves inclusion of a process as a simplified function Involves inclusion of a process as a simplified function

rather than an explicit calculation from first principlesrather than an explicit calculation from first principles Sub-grid scale phenomena, like thunderstorms, Sub-grid scale phenomena, like thunderstorms,

must be parameterizedmust be parameterized Not possible to deal with these explicitlyNot possible to deal with these explicitly

Other processes are parameterized to reduce Other processes are parameterized to reduce computation requiredcomputation required

Certain processes omitted from model if their Certain processes omitted from model if their contribution negligible on time scale of interestcontribution negligible on time scale of interest Role of deep ocean circulation while modeling Role of deep ocean circulation while modeling

changes over time scales of years to decadeschanges over time scales of years to decades Models may handle radiative transfers in detail but Models may handle radiative transfers in detail but

neglect or parameterize horizontal energy neglect or parameterize horizontal energy transporttransport

Models may provide 3-D representation but contain Models may provide 3-D representation but contain much less detailed radiative transfer informationmuch less detailed radiative transfer information

Modeling Climate Response Ultimate purpose of a modelUltimate purpose of a model

Identify response of the climate systemIdentify response of the climate system Change in the parameters and processes Change in the parameters and processes

that control the state of the systemthat control the state of the system Climate response occurs to restore Climate response occurs to restore

equilibrium within the climate systemequilibrium within the climate system If radiative forcing associated with an If radiative forcing associated with an

increase in atmospheric COincrease in atmospheric CO22 perturbs the perturbs the climate systemclimate system

Model will assess how the climate system Model will assess how the climate system responds to this perturbation to restore responds to this perturbation to restore equilibriumequilibrium

Model Equilibrium Model may require many years of simulated Model may require many years of simulated

change to reach equilibriumchange to reach equilibrium Final years of simulation averagedFinal years of simulation averaged

Nature of the Model One of two modesOne of two modes

Equilibrium modeEquilibrium mode No account taken of energy storage processes No account taken of energy storage processes

that control evolution of climate response with that control evolution of climate response with timetime

Assume climate responds instantaneously Assume climate responds instantaneously following system perturbationfollowing system perturbation

Transient modeTransient mode Inclusion of energy storage processesInclusion of energy storage processes Simulate development of a climate response with Simulate development of a climate response with

timetime Models typically run twiceModels typically run twice

In a control run with no forcingIn a control run with no forcing In a test run including forcing and perturbation of In a test run including forcing and perturbation of

the climate systemthe climate system

Climate Sensitivity Critical parametersCritical parameters In the most complex modelsIn the most complex models

Climate sensitivity calculated explicitly Climate sensitivity calculated explicitly through simulations of processes involvedthrough simulations of processes involved

In simpler models In simpler models Climate sensitivity is parameterized by Climate sensitivity is parameterized by

reference to the range of values suggested reference to the range of values suggested by the more complex modelsby the more complex models

This approach, where more sophisticated This approach, where more sophisticated models are nested in less complex models are nested in less complex models, is common in the field of climate models, is common in the field of climate modelingmodeling

Data-Model Comparisons Models constructed to simulate Modern circulationModels constructed to simulate Modern circulation

Changes based on Earth History inserted in modelChanges based on Earth History inserted in model Climate output compared with observationsClimate output compared with observations

One-Dimensional Models Simplified representation Simplified representation

of of entire planetof of entire planet Model driven by global Model driven by global

mean incoming solar mean incoming solar radiation and albedoradiation and albedo

Single vertical column of Single vertical column of air divided into layersair divided into layers Each layer contains Each layer contains

important constituents important constituents (dust, greenhouse (dust, greenhouse gases, etc)gases, etc)

Layers exchange only Layers exchange only verticallyvertically

Types of Models Energy balance models (EBMs)Energy balance models (EBMs)

Simulate two fundamental climate processesSimulate two fundamental climate processesGlobal radiation balanceGlobal radiation balanceLatitudinal (equator-to-pole) energy transferLatitudinal (equator-to-pole) energy transfer

Radiative-convective models (RCMs) Radiative-convective models (RCMs) Simulate detailed energy transfer through the Simulate detailed energy transfer through the

depth of the atmospheredepth of the atmosphereRadiative transformations that occur as Radiative transformations that occur as

energy is absorbed, emitted and scattered energy is absorbed, emitted and scattered Role of convection Role of convection

EBMs 0-D EBMs0-D EBMs

Earth is a single point in spaceEarth is a single point in space Global radiation balance modeledGlobal radiation balance modeled

In 1-D models latitude is includedIn 1-D models latitude is included Temperature for each latitude band is Temperature for each latitude band is

calculatedcalculatedUsing latitudinal value for albedo, energy Using latitudinal value for albedo, energy

flux, etc.flux, etc. Latitudinal energy transfer estimated from Latitudinal energy transfer estimated from

linear empirical relationshipslinear empirical relationshipsDifference between latitudinal Difference between latitudinal

temperature and global average temperature and global average temperaturetemperature

RBMs Surface albedo, cloud amount and Surface albedo, cloud amount and

atmospheric turbidityatmospheric turbidity Used to determine heating rates Used to determine heating rates

atmospheric layersatmospheric layers Imbalance between net radiation at top Imbalance between net radiation at top

and bottom of each layer determinedand bottom of each layer determined If calculated vertical temperature profile If calculated vertical temperature profile

(lapse rate) exceeds some stability criterion (lapse rate) exceeds some stability criterion (critical lapse rate)(critical lapse rate) Convection is assumed to take place (i.e. Convection is assumed to take place (i.e.

the vertical mixing of air) until the stability the vertical mixing of air) until the stability criterion is no longer breachedcriterion is no longer breached

Two-Dimensional Models Multi-layered Multi-layered

atmosphere coupled atmosphere coupled with Earth’s physical with Earth’s physical properties averaged properties averaged by latitudeby latitude

Allows simulations of Allows simulations of climatic processes climatic processes that vary with latitudethat vary with latitude Angle of incoming Angle of incoming

solar radiationsolar radiation Albedo of Earth’s Albedo of Earth’s

surfacesurface Heat capacity Heat capacity

changeschanges

Statistical-Dynamical Models Combine horizontal energy transfer modeled Combine horizontal energy transfer modeled

by EBMs with the radiative-convective by EBMs with the radiative-convective approach of RCMsapproach of RCMs Equator-to-pole energy transfer is more Equator-to-pole energy transfer is more

sophisticatedsophisticatedParameters like wind speed and wind Parameters like wind speed and wind

direction modeled by statistical relationsdirection modeled by statistical relationsLaws of motion are used to obtain a Laws of motion are used to obtain a

measure of energy diffusionmeasure of energy diffusion Particular useful to investigate role of Particular useful to investigate role of

horizontal energy transfer and processes horizontal energy transfer and processes that directly disturb that transferthat directly disturb that transfer

2-D Models AdvantageAdvantage

Simulate long intervals of time Simulate long intervals of time quickly and inexpensivelyquickly and inexpensively

DisadvantageDisadvantage Not sensitive to climate processes Not sensitive to climate processes

that depend on geographic position that depend on geographic position of continents and oceansof continents and oceans

Three-Dimensional Models - GCM

3-D representation of Earth’s 3-D representation of Earth’s surface and atmospheresurface and atmosphere

Most sophisticated attempt to Most sophisticated attempt to simulate the climate systemsimulate the climate system

3-D model based on 3-D model based on fundamental laws of physics:fundamental laws of physics: Conservation of energyConservation of energy Conservation of Conservation of

momentummomentum Conservation of massConservation of mass Ideal Gas LawIdeal Gas Law

GCMs Represent key features affecting Represent key features affecting

climateclimate Spatial distribution of land, water, iceSpatial distribution of land, water, ice Regional variation in heat capacity Regional variation in heat capacity

and albedo of surfaceand albedo of surface Elevation of mountains and glaciersElevation of mountains and glaciers Concentrations of greenhouse gasesConcentrations of greenhouse gases Seasonal variations in solar radiationSeasonal variations in solar radiation

Calculations at interactions of grid Calculations at interactions of grid boxesboxes

GCMs Atmospheric variables at each grid point requires the Atmospheric variables at each grid point requires the

storage, retrieval, recalculation and re-storage of 10storage, retrieval, recalculation and re-storage of 1055 figures figures at every time-stepat every time-step Models contain thousands of grid pointsModels contain thousands of grid points GCMs are computationally expensiveGCMs are computationally expensive

Can provide accurate representations of planetary climateCan provide accurate representations of planetary climate Simulate global and continental scale processes in detailSimulate global and continental scale processes in detail

GCMs cannot simulate synoptic regional meteorological GCMs cannot simulate synoptic regional meteorological phenomena (e.g.,tropical storms)phenomena (e.g.,tropical storms) Play an important part in the latitudinal transfer of energy Play an important part in the latitudinal transfer of energy

and momentumand momentum Spatial resolution of GCMs limited in vertical dimensionSpatial resolution of GCMs limited in vertical dimension

Many boundary layer processes must be Many boundary layer processes must be parameterizedparameterized

Sensitivity Test Control case establishedControl case established

Modern climate simulatedModern climate simulated One boundary condition altered at a timeOne boundary condition altered at a time

Model output compared with present day Model output compared with present day climate simulationclimate simulation

Information reveals impact of that Information reveals impact of that boundary conditionboundary condition

Boundary condition examplesBoundary condition examplesContinental configurationContinental configuration Ice sheet expansionIce sheet expansionSolar radiation influxSolar radiation influxGreenhouse gas concentrationsGreenhouse gas concentrations

Model Resolution Can it image New Zealand? – this is probably Can it image New Zealand? – this is probably

now out of date! (2° lat x 3° long)now out of date! (2° lat x 3° long)

Atmospheric and Ocean GCMs Atmospheric GCM more sophisticatedAtmospheric GCM more sophisticated

Much detail known about Much detail known about atmospheric circulation, elevations, atmospheric circulation, elevations, landmasses, etc.landmasses, etc.

Ocean GCM primitiveOcean GCM primitive Rudimentary knowledge of oceanic Rudimentary knowledge of oceanic

circulationcirculationDeep water formationDeep water formation

Difficult to model important small Difficult to model important small featuresfeaturesFast-moving narrow currentsFast-moving narrow currents

Oceanic GCMs Similar in construction to atmospheric GCMSimilar in construction to atmospheric GCM Lower boundary seafloorLower boundary seafloor Water column divided grid boxesWater column divided grid boxes

Low resolution, fewer layers/boxes, ±biologyLow resolution, fewer layers/boxes, ±biology Output temperature, salinity, sea ice, gasesOutput temperature, salinity, sea ice, gases

Atmospheric and Ocean GCMs Oceanic GCMs simulates circulation over Oceanic GCMs simulates circulation over

several years to decadesseveral years to decades Atmospheric GCMs simulates circulation Atmospheric GCMs simulates circulation

over several hours to weeksover several hours to weeks Basic incompatibility between modelsBasic incompatibility between models

A-GCMs may be used to drive O-GCMsA-GCMs may be used to drive O-GCMs Asynchronous couplingAsynchronous coupling

Atmospheric conditions drive oceanAtmospheric conditions drive oceanOceanic conditions drive Oceanic conditions drive atmosphereatmosphere

Alternation keeps systems from Alternation keeps systems from getting wackygetting wacky

Geochemical Models Mass balance modelsMass balance models

Follow movement of Earth materials Follow movement of Earth materials from one reservoir to anotherfrom one reservoir to anotherPhysical or chemical formPhysical or chemical form

Models focus on sources, rates of Models focus on sources, rates of transfer and depositional fate of transfer and depositional fate of materialsmaterials

Commonly trace fate of materials using Commonly trace fate of materials using a geochemical tracera geochemical tracer Example Example 1818O content of seawaterO content of seawater

One-way Mass Transfer Models Movement from source to sinkMovement from source to sink

Movement from one reservoir to anotherMovement from one reservoir to another If materials transferred has unique If materials transferred has unique

chemical or physical signaturechemical or physical signature Flux rate (mass transfer timeFlux rate (mass transfer time-1-1) can be ) can be

determineddetermined Example calving of icebergsExample calving of icebergs

Influx of ice-rafted debrisInflux of ice-rafted debrisDetermined by physical Determined by physical sedimentologysedimentology

Quantified by point-countsQuantified by point-counts

Mass Balance Equations Simple mass balanceSimple mass balance

FFtotaltotal = F = F11 + F + F22 + F + F33

Tracer mass balanceTracer mass balance TTRR = (F = (F11TT11 + F + F22TT22 + F + F33TT33)/(F)/(F11 + F + F22+ F+ F33)) TTRR is the mean value of tagged inputs is the mean value of tagged inputs

Mass balance of two components in Mass balance of two components in systemsystem Tracer entering = tracer leavingTracer entering = tracer leaving TTRR = ƒ = ƒininTTinin + (1 – ƒ + (1 – ƒoutout)T)Toutout

Tracer Mass Balance Example Global carbon redox balanceGlobal carbon redox balance

Average Average 1313C of carbon on Earth = -4.6C of carbon on Earth = -4.6‰‰COCO22 in hydrothermal vents in hydrothermal vents

Average Average 1313C of carbonates = +0.6C of carbonates = +0.6‰‰ Average Average 1313C of organic carbon = -25.4C of organic carbon = -25.4‰‰

KnowKnow 1313C entering = C entering = 1313C leavingC leaving RR = ƒ = ƒoooo + (1 – ƒ + (1 – ƒoo))carbcarb

––4.6 = ƒ4.6 = ƒoo(-25.4)+ (1 – ƒ(-25.4)+ (1 – ƒoo)0.6)0.620% of carbon buried in marine 20% of carbon buried in marine

sediments is organic carbonsediments is organic carbon

Chemical Reservoirs Earth reservoirsEarth reservoirs

Atmosphere, ocean, ice, vegetation Atmosphere, ocean, ice, vegetation and sedimentsand sediments

Ocean most important reservoirOcean most important reservoirInteracts with other reservoirsInteracts with other reservoirsReceives weathering productsReceives weathering productsNew minerals deposited in New minerals deposited in sedimentssediments

Tracer is carried to ocean, mixed and Tracer is carried to ocean, mixed and trapped in sedimentary mineral archivetrapped in sedimentary mineral archive

Steady State Tub If flux of tracer into and out of reservoir If flux of tracer into and out of reservoir

are equal, the system is at steady stateare equal, the system is at steady state

Residence Time Time it takes for tracer to pass through Time it takes for tracer to pass through

tubtub Residence time = reservoir size/fluxResidence time = reservoir size/flux

Residence time of tracer typically > Residence time of tracer typically > mixing time of the ocean (1500 y)mixing time of the ocean (1500 y) Tracer distribution homogenousTracer distribution homogenous Tracer concentration or isotopic Tracer concentration or isotopic

composition is everywhere equalcomposition is everywhere equal Records whole-ocean chemistry Records whole-ocean chemistry

during depositionduring deposition

Reservoir Exchange Models Models can be designed to track reversible Models can be designed to track reversible

exchange between different sized reservoirsexchange between different sized reservoirs

Reservoir Exchange Monitor cycling of tracers between reservoirs Monitor cycling of tracers between reservoirs

through timethrough time Tracer with distinctive value moves freely Tracer with distinctive value moves freely

between reservoirsbetween reservoirs Typically between small and large reservoirsTypically between small and large reservoirs

Ocean and atmosphere, vegetation, landOcean and atmosphere, vegetation, land Monitors change in size of smaller reservoirMonitors change in size of smaller reservoir Tracer exchange detected in sedimentary Tracer exchange detected in sedimentary

mineralsmineralsExchange produces change in volume and Exchange produces change in volume and

tracer value in oceantracer value in ocean

Reservoir Exchange Example Change in the Change in the 1818O of seawaterO of seawater 1818O of glacial ice and seawater differentO of glacial ice and seawater different

Change in glacial ice volumeChange in glacial ice volumeProduces small changes in the Produces small changes in the oxygen isotopic composition of oxygen isotopic composition of seawaterseawater

Change in seawater Change in seawater 1818O recordedO recordedCalcareous shells or sediment Calcareous shells or sediment porewaterporewater

Glacial ice small reservoir and ocean Glacial ice small reservoir and ocean large reservoirlarge reservoir

Time-Dependent Models Most geochemical models assume steady-Most geochemical models assume steady-

state conditionsstate conditions Time-dependent models assume steady-state Time-dependent models assume steady-state

only during equilibrium conditionsonly during equilibrium conditions Steady-state conditions imply no change in Steady-state conditions imply no change in

reservoir sizereservoir size Time-dependent models allow changes in Time-dependent models allow changes in

reservoir sizereservoir size From one equilibrium state to anotherFrom one equilibrium state to another Under equilibriumUnder equilibrium

Steady-state conditions prevailSteady-state conditions prevail

CO2 and Long-Term Climate What has moderated Earth surface What has moderated Earth surface

temperature over the last 4.55 by so temperature over the last 4.55 by so thatthat All surface vegetation did not All surface vegetation did not

spontaneously catch on fire and all spontaneously catch on fire and all lakes and oceans vaporize?lakes and oceans vaporize?

All lakes and ocean did not freeze All lakes and ocean did not freeze solid?solid?

Greenhouse Worlds Why is Venus so much hotter than Earth?Why is Venus so much hotter than Earth?

Although solar radiation 2x Earth, most is Although solar radiation 2x Earth, most is reflected but 96% of back radiation absorbedreflected but 96% of back radiation absorbed

What originally controlled C? In solar nebula most carbon was CHIn solar nebula most carbon was CH44

Lost from Earth and VenusLost from Earth and Venus Earth captured 1 in 3000 carbon atomsEarth captured 1 in 3000 carbon atoms

Tiny carbon fraction in the atmosphere as COTiny carbon fraction in the atmosphere as CO22

•60 out of every million C atoms60 out of every million C atoms Bulk of carbon in sediments on EarthBulk of carbon in sediments on Earth

•CaCOCaCO33 (limestone and dolostone) and organic (limestone and dolostone) and organic residues (kerogen)residues (kerogen)

Venus probably had similar early planetary historyVenus probably had similar early planetary history Most carbon is in atmosphere as COMost carbon is in atmosphere as CO22

Venus has conditions that would prevail on EarthVenus has conditions that would prevail on Earth All COAll CO22 locked up in sediments were released to locked up in sediments were released to

the atmospherethe atmosphere

Earth and Venus Water balance different on Earth and VenusWater balance different on Earth and Venus If Venus and Earth started with same If Venus and Earth started with same

components components Venus should have eitherVenus should have either

Sizable oceansSizable oceansAtmosphere dominated by steamAtmosphere dominated by steam

H present initially as HH present initially as H22O escaped to spaceO escaped to spaceHH22O transported "top" of the Venusian O transported "top" of the Venusian

atmosphereatmosphereDisassociated forming H and O atomsDisassociated forming H and O atomsH escaped the atmosphereH escaped the atmosphereOxygen stirred back to surfaceOxygen stirred back to surface

•Reacted with iron forming iron oxideReacted with iron forming iron oxide

Planetary Evolution Similar Although Earth and Venus started with Although Earth and Venus started with

same componentssame components Earth evolved such that carbon safely Earth evolved such that carbon safely

buried in early sedimentsburied in early sedimentsAvoiding runaway greenhouse effectAvoiding runaway greenhouse effect

Venus built up COVenus built up CO22 in the atmosphere in the atmosphere Build-up led to high temperatureBuild-up led to high temperature

High enough to kill all lifeHigh enough to kill all life•If life ever did get a footholdIf life ever did get a foothold

Once hot, could not coolOnce hot, could not cool

Why Runaway Greenhouse? Don't know why Venus Don't know why Venus

climate went haywireclimate went haywire Extra sunlight Venus Extra sunlight Venus

receives?receives? Life perhaps never Life perhaps never

got started?got started? No sink for carbon No sink for carbon

in organic matterin organic matter Was the initial Was the initial

component of water component of water smaller than that on smaller than that on Earth?Earth?

Did God make Venus Did God make Venus as a warning sign?as a warning sign?

Early Earth: Faint Young Sun Solar Luminosity 4.55 bya Solar Luminosity 4.55 bya

25% lower than today25% lower than today Faint young Sun paradoxFaint young Sun paradox

If early Earth had no If early Earth had no atmosphere or today’s atmosphere or today’s atmosphereatmosphere

Radiant energy at Radiant energy at surface well below 0°C surface well below 0°C for first 3 billion years of for first 3 billion years of Earth historyEarth history

No evidence in scant No evidence in scant Archean rock record that Archean rock record that planet was frozenplanet was frozen

Early Earth: A Greenhouse World

Earth was more Venus-Earth was more Venus-like during Archeanlike during Archean

Models indicate that Models indicate that greenhouse requiredgreenhouse required

Several greenhouse Several greenhouse gasesgases HH22O, COO, CO22, CH, CH44, NH, NH33, ,

NN22OO HH22O and COO and CO22 most most

likelylikely101022-10-1033 x PAL CO x PAL CO22

Archean Atmosphere Faint young Sun paradox presents dilemmaFaint young Sun paradox presents dilemma

1) What is the source for high levels of 1) What is the source for high levels of greenhouse gases in Earth’s earliest greenhouse gases in Earth’s earliest atmosphere?atmosphere?

2) How were those gases removed with 2) How were those gases removed with time?time?Models indicate Sun’s strength increased Models indicate Sun’s strength increased

slowly with timeslowly with timeGeologic record strongly suggests Earth Geologic record strongly suggests Earth

maintained a moderate climate maintained a moderate climate throughout Earth history (throughout Earth history (i.e.i.e., no , no runaway greenhouse like on Venus)runaway greenhouse like on Venus)

Source of Greenhouse Gases Input of COInput of CO22 and other greenhouse gases from and other greenhouse gases from

volcanic emissionsvolcanic emissions Most likely cause of high levels in ArcheanMost likely cause of high levels in Archean