Radiative ForcingRadiative Forcing

Definition: A change in the net radiation at Definition: A change in the net radiation at the top of the atmosphere due to some the top of the atmosphere due to some external factor.external factor.

Net RadiationNet Radiation

Net radiation = Incoming - Outgoing Net radiation = Incoming - Outgoing

Positive net radiationPositive net radiation Incoming > OutgoingIncoming > Outgoing

Negative net radiationNegative net radiation Outgoing > IncomingOutgoing > Incoming

Positive & Negative ForcingPositive & Negative Forcing

Positive forcing Positive forcing warming warming

Negative forcing Negative forcing cooling cooling

Forcing and FeedbacksForcing and Feedbacks

““Forcing” is produced by an Forcing” is produced by an externalexternal process, e.g. process, e.g. Changes in solar fluxChanges in solar flux Volcanic eruptionsVolcanic eruptions Human actionsHuman actions

A feedback is a A feedback is a responseresponse to temperature changes to temperature changes

Example: Increased water vapor due to warmingExample: Increased water vapor due to warming

Anthropogenic increases in greenhouse gases are Anthropogenic increases in greenhouse gases are considered forcingsconsidered forcings

Increases in greenhouse gases that are Increases in greenhouse gases that are caused bycaused by temperature changes are feedbacks temperature changes are feedbacks

The same gas can be involved in forcings The same gas can be involved in forcings and feedbacks, e.g., COand feedbacks, e.g., CO22

Forcing: Forcing: COCO22 increase from burning of fossil fuels increase from burning of fossil fuels Largest – by Largest – by farfar: increased greenhouse gases : increased greenhouse gases Increase is almost entirely Increase is almost entirely anthropogenicanthropogenic

FeedbackFeedback temp temp decay decay COCO22

Initial EquilibriumInitial Equilibrium

Absorbed Shortwave

OLR

Top of atmosphere

Now, add greenhouse gas

Keep temperatures fixed

Reduced Upward FluxReduced Upward Flux

Absorbed Shortwave

OLR

Top of atmosphere

Net Downward FluxNet Downward Flux

Net Flux

Top of atmosphere

Result: A positive radiative forcing

Negative Radiative ForcingsNegative Radiative Forcings

Largest: Increase in sulfate aerosolsLargest: Increase in sulfate aerosols Mostly anthropogenic Mostly anthropogenic

Effect of Anthropogenic Sulfate Effect of Anthropogenic Sulfate Aerosols on TemperatureAerosols on Temperature

Direct effectDirect effect The aerosols themselves reflect sunlightThe aerosols themselves reflect sunlight This is similar to the effect of volcanic aerosolsThis is similar to the effect of volcanic aerosols

Indirect effectIndirect effect Sulfate aerosols act as Sulfate aerosols act as condensation nuclei This increases the droplet concentration in cloudsThis increases the droplet concentration in clouds Result: Increased cloud albedoResult: Increased cloud albedo

Both effects tend to increase the Earth’s Both effects tend to increase the Earth’s albedoalbedo

Solar IrradianceSolar Irradiance

Some evidence suggests solar irradiance Some evidence suggests solar irradiance may have increased latelymay have increased lately

Current estimate of forcing: very smallCurrent estimate of forcing: very small

Note: Evidence is very weak!Note: Evidence is very weak!

Climate SensitivityClimate Sensitivity

The change in equilibrium temperature The change in equilibrium temperature per unit of radiative forcingper unit of radiative forcing

Tem

per

atu

re

TimeStart in equilibriumApply

radiative forcing

Temp. risesChange in equilibrium temp

New Equilibrium Temp

ExampleExample

Suppose Sensitivity = 2Suppose Sensitivity = 2C per unit of C per unit of forcing (1 Wmforcing (1 Wm-2-2))

Radiative forcing = 3 WmRadiative forcing = 3 Wm-2-2

Then, Then, eventualeventual warming = 2 x 3 = 6 warming = 2 x 3 = 6C C

Differing SensitivitiesDiffering Sensitivities

Same radiative forcing applied at t= 0

System 2 is twice as sensitive

1 C

2 C

Comparing ModelsComparing Models

Double CODouble CO22 content of model atmosphere content of model atmosphere Radiative forcing ~ 4 W/mRadiative forcing ~ 4 W/m22

IPCC has compared many climate modelsIPCC has compared many climate models

Results used to estimate actual climate Results used to estimate actual climate sensitivity of Earthsensitivity of Earth

Sensitivity EstimatesSensitivity Estimates

Model sensitivities have a range of 2Model sensitivities have a range of 2C to C to 4.54.5C for a doubling of COC for a doubling of CO22

(A technical point – don’t memorize.)(A technical point – don’t memorize.)

The Role of FeedbacksThe Role of Feedbacks

Model sensitivity is determined by the Model sensitivity is determined by the strength of the strength of the feedbacksfeedbacks in the model in the model

PositivePositive feedbacks feedbacks increase increase sensitivitysensitivity

NegativeNegative feedbacks feedbacks decreasedecrease sensitivitysensitivity

Differences in Model SensitivityDifferences in Model Sensitivity

Main Cause of Variation: Cloud Main Cause of Variation: Cloud FeedbacksFeedbacks

In most models, cloud feedback is In most models, cloud feedback is positivepositive However, magnitude varies a lot from one However, magnitude varies a lot from one

model to anothermodel to another

Thermal InertiaThermal Inertia

Determines Determines raterate of temperature of temperature changechange

RateRate of Warming of Warming

Thermal inertiaThermal inertia: resistance of system to : resistance of system to temp. changetemp. change Measured by Measured by heat capacityheat capacity

Higher heat capacity Higher heat capacity slower warming slower warming

System 1: 70% of warming has occurred at t = 1.2Time

Tem

per

atu

re

Ch

ang

e (C

)

System 2: 70% of warming has occurred at t = 2.4

Earth-Atmosphere SystemEarth-Atmosphere System

Most of the heat capacity is in oceansMost of the heat capacity is in oceans

Presence of oceans slows down warmingPresence of oceans slows down warming

ComparisonComparison

Look at two systems with same radiative Look at two systems with same radiative forcing and sensitivity, but different heat forcing and sensitivity, but different heat capacitiescapacities

SummarySummary

Positive (negative) radiative forcing causes Positive (negative) radiative forcing causes warming (cooling)warming (cooling)

System warms (cools) until equilibrium is System warms (cools) until equilibrium is restoredrestored

Amount of eventual warming (cooling) Amount of eventual warming (cooling) depends on radiative forcing and sensitivitydepends on radiative forcing and sensitivity Eventual warming (cooling) = sensitivity x rad. Eventual warming (cooling) = sensitivity x rad.

forcingforcing

RateRate of warming is inversely proportional to of warming is inversely proportional to heat capacityheat capacity

More Realistic SituationMore Realistic Situation

Previous examples assumed radiative Previous examples assumed radiative forcing applied instantaneouslyforcing applied instantaneously i.e., all GHG & aerosols added i.e., all GHG & aerosols added

instantaneouslyinstantaneously

Real life: GHG & aerosols added Real life: GHG & aerosols added graduallygradually

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2121stst Century Climate Change Century Climate Change

Dominant influence likely to be increase in Dominant influence likely to be increase in greenhouse gases (anthropogenic)greenhouse gases (anthropogenic)

Projections of temperature change are Projections of temperature change are made using made using climate modelsclimate models

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Climate Models – 3Climate Models – 3

Climate Model

Anthropogenic forcing

Climate change

Input

Output

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Calculation of Future COCalculation of Future CO22

Concentrations -- MethodConcentrations -- Method

Carbon Cycle Model

Anthropogenic Emissions

CO2 Concentration increase

Model Input

Model output

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Climate Models -- 5Climate Models -- 5

Complication: Models have differing Complication: Models have differing sensitivitiessensitivities

models produce different results for same models produce different results for same emission scenariosemission scenarios

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Differing Response of Models Differing Response of Models for Same Scenariofor Same Scenario

Time

Glo

bal M

ean

Tem

pera

ture

High Sensitivity

Low Sensitivity

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Summary: Two Causes for Summary: Two Causes for Large Range in ProjectionsLarge Range in Projections

1.1. Wide range in emission scenariosWide range in emission scenarios2.2. Wide range in model sensitivitiesWide range in model sensitivities

#1 due to uncertainty in future human #1 due to uncertainty in future human actions (i.e., it is not a fault of the actions (i.e., it is not a fault of the models)models)

#2 is due to our imperfect understanding #2 is due to our imperfect understanding of the climate system (i.e., it is a fault of the climate system (i.e., it is a fault of the models)of the models)

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Impacts Impacts

Arctic: large reduction in summer sea Arctic: large reduction in summer sea iceice Arctic could be ice-free in summer by end Arctic could be ice-free in summer by end

of centuryof century

Permafrost = soil that remains frozen Permafrost = soil that remains frozen throughout the yearthroughout the year Warming Warming softening of permafrost softening of permafrost

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ImpactsImpacts

Glaciers and Ice SheetsGlaciers and Ice Sheets Mountain glaciers will continue to shrinkMountain glaciers will continue to shrink Greenland ice sheet will very probably lose Greenland ice sheet will very probably lose

mass mass

Antarctica (?) Antarctica (?)

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Impacts: Sea LevelImpacts: Sea Level

Melting glacial ice and thermal Melting glacial ice and thermal expansion will cause sea level to riseexpansion will cause sea level to rise

Estimated rise Estimated rise Low-emission scenario: 18 – 38 cmLow-emission scenario: 18 – 38 cm High-emission scenario: 26 – 59 cmHigh-emission scenario: 26 – 59 cm

Estimates are probably too lowEstimates are probably too low Contribution from ice sheets was not taken Contribution from ice sheets was not taken

into account!into account!

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Impact of Rising Sea LevelImpact of Rising Sea Level

Greatest in countries with heavily Greatest in countries with heavily populated coastal regions, e.g. populated coastal regions, e.g. Bangladesh and in small-island nationsBangladesh and in small-island nations

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Fresh Water SuppliesFresh Water Supplies

Warming Warming shrinking glaciers, reduced shrinking glaciers, reduced snowfall in mountainssnowfall in mountains

Problem: 1/6 of world population depends Problem: 1/6 of world population depends on glacial & snow melt for drinking wateron glacial & snow melt for drinking water

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PrecipitationPrecipitation

Models project increases in precipitation in Models project increases in precipitation in some regions, decreases in otherssome regions, decreases in others

Regions of decrease include:Regions of decrease include: Southwestern U. S., Mexico, Central America, Southwestern U. S., Mexico, Central America,

CaribbeanCaribbean MediterraneanMediterranean

Regions of increase include:Regions of increase include: Canada, most of AsiaCanada, most of Asia

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Soil Moisture, RunoffSoil Moisture, Runoff

precip. precip. soil moisture and runoffsoil moisture and runoff

But, can have But, can have soil moisture even with soil moisture even with precip.precip.

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Effect on CaliforniaEffect on California

Warming Warming less snowfall in mountains less snowfall in mountains less summer runoffless summer runoff less water in summer for less water in summer for irrigationirrigation hydroelectric powerhydroelectric power drinking water drinking water

Loss of salmon habitatLoss of salmon habitat

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More about precipitationMore about precipitation

Models project increased Models project increased variabilityvariability

increased flooding increased flooding andand increased increased droughts!droughts!

Another problem: increased demand for Another problem: increased demand for water.water.

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AgricultureAgriculture

Reductions in soil moisture Reductions in soil moisture reduced reduced crop yieldscrop yields

However, areas with increased soil However, areas with increased soil moisture could benefit moisture could benefit (If warming isn’t too large.)(If warming isn’t too large.)

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Other Potential Agricultural Other Potential Agricultural BenefitsBenefits of of Warming Warming

Increased growing season in higher latitudesIncreased growing season in higher latitudes Could benefit Canada, Russia Could benefit Canada, Russia

Beneficial effects of increased COBeneficial effects of increased CO22 could could

offset damaging effects of reduced soil offset damaging effects of reduced soil moisturemoisture Called “COCalled “CO22 fertilization” fertilization” Only works if warming is relatively smallOnly works if warming is relatively small

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EcosystemsEcosystems

In past, ecosystems have been able to In past, ecosystems have been able to adapt, but …adapt, but …

“ “ resilience of many ecosystems is likely to resilience of many ecosystems is likely to be exceeded by 2100”be exceeded by 2100”

Effects of climate change aggravated by Effects of climate change aggravated by increased human demandsincreased human demands fragmentation of habitatsfragmentation of habitats

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Ecosystems, continuedEcosystems, continued

Up to 30% of species at “increasingly high Up to 30% of species at “increasingly high risk of extinction” if average global temp risk of extinction” if average global temp increase above 2 -3increase above 2 -3C C

Oceans becoming more acidic Oceans becoming more acidic Will hurt organisms that make shellsWill hurt organisms that make shells

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Carbon CycleCarbon Cycle

Now, biosphere is a net “sink” of carbonNow, biosphere is a net “sink” of carbon i.e., carbon uptake > carbon releasedi.e., carbon uptake > carbon released

By mid-century, biosphere likely to By mid-century, biosphere likely to become a net source of carbonbecome a net source of carbon i.e., carbon release > carbon uptakei.e., carbon release > carbon uptake (mainly due to increased rate of decay)(mainly due to increased rate of decay)

Ocean carbon uptake will diminishOcean carbon uptake will diminish

Result: Faster rise of COResult: Faster rise of CO22

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Impacts on U. S. ForestsImpacts on U. S. Forests

Each tree species requires a specific Each tree species requires a specific environment for optimum growthenvironment for optimum growth

Climate change will cause a shift in tree Climate change will cause a shift in tree habitatshabitats

Projections of habitat changesProjections of habitat changes http://www.fs.fed.us/ne/delaware/atlas/web_at

las.html#

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Tropical Cyclones – basic infoTropical Cyclones – basic info

Called hurricanes in Atlantic, eastern Called hurricanes in Atlantic, eastern PacificPacific

Called typhoons in western Pacific (north of Called typhoons in western Pacific (north of equator)equator)

Energy source: heat stored in oceansEnergy source: heat stored in oceans

Theory: warmer oceans Theory: warmer oceans stronger storms stronger storms (There is evidence this already happening)(There is evidence this already happening)

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Forest FiresForest Fires

In western U. S., warming In western U. S., warming more forest more forest firesfires

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Human HealthHuman Health

More deaths from heatwavesMore deaths from heatwaves Like 1995 Chicago heat waveLike 1995 Chicago heat wave

Increases in some tropical diseasesIncreases in some tropical diseases

AdaptationAdaptation Strategies for coping with climate changeStrategies for coping with climate change

MitigationMitigation Strategies for reducing the rate of climate Strategies for reducing the rate of climate

changechange

Why Adaptation?Why Adaptation?

Some warming is inevitable because of Some warming is inevitable because of thermal inertiathermal inertia

Adaptation Examples Adaptation Examples Sea-level riseSea-level rise Build coastal defenses Build coastal defenses (e.g., dikes, as in the Netherlands)(e.g., dikes, as in the Netherlands)

Expensive!Expensive! Relocation of affected populationsRelocation of affected populations

(Millions of refugees possible.)(Millions of refugees possible.)

More ExamplesMore Examples

AgricultureAgriculture Increased use of irrigationIncreased use of irrigation Development of heat- and drought-Development of heat- and drought-

resistance crops resistance crops (Genetic engineering?)(Genetic engineering?)

Human health and comfortHuman health and comfort Increased use of air conditioningIncreased use of air conditioning

(Would require increased electricity)(Would require increased electricity)

MitigationMitigation

Reduction of greenhouse-gas emissionsReduction of greenhouse-gas emissions Probably the most effective wayProbably the most effective way

Carbon capture and storageCarbon capture and storage Send emissions into ground or deep oceanSend emissions into ground or deep ocean

Geoengineering (more later)Geoengineering (more later) Increase albedo (e.g., sulfur in stratosphere)Increase albedo (e.g., sulfur in stratosphere) Remove CORemove CO22 (e.g., artificial trees) (e.g., artificial trees)

Reducing Global EmissionsReducing Global Emissions

This is a massive problemThis is a massive problem

Very large reductions are required to make Very large reductions are required to make much differencemuch difference

International cooperation is requiredInternational cooperation is required

Some Methods to Reduce Some Methods to Reduce EmissionsEmissions

Using alternative energy sources, e.g., Using alternative energy sources, e.g., solar, wind, nuclearsolar, wind, nuclear (France: 80% of power generation is nuclear)(France: 80% of power generation is nuclear)

Increased efficiencyIncreased efficiency Improved gas mileage in carsImproved gas mileage in cars Compact fluorescents instead of incandescent Compact fluorescents instead of incandescent

light bulbslight bulbs(Problem: CFs contain mercury)(Problem: CFs contain mercury)

Geoengineering SchemesGeoengineering Schemes

Disposal of CO2 in the Deep OceanDisposal of CO2 in the Deep OceanCarbon Capture and StorageCarbon Capture and StorageAerosol Injection into the StratosphereAerosol Injection into the StratosphereInjection of other small reflectors into Injection of other small reflectors into StratosphereStratosphereShielding the Earth from AfarShielding the Earth from AfarPlacing reflective plastic over desertsPlacing reflective plastic over desertsModify ocean reflectivityModify ocean reflectivityDamming the Bering StraitDamming the Bering Strait